control systems using the three test maneuvers developed at VRTC.

                         Table 2--EMA Test Tractors Including Type, GVWR, and Wheelbase
----------------------------------------------------------------------------------------------------------------
                                                                                Wheelbase
   Tractor configuration (EMA Vehicle I.D.)          Stability control type          GVWR (lb)       (inches)
----------------------------------------------------------------------------------------------------------------
6x4 Typical Tractor (Vehicle A)...............  ESC.............................          52,000             228
4x2 (Vehicle B)...............................  ESC.............................          32,000             140
4x2 (Vehicle C)...............................  RSC with steering wheel angle             34,700             152
                                                 sensor.
6x4 Severe Service (Vehicle D)................  ESC.............................          66,000             220
6x4 w/Pusher Axle (Vehicle E).................  ESC.............................          86,000             270
8x6 Tridem Drive Axle (Vehicle F).............  ESC.............................          89,000             263
6x4 w/Pusher Axle (Vehicle G).................  ESC.............................          92,000             243
6x4 Severe Service (Vehicle H)................  RSC.............................          60,600             246
6x4 (Vehicle I)...............................  ESC.............................          52,000             232
6x4 (Vehicle J)...............................  ESC.............................          52,350             245
----------------------------------------------------------------------------------------------------------------

    EMA provided its test data to the agency.\27\ Although the tractors
were not identified by make or model, EMA provided the configuration
and weight ratings for each tractor. Eight tractors were subjected to
the SIS and RSM to evaluate rollover prevention, and three tractors
were subjected to the SWD maneuver, and the ramp with dwell (RWD)
maneuver on a low-friction surface to evaluate yaw stability. Two of
the tractors were equipped with RSC systems and seven tractors were
equipped with ESC systems. EMA also submitted test data for several
maneuvers in which the test parameters were varied. With the exception
of Vehicle J, EMA did not submit baseline test data--that is, EMA
submitted data only for maneuvers with ESC or RSC systems enabled.
---------------------------------------------------------------------------

    \27\ Data from Vehicles A through I are included have been
placed in the docket. Docket Nos. NHTSA-2010-0034-0011 through
NHTSA-2010-0034-0021 and Docket No. NHTSA-2010-0034-0024. Vehicle J
testing is discussed in detail in a later section.
---------------------------------------------------------------------------

1. Slowly Increasing Steer Maneuver
    For all tractors, test data were provided for the SIS tests used to
derive the steering wheel angle with each tractor in the bobtail
condition. In the first SIS series conducted on eight of the tractors,
three SIS tests were conducted in each direction on a dry road surface,
and a best fit linear regression was used to project the steering wheel
angle for a lateral acceleration of 0.5g. The average of the absolute
value of each of the six runs was calculated for the final angle.
    Compared to the steering wheel angles that were derived for the
three VRTC tractors, a much wider range in SWA was seen among EMA's
results. The steering wheel angles generally increased with the
tractor's wheelbase from an angle of 126 degrees for the 140-inch
wheelbase 4x2 to an angle of 291 degrees for the 270-inch wheelbase 6x4
with a pusher axle. For Vehicle H, EMA also provided data from direct
measurement of the steering wheel angle from driving the tractor at
0.5g of lateral acceleration. This angle was 290 degrees, which is
slightly larger than the calculated value of 281 degrees extrapolated
from the SIS test data in the 0.05 to 0.30g operating region. The EMA
data provided for these SIS tests did not include information on
stability control engine torque reduction.
    Additional SIS tests were conducted on three tractors that were to
be subsequently tested using the SWD maneuver to evaluate tractor yaw
stability. The SIS test conditions were identical to the prior SIS
tests. A best fit linear regression was used to project the steering
wheel angle for a lateral acceleration of 0.5g, and the average of the
absolute value of each of the six runs was calculated for the final
angle as in the prior SIS tests. Comparing these data to the prior SIS
test results, Vehicle B, which had the smallest angle of 126 degrees in
the prior SIS tests, showed a ten degree reduction of its angle in this
test series. Vehicle G's angle was nearly identical (203 degrees in the
first series vs. 205 degrees in the second series).
2. Ramp Steer Maneuver
    For the RSM tests on eight tractors, the tractors were attached to
a FMVSS No. 121 control trailer and were loaded to their GVWR by
placing the ballast over the fifth wheel, with the ballast placed
directly on the trailer deck resulting in a low center of gravity
height. The weight on the FMVSS No. 121 control trailer's single axle
ranged between 5,720 and 5,930 lb for all eight tractor tests, and the
trailer brakes were not enabled. While the weight on the trailer axle
is nominally 4,500 lb when the trailer is used for FMVSS No. 121
stopping distance tests, the increased weight in these RSM tests
reflects the added weight of the outriggers installed on the trailer.
In general, each of the tractors was loaded to its GVWR with the steer,
drive, and auxiliary axles loaded to, or very close to, their
respective GAWRs. The only exception was the 140-inch wheelbase 4x2
which only had 9,950 lb on the steer axle, although it was rated for
12,000 lb.
    In the tests, the stability control systems automatically applied
the tractor's foundation brakes to reduce speed and lateral
acceleration. The initial vehicle deceleration generally coincided with
the end of ramp steer input, indicating that the stability control
systems were effective at reducing the lateral acceleration. The speed
at wheel lift for EMA's tests ranged from 33 to 38 mph, as compared to
31 to 39 mph for the VRTC tests that used a similar unbraked trailer,
but with a higher center of gravity loading condition and a higher
overall vehicle test weight. Both 4x2 tractors tested by EMA
experienced oversteer in addition to the wheel lift.
3. Sine With Dwell Maneuver
    EMA provided test results for the SWD maneuver for four tractors
equipped with ESC systems. The sinusoidal steering frequency used for
testing was 0.5 Hz and the dwell time was one second. The amplitude of
the steering wheel inputs started at 30 percent of the steering wheel
angle derived from SIS testing, and in subsequent test runs was
increased by 10 percent increments up to 130 percent of the steering
angle. The SWD tests were conducted with two tractor loading
conditions: Loaded to 60 percent of the drive axle(s) GAWR with the
FMVSS No. 121 unbraked control trailer attached (loaded tests), and in
the unloaded condition with no trailer attached (bobtail tests). The
maneuver entrance speed was 45 mph and the test was conducted on dry
pavement.
    The results of the loaded tests for Vehicles G and I indicated that
both tractors remained roll and yaw stable through the full range of
testing, and there were no indications of tractor wheel lift in the
test comments or the unprocessed data. The largest steering wheel angle
produced the highest peak lateral acceleration, which occurred during
the dwell portion of the maneuver for both tractors. Vehicle I reached
approximately 0.75g and Vehicle G reached just under 0.6g. Although
both tractors were close in wheelbase and tested with similar steering
wheel angles, Vehicle G, tested with its liftable axle in the lowered
position, was either less responsive in the SWD maneuver or its ESC
performed slightly better than the ESC on Vehicle I. Both tractors had
similar overall vehicle decelerations; however, the ESC on Vehicle G
commanded higher steer axle braking pressures than the ESC on Vehicle
I. Vehicle I appeared to have more lateral sliding in the maneuver, as
its yaw rate decay was slower at the end of steering input.
    Vehicle B (140-inch wheelbase 4x2) exhibited yaw instability in the
SWD maneuver. This tractor had high lateral acceleration that was
attained at lower steering wheel angles than for the 6x4 tractors. For
example, the peak tractor lateral acceleration was already reaching
0.70g at 80 percent of the SIS-derived steering wheel angle, compared
to Vehicle I which reached 0.60g and Vehicle G which reached 0.45g at
this steering wheel angle scalar. The yaw rate decay after completion
of steer was also much slower than for the 6x4 tractors, which appears
to indicate that the vehicle was sliding much more and taking longer to
return to the straight-ahead position. This is most evident in the
testing at 130 percent of the SIS-derived steering wheel angle, in
which the decay yaw rate decay was about 3.5 seconds.
    The maneuver entrance speed was reduced to 30 mph in the bobtail
SWD tests, which were conducted on a low-friction wet Jennite surface.
The short wheelbase 4x2 tractor, Vehicle B, appeared to complete all of
the test series without any observed instability

or control issues, and the peak tractor lateral acceleration was
limited to approximately 0.3g in all tests. However, both 6x4 tractors
(Vehicles G and I) appeared to have steering responsiveness issues that
were particularly noticeable at higher steering wheel angles. At the
reversal in steering wheel angle direction, the yaw rate and lateral
acceleration response was delayed, indicating severe understeer. During
the dwell portion of the maneuver at higher steering wheel angles,
Vehicle I slowly built lateral acceleration up to 0.3g, while Vehicle G
achieved similar but slightly lower acceleration levels. Vehicle G's
yaw rate also was slower to respond at the completion of steer, taking
as long as 2.5 seconds to decay to zero for the test conducted at the
highest steering wheel angle tested.
4. Ramp With Dwell Maneuver
    The three tractors equipped with ESC systems tested in the SWD
maneuvers were also tested to the RWD maneuver. Once the initial
steering wheel angle and test speed were attained, the steering machine
increased the steering wheel angle to 180 degrees in one second, held
that steering wheel angle constant for three seconds (the dwell portion
of the maneuver), and then reduced the steering wheel angle to zero in
one second. In subsequent RWD test runs, the steering wheel angle was
increased in 90 degree increments up to 540 degrees.
    The test results show that for Vehicles B and I, the steady-state
lateral acceleration (prior to the ramp steer) was approximately 0.2g,
and for Vehicle G the steady-state tractor lateral acceleration was
approximately 0.1g. When the steering wheel angle was increased during
the initial steering ramp input, the lateral acceleration and yaw rate
increased slightly and in many of the test runs was then observed to
drop off, indicating that the tractor was not responsive to the
steering input. During the first two seconds of the steering dwell
portion of the maneuver, the tractor lateral acceleration typically
remained at 0.25g or less for all tests. During the last one second of
the steering dwell, all of the test runs for Vehicles G and I showed
steadily increasing lateral acceleration, as high as 0.5g, even as the
steering wheel angle was reduced to zero. This indicates that the
tractors were in a severe oversteer condition, and the agency
speculates that the relatively high lateral acceleration may have been
a result of the tractor running off of the low friction wet Jennite
surface and onto a higher friction road surface. The test data show
that this was always accompanied by braking on the steer axle, which is
indicative of oversteer corrections being commanded by the ESC. Vehicle
B had much less increase in lateral acceleration at the end of the
maneuver and appeared to be under control. Late in the maneuver the
commanded brake pressures for Vehicle B showed that both front and rear
brake applications were made on the right side of the tractor, and the
application pressures were nearly identical. Whether this is a data
collection anomaly or stability control braking strategy is not
certain, but Vehicle B was the vehicle that exhibited the least amount
of oversteer.
    The RWD test results demonstrated that the stability control
systems on these tractors correctly identified the vehicle loss of
control problems (severe oversteer and understeer) and took corrective
action, including engine output torque intervention and commanding
individual applications of the tractor's foundation brakes. However,
the severity of the RWD test maneuver was sufficiently high to
overdrive the capability of the stability control systems to mitigate
severe understeer.
    In summary, EMA provided test data for nine tractors each tested
for the three maneuvers developed by NHTSA researchers. The nine
tractors included a wider variety of tractor configurations than those
tested by the agency, and included severe service tractors, tractors
with auxiliary lift axles, a tridem drive axle tractor, and a very
short wheelbase two-axle tractor. Slowly increasing steer vehicle
characterization tests were conducted on all nine tractors (two with
RSC and seven with ESC) in the bobtail condition and the test data were
used to extrapolate the steering wheel angle that would provide 0.5g of
lateral acceleration at 30 mph. These data produced a wider range of
steering wheel angles than had been seen from the agency's tests on its
three tractors, with the short wheelbase 4x2 having an angle of only
116 degrees, and a 6x4 tractor with a liftable pusher axle having the
highest angle at 291 degrees.
    EMA provided ramp steer maneuver test results for eight tractors
that were loaded to their GVWRs using an unbraked 28-foot control
trailer. Data were only provided for tests with the stability control
system enabled, and the RSM was conducted up to speeds at which the
system could successfully intervene. The range of speeds achieved at
the point of overdriving the stability control systems was similar to
the range of speeds from the VRTC RSM tests, although the loading
conditions were slightly different. The two 4x2 tractors (one with RSC,
and one with ESC) tested by EMA experienced oversteer and wheel lift,
while the other tractors all experienced wheel lift.
    SWD test results were provided for three tractors, each equipped
with ESC, using a 0.5 Hz sinusoidal steering input frequency and a 1.0
second dwell time, and the tractors were tested in the bobtail
condition and loaded to 60 percent of drive axle(s) GAWR. In the tests
on dry pavement at a maneuver entrance speed of 45 mph, the typical 6x4
completed all tests, while the 6x4 equipped with a lift axle (tested in
the lowered position) also completed all tests but appeared to be
slower to respond to the steering inputs. The short wheelbase 4x2
tractor appeared to exhibit control problems and at the highest
steering wheel angle tested. The sine with dwell tests on the three
tractors in the bobtail condition were conducted on a low-friction wet
Jennite test surface with a lower maneuver entrance speed of 30 mph. In
these tests, the short wheelbase 4x2 tractor completed all tests, while
the two 6x4 tractors appeared to experience severe understeer at the
higher steering wheel angles tested.
5. Vehicle J Testing
(a) EMA Testing of Vehicle J
    In December 2010, EMA provided testing data on a tenth vehicle they
tested.\28\ Vehicle J was intended to be representative of a typical
6x4 tractor, with a 245 inch wheelbase and a GVWR of 52,350 pounds. EMA
subjected Vehicle J to four different test maneuvers: The slowly
increasing steer test; the sine with dwell test; a J-turn maneuver, and
a wet Jennite drive through test.
---------------------------------------------------------------------------

    \28\ Vehicle J data provided to the agency has been placed in
Docket No. NHTSA-2010-0034-0022 and Docket No. NHTSA-2010-0034-0023.
---------------------------------------------------------------------------

    EMA first conducted the slowly increasing steer test maneuver with
a steering controller on Vehicle J to determine the steering wheel
angle that would produce a lateral acceleration of 0.5g. EMA conducted
two series of test runs, one in each direction. A best fit linear
regression was used to determine that the average steering angle on the
six runs that would produce a lateral acceleration of 0.5g was 197
degrees. This value was used for subsequent testing.
    EMA next conducted sine with dwell testing. EMA conducted two
series of SWD tests--one with the ESC system on and one with the ESC
system off. EMA equipped the vehicle with an FMVSS No. 121 control
trailer and loaded the

vehicle so that the drive axles were loaded to 60 percent of the GAWR,
which resulted in the vehicle being loaded to approximately 78.6
percent of its GVWR.
    EMA provided data on six runs of the SWD maneuver. EMA conducted
the test at scalars from 0.8 to 1.3 of the SIS-derived steering wheel
angle. EMA also provided data on three runs of the SWD maneuver with
the system deactivated. Those tests were conducted at scalars of 1.0
and 1.3, and 1.5.
    Each test run with the system enabled showed a 20- to 25-mph
reduction of speed during the test maneuver. In contrast, tests
conducted with the system off indicated only limited speed reduction of
less than five mph. This indicated that the ESC system acted to reduce
vehicle speed.
    Each test run with the system enabled conducted at scalars between
0.8 and 1.2 resulted in a peak lateral acceleration between 0.6g and
0.7g. The lateral acceleration then quickly dropped to zero within 0.3
to 0.4 seconds after the completion of the steer. Yaw rate during the
dwell portion of the maneuver peaked at approximately 18 to 22 degrees
per second, except at a scalar of 1.2 where yaw rate peaked at
approximately 24 degrees per second) and showed a downward trend during
the dwell, dropping by approximately five degrees per second. The yaw
rate dropped to zero within 0.2 seconds after completion of steer. The
vehicle's ESC system used selective braking to reduce the speed,
lateral acceleration, and yaw rate responses.
    With the system disabled, the test run at a scalar of 1.0 resulted
in a peak lateral acceleration of approximately 0.8g. A 0.2g drop in
lateral acceleration was observed at the beginning of the dwell portion
of the maneuver followed by a sudden rise of the same amount,
indicating possible oversteer. The lateral acceleration dropped to zero
less quickly than in tests with the system on (approximately 0.5
seconds) after completion of steer. This was largely due to the drop in
lateral acceleration starting later with the system off than with the
system on. The yaw rate peaked at approximately 21 degrees per second.
Unlike with the system on, there was not a clear drop in yaw rate
during the dwell portion of the maneuver. The yaw rate also dropped to
zero slower than in tests with the system off (approximately 0.25
seconds after completion of steer).
    For test runs at steering wheel angle scalars of 1.3, the peak
lateral acceleration was slightly lower with the system on
(approximately 0.75g) in comparison to the test run with the system off
(over 0.8g). Momentary variability in lateral acceleration was observed
in both tests, indicating possible tractor instability. Again, with the
system on, the lateral acceleration decayed faster at the completion of
steer (approximately 0.4 seconds) than it did with the system off (over
0.6 seconds). This was largely due to the reduction in lateral
acceleration starting later with the system off than with the system
on. The yaw rate peaked for both tests at approximately 25 degrees per
second. Again, however, the yaw rate decreased by approximately five
degrees during the dwell portion of the maneuver with the system on
while no clear decay was observed with the system off. Also, the yaw
rate decreased to zero slower after completion of steer with the system
off (0.25 seconds) than it did with the system on (less than 0.2
seconds).
    EMA also submitted data on one SWD test run with the system off at
a steering wheel angle scalar of 1.5. Peak lateral acceleration
observed during this test run was nearly 0.9g. The lateral acceleration
rate dropped to zero in slightly over 0.5 seconds after completion of
steer. The yaw rate peaked at approximately 24 degrees per second.
Unlike in runs with lower steering wheel angles, a reduction in yaw
rate was observable during the dwell portion. However, that reduction
was much sharper, occurring entirely within a 0.5 second period rather
than throughout the entire 1.0 second dwell period. Like in prior
tests, the yaw rate dropped to zero within approximately 0.25 seconds.
    EMA's SWD maneuver test data from Vehicle J demonstrated that the
ESC system activated to lower lateral acceleration and yaw rate during
the SWD maneuver. However, even with the ESC system turned off, the
lateral acceleration and yaw rates dropped relatively quickly at the
end of the test maneuver, indicating that the vehicle did not become
unstable during testing. Although EMA only provided test data from
three runs with the system off compared to six runs with the system
enabled, the runs with the system off did include a run with a steering
wheel angle scalar of 1.5, which was higher than any run in NHTSA's
testing, and no severe incidents of instability were observed.
    EMA next conducted J-turn testing both with the system enabled and
disabled. The test was conducted on a 150-foot fixed radius curve. The
vehicle was tested with an FMVSS No. 121 control trailer and was loaded
to the FMVSS No. 121 loading conditions. The tests were conducted at
initial entry speeds of 30 to 36 mph, in increments of two mph.
    In tests conducted with the ESC system enabled, system activation
occurred at each test speed. The system commanded brake activations to
reduce vehicle speed to 18 mph from initial speeds of 30 mph and 32
mph, down to 10 mph from an initial speed of 34 mph, and down to 6 mph
at an initial speed of 36 mph. The vehicle was able to maintain the
lane at all speeds tested. Lateral acceleration peaked at 0.4 to 0.5g
at 30 and 32 mph and peaked at 0.6g at 34 mph and 36 mph. Yaw rate
peaked at approximately 15 degrees per second at 30 and 32 mph and
peaked at approximately 20 degrees per second at 34 mph and 36 mph. At
the higher speeds tested, lateral acceleration and yaw rate were
observed to drop coincident with speed.
    With the system disabled, no reduction in speed during the maneuver
was observed. Thus, lateral acceleration and yaw rates remained
relatively constant throughout the maneuver. At test speeds of 30 and
32 mph, lateral acceleration peaked at approximately 0.55 to 0.65g and
yaw rate peaked at approximately 20 degrees per second. At 34 mph, the
lateral acceleration peaked at approximately 0.9g and the steering
wheel angle necessary to maintain the lane decreased substantially. Yaw
rate peaked at approximately 22 degrees per second and dropped to
approximately 15 degrees per second, indicating the vehicle was
starting to plow out. At 36 mph, the vehicle plowed out of the lane.
    The fourth maneuver EMA performed on Vehicle J was a wet Jennite
drive-through (WJDT) maneuver. This maneuver was intended to test yaw
stability. The WJDT maneuver is identical to method for determining the
maximum drive-through speed when testing vehicles for compliance with
S<IP_ADDRESS> of FMVSS No. 121. The vehicle is driven through a 500-foot
radius curve with a wet surface having a peak coefficient of friction
of approximately 0.5 at successively increasing speeds (up to 40 mph)
to determine the maximum speed at which the vehicle can maintain the
curve.\29\
---------------------------------------------------------------------------

    \29\ To conduct the FMVSS No. 121 stability and control during
braking compliance test, the vehicle is driven at the lesser of 30
mph or 75 percent of the maximum drive-through speed. A full brake
application is made and a vehicle must stop at least three times out
of four within the 12-foot lane.
---------------------------------------------------------------------------

    EMA performed this test with both the stability control system
enabled and disabled in two load configurations. First, the vehicle was
tested in the bobtail (unloaded) configuration.

Second, the vehicle was loaded to the FMVSS No. 121 test loading
condition.
    In the bobtail configuration with the ESC system enabled, test runs
at 30 and 32 mph yielded no system activation. At 33 mph, system
activation occurred as both engine torque reduction and selective
braking to improve yaw stability occurred. As a result, the vehicle
speed decreased to approximately 29 mph during the maneuver and the
driver responded by rapidly straightening the steering wheel. Vehicle
yaw rate peaked at approximately 10 degrees per second. A second run at
33 mph showed only brief system activation and a minimal reduction in
speed. During two runs at 34 mph, ESC system intervention was again
observed as torque reduction and selective braking reduced vehicle
speed to 28 to 29 mph and the driver again responded by rapidly
straightening the steering wheel. Yaw rate peaked at near 10 degrees
per second and again, as the driver responded, decreased. During two
runs at 35 mph, the vehicle was unable to maintain the lane due to
understeer, despite system intervention.
    In the bobtail configuration with the system disabled, at 32 mph,
the driver had to adjust steering by adding steering input during both
runs attempted at this speed, indicating substantial understeer. During
two runs at 33 mph, the vehicle was unable to maintain the lane,
despite large steering inputs from the driver.
    In the loaded configuration with the ESC system enabled, system
activation occurred at a speed of 30 mph, though only slight (1 to 2
mph) reduction in speed was observed. The driver had to increase his
steering input, but there was no corresponding increase in yaw rate,
indicating understeer. At 32 mph, both engine torque reduction and
selective braking occurred to improve yaw stability occurred. As a
result, the vehicle speed decreased to approximately 27 to 28 mph
during the maneuver. At 34 mph, the ESC system intervened more
substantially, resulting in a reduction of speed to approximately 26
mph. Nevertheless, the vehicle was able to maintain the lane. At 35
mph, the vehicle was unable to maintain the lane due to understeer,
despite system intervention.
    In the loaded configuration with the system disabled, understeer
was observed at 32 mph, as evident by substantial increase in steering
input by the driver; however, the vehicle was able to maintain the
lane. At 33 mph, the vehicle was unable to maintain the lane.
    The maximum drive through speed in both vehicle configurations was
only 32 mph with the system off, compared to 34 mph with the system on.
This demonstrates that an ESC system has some ability to mitigate
understeer when navigating a curve on a low-friction surface, and allow
the driver to maintain control at higher curve entrance speeds.
(b) NHTSA Testing of EMA's Vehicle J
    At NHTSA's request, EMA provided Vehicle J to NHTSA for NHTSA to
duplicate EMA's testing.\30\ In particular, the agency was interested
in the performance of Vehicle J during the sine with dwell maneuver.
NHTSA's two 6x4 tractors that were tested in with the SWD represented
the upper and lower size bounds of what would be considered a typical
6x4 tractor and both tractors could not maintain stability during a SWD
maneuver with the ESC system disabled. Vehicle J's size is within the
bounds of the two typical 6x4 tractors tested by NHTSA.
---------------------------------------------------------------------------

    \30\ A copy of NHTSA's Vehicle J test data has been placed in
the docket. Docket No. NHTSA-2010-0034-0044.
---------------------------------------------------------------------------

    NHTSA conducted 20 test runs of Vehicle J in the SWD maneuver at
steering wheel angle scalars of 0.4 to 1.3 of the SIS-derived steering
wheel angle attached to VRTC's FMVSS No. 121-style control trailer.
When tested with the ESC system disabled at a steering wheel angle
scalar of 1.2, NHTSA was able to detect lateral instability that
continued for almost two seconds after completion of the SWD
maneuver.\31\
---------------------------------------------------------------------------

    \31\ NHTSA was able to conduct 19 test maneuvers with Vehicle J
that did not result in substantial roll instability. NHTSA did not
find any yaw instability in any of the 20 test maneuvers.
---------------------------------------------------------------------------

    It was discovered that EMA conducted its testing of Vehicle J with
a control trailer with different specifications than NHTSA used. NHTSA
then attempted to duplicate EMA's Vehicle J's testing using the control
trailer used by EMA.\32\ The results of NHTSA's tests with EMA's
control trailer were not meaningfully different than the results of
EMA's testing. That is, there were no instances of substantial roll or
yaw instability in 20 test runs conducted by NHTSA.
---------------------------------------------------------------------------

    \32\ NHTSA's test data identifies the trailer used by EMA as a
``Link'' trailer and the trailer used by NHTSA as the ``NHTSA'' or
``VRTC'' trailer.
---------------------------------------------------------------------------

    As a result of NHTSA's testing of Vehicle J, the agency discovered
that there exist three areas of variability in FMVSS No. 121-style
control trailers and loading which, while not necessarily relevant to
FMVSS No. 121 testing, could affect the results of stability control
system testing if the specifications for an FMVSS No. 121-style control
trailer were simply carried over to a stability control standard.
First, EMA's control trailer had a wider track width \33\ than NHTSA's
trailer, which made EMA's trailer, and thereby the combination vehicle,
more stable during SWD testing. Second, EMA's control trailer had a
lower deck height than NHTSA's trailer, which contributed to a lower
center of gravity on EMA's trailer. Third, EMA loaded its trailer with
steel for ballast, whereas NHTSA loaded its trailer with concrete for
ballast, which also contributed to the lower center of gravity on EMA's
trailer because steel would not have to be stacked as high to achieve a
full load.
---------------------------------------------------------------------------

    \33\ The track width is the distance between the centerlines of
a vehicle's left and right tires. In vehicles with dual tires, the
track width would be measured from between the dual tires on each
side of the vehicle.
---------------------------------------------------------------------------

E. Other Industry Research

    The SAE Truck and Bus Control Systems Task Force (renamed as the
Truck and Bus Stability Control Committee) was formed in 2007 to
facilitate information sharing among the industry and government
regarding heavy vehicle stability control systems.\34\ The information
shared included proposed test maneuvers that could potentially be used
to evaluate the performance of stability control systems. Although the
Task Force has not published any formal documents describing these test
maneuvers, the following provides an overview of the maneuvers that
have been discussed.
---------------------------------------------------------------------------

    \34\ See http://www.sae.org/events/cve/presentations/2007truckbus.pdf for an overview of the SAE Truck and Bus Council
organizational chart.
---------------------------------------------------------------------------

1. Decreasing Radius Test
    A decreasing radius test (DRT) was developed to evaluate the roll
stability performance of a heavy vehicle stability control system.\35\
With the DRT, the test conditions could also be adjusted to evaluate
yaw stability as well. In the DRT, the vehicle is accelerated to a
constant speed of 29 mph on a dry road surface, and an initial steering
input is made to follow a curve with a 150-foot radius. Once the
initial curve radius is achieved, the radius is linearly reduced to a
radius of 90 feet as the vehicle negotiates 120 degrees of arc. Thus,
it is similar to the J-turn maneuver. The speed of 29 mph was derived
based on a vehicle dynamics simulation, which estimated that the
maneuver would produce 0.3g of lateral acceleration during the initial
steering input and this would steadily increase to 0.6g at the 90-foot
radius curve.
---------------------------------------------------------------------------

    \35\ See Docket No. NHTSA-2010-0034-0036.
---------------------------------------------------------------------------

    Tests would be conducted in a loaded condition with the tractor
coupled to a trailer and an unloaded condition in a

bobtail configuration. Because actual vehicle testing had not been
conducted using this maneuver, pass/fail criteria have not yet been
developed. Simulations of this test have been run using driver-
controlled steering inputs; however, parameters could also be developed
to conduct this maneuver using an automated steering controller.
2. Lane Change on a Large Diameter Circle
    Volvo provided information on the Lane Change on a Large Diameter
Circle (LC-LDC) maneuver that they have used to evaluate stability
control system performance.\36\ In this maneuver the vehicle is driven
at a constant speed, just below the threshold speed for rollover or
loss of control, around the inside lane of an 800-foot radius curve
that has two lanes. The driver then drifts to the outside lane, and
steers back into the inside lane. For rollover testing the asphalt road
surface is dry and for yaw testing the surface is wet. The test can be
conducted using a bobtail tractor, a tractor towing an FMVSS No. 121
control trailer, or a tractor towing any other type of trailer in a
fully loaded condition. Volvo evaluated the roll stability performance
during this maneuver based on whether the trailer outrigger made
contact with the ground. Volvo considers this maneuver to be
representative of certain highway segments that are encountered, and
that the maneuver is severe enough to fully challenge a stability
control system.
---------------------------------------------------------------------------

    \36\ See Docket No. NHTSA-2010-0034-0042.
---------------------------------------------------------------------------

3. Yaw Control Tests
    Bendix developed two yaw stability test maneuvers to evaluate the
ability of stability control systems to prevent severe oversteer and
understeer conditions. The first test maneuver is a Sinusoidal Steering
Maneuver (SSM) to evaluate oversteer prevention.\37\ The first step in
this test is to identify the steering wheel angle that produces a
tractor lateral acceleration of 0.5g at 30 mph on dry pavement with the
tractor in the bobtail condition. Bendix recommended that this angle be
derived by either a slowly increasing steer test (SIS test described in
section IV.D.2 above) or an equation developed by Bendix for estimating
the angle based on the tractor's wheelbase:

    \37\ See Docket No. NHTSA-2010-0034-0037.
---------------------------------------------------------------------------

Steering Wheel Angle ([delta]) = (35.5 x (tractor wheelbase in meters))
+ 30.94

    The Sinusoidal Steering Maneuver test is then conducted with the
tractor in the bobtail condition using a low-friction wet Jennite road
surface (nominal peak friction coefficient of 0.5). The vehicle is
driven at a constant speed of approximately 30 mph and, as a sinusoidal
steering input is initiated (continuous left and right steering inputs
using the steering wheel angle determined above), the driver increases
the throttle position to request 100 percent of engine torque.
    The second test maneuver developed by Bendix was the ramp with a
dwell maneuver discussed in section IV.D.4 above.\38\ The RWD maneuver
is intended to evaluate understeer prevention, though oversteer can
also occur during the maneuver. The RWD test is conducted with the
tractor in the bobtail condition and using a wet Jennite road surface.
The first step in this test is to characterize the vehicle's steering
by conducting a series of drive-through speed evaluations at a constant
speed on a 500-foot radius curve. Once the maximum constant travel
speed is determined (typically between 28 and 32 mph, but not to exceed
35 mph), the steering wheel angle is measured for negotiating the curve
at that speed. The RWD test maneuver speed is then conducted at the
maximum drive-through speed. Bendix suggested that manual steering by a
test driver or an automated steering machine could be used. Once the
vehicle has been accelerated to the test maneuver speed, the speed is
held constant by the driver and he inputs the drive-through steering
wheel angle. After the vehicle reaches a constant lateral acceleration
condition, the steering wheel angle is increased to 180 degrees in a
period of one second. That increased angle is held constant for three
seconds, and then the angle is reduced to zero in a period of one
second. Subsequent test runs are conducted by increasing the steering
wheel angle in increments of 90 degrees up to 540 degrees.
---------------------------------------------------------------------------

    \38\ See Docket No. NHTSA-2010-0034-0038.
---------------------------------------------------------------------------

    The RWD test performance measures would be based upon test data
showing that the vehicle's stability control system successfully
identified a vehicle control problem (understeer or oversteer) and
intervened by reducing the engine torque output and commanding the
application of individual foundation brakes in a manner that is
suitable to mitigate the control problem. Bendix did not believe that
vehicle yaw or path-following pass/fail criteria would be appropriate
for this test maneuver.
    Two maneuvers that the industry has developed to evaluate the
performance of stability control systems, lane change on a large
diameter circle and sinusoidal steering, can be used to demonstrate
that a stability control system is capable of preventing a rollover or
a yaw instability condition. The RWD maneuver may exceed the
capabilities of stability control systems but provides brake
application data that can be reviewed to determine if a stability
control system provides the correct control responses to address a
severe oversteer or understeer condition.

V. Agency Proposal

    Based upon the foregoing research, the agency is proposing a new
FMVSS to require ESC systems be installed on truck tractors and buses
with a GVWR of greater than 11,793 kilograms (26,000 pounds).\39\ There
are several issues raised by this proposed rule on which the agency
seeks public comment, each of which is discussed in detail in the
following sections.
---------------------------------------------------------------------------

    \39\ To distinguish this new FMVSS from the light vehicle ESC
requirement in FMVSS No. 126, we are proposing to revise the title
FMVSS No. 126 to reflect that it is applicable only to light
vehicles.
---------------------------------------------------------------------------

A. NHTSA's Statutory Authority

    NHTSA is proposing today's NPRM under the National Traffic and
Motor Vehicle Safety Act (``Motor Vehicle Safety Act''). Under 49
U.S.C. Chapter 301, Motor Vehicle Safety (49 U.S.C. 30101 et seq.), the
Secretary of Transportation is responsible for prescribing motor
vehicle safety standards that are practicable, meet the need for motor
vehicle safety, and are stated in objective terms. ``Motor vehicle
safety'' is defined in the Motor Vehicle Safety Act as ``the
performance of a motor vehicle or motor vehicle equipment in a way that
protects the public against unreasonable risk of accidents occurring
because of the design, construction, or performance of a motor vehicle,
and against unreasonable risk of death or injury in an accident, and
includes nonoperational safety of a motor vehicle.'' ``Motor vehicle
safety standard'' means a minimum performance standard for motor
vehicles or motor vehicle equipment. When prescribing such standards,
the Secretary must consider all relevant, available motor vehicle
safety information. The Secretary must also consider whether a proposed
standard is reasonable, practicable, and appropriate for the types of
motor vehicles or motor vehicle equipment for which it is prescribed
and the extent to which the standard will further the statutory purpose
of reducing traffic accidents and associated deaths. The

responsibility for promulgation of Federal motor vehicle safety
standards is delegated to NHTSA. In making the proposals in today's
NPRM, the agency carefully considered all the aforementioned statutory
requirements.

B. Applicability

1. Vehicle types
    Vehicles with a GVWR greater than 10,000 pounds include a large
variety of vehicles ranging from medium duty pickup trucks to different
types of single unit trucks, buses, trailers and truck tractors.
Vehicles with a GVWR of greater than 10,000 pounds are divided into
Classes 3 through 8. Class 7 vehicles are those with a GVWR greater
than 11,793 kilograms (26,000 pounds) and up to 14,969 kilograms
(33,000 pounds), and Class 8 vehicles are those with a GVWR greater
than 14,969 kilograms (33,000 pounds).
    The vast majority of vehicles with a GVWR of greater than 4,536
kilograms (10,000 pounds) for which stability control systems are
currently available are truck tractors. Approximately 150,000 truck
tractors with a GVWR of greater than 11,793 kilograms (26,000 pounds)
are manufactured each year. In 2009, about 20 percent of Class 7 and 8
truck tractors were equipped with a stability control system.
    About 85 percent of truck tractors sold annually in the U.S. are
air-braked three-axle (6x4) tractors with a front axle that has a GAWR
of 14,600 pounds or less and with two rear drive axles that have a
combined GAWR of 45,000 pounds or less, which we will refer to as
``typical 6x4 tractors.'' Two-axle (4x2) tractors and severe service
tractors (those with three axles that are not ``typical 6x4 tractors''
or those with four or more axles) represent about 15 percent of the
truck-tractor market in the U.S.
    The majority of the research on the effectiveness of stability
control systems to date has been performed on typical 6x4 tractors. As
a result, the agency's research included two typical 6x4 tractors. The
agency also included one 4x2 tractor in its testing because two-axle
tractors represent the next largest segment of the truck-tractor
market. No severe service tractors were tested. EMA performed tests on
nine tractors equipped with stability control systems. The tractors
included two 4x2 tractors, two typical 6x4 tractors, two severe service
6x4 tractors, two 6x4 tractors with a liftable auxiliary axle in front
of the drive axles, and one 8x6 tractor.
    This proposal would also require certain buses to be equipped with
an ESC system. We intend the applicability of this proposed requirement
to be similar to the applicability of the agency's proposal that
certain buses be equipped with seat belts.\40\ That proposal was
applicable to buses with a gross vehicle weight rating (GVWR) of 11,793
kilograms (26,000 pounds) or greater, 16 or more designated seating
positions (including the driver), and at least 2 rows of passenger
seats that are rearward of the driver's seating position and are
forward-facing or can convert to forward-facing without the use of
tools.'' That proposal excluded school buses and urban transit buses
sold for operation in urban transportation along a fixed route with
frequent stops. The agency is proposing a very similar applicability in
this NPRM. We have not made this proposal applicable to buses with a
GVWR of exactly 11,793 kilograms (26,000 pounds) in order to exclude
Class 6 vehicles from this proposal. We believe that this proposal
encompasses the category of ``cross-country intercity buses''
represented in the FARS and FMCSA data (identified in section II.A
above) that had a higher involvement of crashes that ESC systems are
capable of preventing.
---------------------------------------------------------------------------

    \40\ 75 FR 50,958 (Aug. 18, 2010).
---------------------------------------------------------------------------

    The agency tested three buses, all of which had a GVWR over 14,969
kg (33,000 pounds). There are seven manufacturers or distributors of
Class 8 buses covered by this proposal for the U.S. market: Prevost,
MCI, VanHool, Daimler/Setra, CAIO, BlueBird, and BCI. Three of them
(Prevost, MCI, and VanHool), have stated that an ESC system is a
standard feature on their buses sold in the U.S. Daimler/Setra
indicated that an ESC system will be available as an option on its
buses beginning in model year 2011 and that no decision has been made
to make it a standard feature. No official information is available
from CAIO, Bluebird, and BCI regarding ESC system availability.
    There are also at least nine manufacturers of Class 7 buses covered
by this proposal for the U.S. market: Champion, ElDorado National,
Federal Coach, Glaval, IC Bus, MCI, Rexhall, Stallion, and VanHool.
Many Class 7 buses are built on chassis similar to those of single unit
trucks for which ESC has not been widely developed, and we are not
aware of any Class 7 bus that is equipped or currently available with
ESC. Class 7 buses represent less than 20 percent of the market.
Although the agency is not aware of any Class 7 bus currently available
with ESC, we are aware that stability control systems are available on
a limited number of Class 8 single unit trucks, such as ready mix
concrete trucks, refuse trucks, and other air-braked trucks, and that
the same technology could be developed for use on Class 7 buses, which
we believe are also air-braked vehicles.
    Although this proposal would not apply to all buses with a GVWR of
greater than 11,793 kilograms (26,000 pounds), we seek comment on
whether this proposal should be applied to the types of buses that are
excluded from the proposed rule such as school buses and transit buses.
We also seek comment on the feasibility of including the Class 7 buses
described in the prior paragraph that are built on chassis similar to
those of single unit trucks within two years. In particular, we believe
that ESC systems are readily available for air-braked buses; however,
system availability for any hydraulically braked buses that may be
covered by this proposed rule may be more limited. If hydraulically
braked buses are covered by this proposal, we request comment on
manners in which hydraulically braked buses may be differentiated for
exclusion or a different phase-in period.
    The agency is not proposing to include single unit trucks with a
GVWR over 4,536 kg (10,000 pounds) at this time. There are substantial
differences in the complexity of the single unit truck population
compared to the truck-tractor population. The single unit truck
population has wide variations in vehicle weight, wheelbase, number of
axles, center of gravity height, and cargo type, among other things
that affect the calibration and performance of stability control
systems. While some variation exists in the truck tractor market, the
degree of complexity and diversity is substantially less.
    Further, the single unit truck market is structurally different
than the truck tractor market in that the chassis supplier, who is
generally responsible for the brake systems and therefore would likely
provide stability control systems, is often different than the final
body builder. Hence, the chassis supplier may not have knowledge of
critical vehicle design parameters that would affect stability control
system calibration. In contrast, manufacturers of truck tractors have
more complete control of the final, delivered vehicle.
    The complexity of the single unit truck population and the limited
crash data available present a significant challenge to determining the
effectiveness of stability control on these vehicles. We believe that
approximately 1 percent of newly manufactured single-unit trucks are
equipped with stability control systems, and that few, if any, of those
are for

vehicles with hydraulic brakes. The development of stability control
system for vehicles over 10,000 pounds GVWR has been focused on air-
braked vehicles, which include the truck tractors and buses addressed
in this proposal. Because we are concerned about the availability of
production-ready systems on these vehicles, they are not included in
the proposal. However, we seek comment on these observations.
    The agency has initiated a safety benefit study to determine the
safety need for stability control on single-unit trucks, and has also
initiated vehicle research, similar to the research conducted on truck
tractors and large buses described in part IV.C above, which is
expected to be completed in 2012. However, the agency proposes to
require stability control systems on truck tractors without waiting for
the study on the effectiveness of stability control systems on single-
unit trucks to be completed. Waiting for that study to be completed
would unnecessarily delay the benefits of having stability control
systems on truck tractors and large buses, for which testing has been
completed the benefits of stability control systems identified.
    The agency is not proposing to include a requirement for stability
control systems on trailers, primarily because trailer-based RSC
systems were judged by the agency research to be much less effective
than tractor-based RSC or ESC systems in preventing rollover. Trailer-
based RSC systems are capable of applying braking only on the trailer's
brakes. Tractor-based systems can command more braking authority by
using both the tractor and trailer brakes. As a result, trailer-based
RSC systems do not appear to provide additional safety benefits when
used in combination with tractor-based RSC or ESC systems. The trailer-
based RSC systems provide some improvement in roll stability compared
to a base trailer without an RSC system, but a vehicle could still be
overdriven at a lower speed with trailer-based RSC systems than with a
tractor-based system. This means that the maneuver entrance speed
beyond which the stability control system is unable to reduce the
vehicle speed to prevent a rollover was lower for the trailer-based
system than for the tractor-based system. In addition, the typical
service life of a trailer is 20 to 25 years compared with about 8 to 10
years for a truck tractor. Because new tractors are added to the U.S.
fleet at a faster rate than new trailers, the safety benefits from
stability control systems would be achieved at a faster rate by
requiring stability control systems to be installed on a tractor.
    Therefore, the agency proposes to require stability control systems
on truck tractors and buses with a GVWR of greater than 11,793
kilograms (26,000 pounds).
2. Retrofitting In-Service Truck Tractors, Trailers, and Buses
    NHTSA has considered proposing to require retrofitting of in-
service truck tractors, trailers, and large buses with stability
control systems proposed to be required by this NPRM. The Secretary has
the statutory authority to promulgate safety standards for ``commercial
motor vehicles and equipment subsequent to initial manufacture.'' \41\
The Secretary has delegated authority to NHTSA to ``promulgate safety
standards for commercial motor vehicles and equipment subsequent to
initial manufacture when the standards are based upon and similar to
[an FMVSS] promulgated, either simultaneously or previously, under
chapter 301 of title 49, U.S.C.'' \42\ Additionally, the Federal Motor
Carrier Safety Administration (FMCSA) is authorized to promulgate and
enforce vehicle safety regulations, including those aimed at
maintaining commercial motor vehicles so they continue to comply with
the safety standards applicable to commercial motor vehicles at the
time they were manufactured. Although this NPRM does not propose
requiring truck tractors, trailers, or large buses to be equipped with
stability control systems ``subsequent to initial manufacture,'' we are
requesting public comment on several issues related to retrofitting in-
service truck tractors, trailers, and buses:
---------------------------------------------------------------------------

    \41\ See Motor Carrier Safety Improvement Act of 1999, sec.
101(f), Pub. L. 106-159 (Dec. 9, 1999).
    \42\ See 49 CFR 1.50(n).
---------------------------------------------------------------------------

     The extent to which a proposal to retrofit in-service
vehicles with stability control systems would be complex and costly
because of the integration between a stability control system and the
vehicle's chassis, engine, and braking systems.
     The changes necessary to an originally manufactured
vehicle's systems that interface with a stability control system, such
as plumbing for new air brake valves and lines and a new electronic
control unit for a revised antilock brake system.
     The additional requirements that would have to be
established to ensure that stability control components are at an
acceptable level of performance for a compliance test, given the
uniqueness of the maintenance condition for vehicles in service,
particularly for items such as tires and brake components that are
important for ESC performance.
     The original manufacture date of vehicles that should be
subject to any retrofitting requirements.
     Whether the performance requirements for retrofitted
vehicles should be less stringent or equally stringent as for new
vehicles, and, if less stringent, the appropriate level of stringency.
     The cost of retrofitting a stability control system on a
vehicle, which we believe would exceed the cost of including stability
control on a new vehicle.
    In light of these questions, the agency is not proposing that in-
service vehicles be required to be retrofitted with stability control
systems. Instead, this proposed requirement would be applicable only to
newly manufactured vehicles. However, the comments we receive on the
issue of retrofitting will help us determine whether we should issue a
separate supplemental NPRM to require a retrofit.
3. Exclusions From Stability Control Requirement
    Our proposed rule excludes certain types of low-volume, highly
specialized vehicle types. In these cases, the vehicle's speed
capability does not allow it to operate at speeds where roll or yaw
instability is likely to occur.
    Specifically, FMVSS No. 121, Air brake systems, excludes certain
heavy air-braked heavy vehicles from that standard. For truck tractors
and buses, these exclusions include:
     Any vehicle equipped with an axle that has a gross axle
weight rating of 29,000 pounds or more.
     Any truck or bus that has a speed attainable in two miles
of not more than 33 mph.
     Any truck that has a speed attainable in two miles of not
more than 45 mph, an unloaded vehicle weight that is not less than 95
percent of its GVWR, and no capacity to carry occupants other than the
driver and operating crew.
    We believe that the vehicles that are excluded from the
requirements of FMVSS No. 121 should also be excluded from the proposed
stability control requirements because the speed at which these
vehicles operate would make it unlikely that roll or yaw instability
would occur. Accordingly, the proposed stability control requirement
excludes these vehicles.

C. ESC System Capabilities

1. Choosing ESC vs. RSC
    We are proposing to require that truck tractors and large buses be
equipped

with ESC systems rather than RSC systems. An ESC system is capable of
all of the functions of an RSC system. In addition, an ESC system has
the additional ability to detect yaw instability, provide braking at
front wheels, and detect the steering wheel angle. These additions, as
demonstrated by NHTSA's testing, allow an ESC system to have better
rollover prevention performance than an RSC system in addition to the
yaw instability prevention component. This is because the steering
wheel angle sensor allows the ESC system to anticipate changes in
lateral acceleration based upon driver input and to intervene with
engine torque reduction or selective braking sooner, rather than
waiting for the lateral acceleration sensors to detect potential
instability.
    As discussed in greater length in Section VI, mandating ESC systems
rather than RSC systems will prevent more crashes, injuries, and
fatalities. The additional benefits from ESC systems can be attributed
to both the ESC's system's ability to intervene sooner and its ability
to prevent yaw instability that would lead to loss-of-control crashes.
    Mandating ESC systems rather than RSC systems will result in higher
costs to manufacturers. Moreover, our benefit and cost estimates lead
to the preliminary conclusion that mandating RSC systems would be more
cost-effective than mandating ESC systems. However, these extra costs
are more than offset by higher net benefits that would accrue by
mandating ESC systems rather than RSC systems.
2. Definition of ESC
    Definitional requirements in an FMVSS define and describe the type
of system that can be used to meet the performance requirements of a
particular FMVSS. However, the inclusion of a definitional requirement
in an FMVSS may be design restrictive because it would be based on
currently available technology. Limiting the equipment that can be used
to satisfy an FMVSS may limit future technological advancements and
innovation. As stability control technologies are developed even
further, a definitional requirement could be a hindrance to safety
improvements if it limits the use of a newly developed equipment or
technology that is not addressed by the specified definitional
requirement. On the other hand, relying solely on performance-based
tests without mandating any specific equipment may require a battery of
tests to cover the complete operating range of the vehicle. Given the
wide array of possible configurations and operating ranges for heavy
vehicles, the agency does not believe it is practical to develop
performance tests that would address the full range of possibilities
and remain cost-effective. Accordingly, the agency is proposing to
include a definitional requirement in this proposed rule that includes
equipment that would be required as part of a compliant ESC system. We
note that, when developing the ESC requirement for light vehicles, the
agency chose to include such a requirement in FMVSS No. 126.
    SAE International has a Recommended Practice on Brake Systems
Definitions--Truck and Bus, J2627 (Aug. 2009), which includes a
definition of Electronic Stability Control and Roll Stability Control.
SAE International's definition of an ESC system requires that a system
have an electronic control unit that considers wheel speed, yaw rate,
lateral acceleration, and steering angle and that the system must
intervene and control engine torque and auxiliary brake systems to
correct the vehicle's path.
    The UN ECE Regulation 13 definition for the electronic stability
control system, promulgated in Annex 21, includes the following
functional attributes for directional control: sensing yaw rate,
lateral acceleration, wheel speeds, braking input and steering input;
and the ability to control engine power output. For vehicles with
rollover control, the functions required by the stability control
include: sensing lateral acceleration and wheel speeds; and the ability
to control engine power output.
    In developing a definition for ESC, the agency has reviewed the
functional attributes contained in the SAE and the ECE definitions, and
has incorporated portions of both of these definitions in this NPRM. We
have developed a definition that is similar in wording to the
definition from FMVSS No. 126, which specifies certain features that
must be present, that ESC be capable of applying all the brakes
individually on the vehicle, and that it have a computer using a
closed-loop algorithm to limit vehicle oversteer and understeer when
appropriate. Unlike the light vehicle standard, which focuses on yaw
stability, this NRPM proposes to require a stability control system
that also helps to mitigate roll instability conditions. As a result,
we have expanded the definition from the one in FMVSS No. 126 to
include a requirement that the system be capable of sensing impending
rollover and reducing the vehicle's lateral acceleration to prevent
rollover.
    Furthermore, we believe that the ESC system must be operational
during all phases of driving, including acceleration, coasting,
deceleration, and braking, except when the vehicle is below a low-speed
threshold where loss of control or rollover is unlikely. According to
information the agency has obtained from vehicle manufacturers and ESC
suppliers, this low speed threshold for a stability control system is
10 km/h (6.2 mph) for yaw stability control and 20 km/h (12.4 mph) for
roll stability control. For the purposes of a proposed regulation, we
believe that setting a single low speed threshold would be preferable
since the yaw and roll stability functions during a test maneuver are
closely intertwined, which could make it difficult to differentiate
when the roll or yaw function ends. Therefore, we propose a single
threshold of 20 km/h (12.4 mph) as the speed below which ESC is not
required to be operational.
    Therefore, the agency proposes to require the installation of an
ESC system on truck tractors and large buses, which has all of the
following attributes:
    1. Augments vehicle directional stability by applying and adjusting
vehicle brake torques individually at each wheel position on at least
one front and at least one rear axle of the vehicle to induce
correcting yaw moment to limit vehicle oversteer and to limit vehicle
understeer;
    2. Enhances rollover stability by applying and adjusting the
vehicle brake torques individually at each wheel position on at least
one front and at least one rear axle of the vehicle to reduce lateral
acceleration of a vehicle;
    3. Computer-controlled with the computer using a closed-loop
algorithm to induce correcting yaw moment and enhance rollover
stability;
    4. Has a means to determine the vehicle's lateral acceleration;
    5. Has a means to determine the vehicle's yaw rate and to estimate
its side slip or side slip derivative with respect to time;
    6. Has a means to estimate vehicle mass or, if applicable,
combination vehicle mass;
    7. Has a means to monitor driver steering input;
    8. Has a means to modify engine torque, as necessary, to assist the
driver in maintaining control of the vehicle; and
    9. When installed on a truck tractor, has the means to provide
brake pressure to automatically apply and modulate the brake torques of
a towed semi-trailer.
    The benefit of an ESC system is that it will reduce vehicle
rollovers and loss of control under a wide variety of vehicle
operational and environmental conditions. However, the performance

tests proposed in this NPRM would only evaluate ESC system performance
under very specific environmental conditions. To ensure that a vehicle
is equipped with an ESC system that meets the proposed definition, we
are proposing that vehicle manufacturers make available to the agency
documentation that would enable us to ascertain that the system
includes the components and performs the functions of an ESC system.
    We are proposing that the vehicle manufacturer provide a system
diagram that identifies all ESC system hardware; a written explanation,
with logic diagrams included, describing the ESC system's basic
operational characteristics; and a discussion of the pertinent inputs
to the computer and how its algorithm uses that information to prevent
rollover and limit oversteer and understeer. Because the proposed
definition for ESC systems on truck tractors includes the capability to
provide brake pressure to a towed vehicle, the agency is proposing to
require that, as part of the system documentation, the manufacturer
include the information that shows how the tractor provides brake
pressure to a towed trailer under the appropriate conditions.
    It is common practice for the NHTSA's Office of Vehicle Safety
Compliance to request relevant technical information from a
manufacturer prior to conducting many of its compliance test programs.
The agency included such a requirement in the light vehicle ESC
standard. Prior to conducting any of the FMVSS No. 126 compliance
tests, NHTSA requires manufacturers to provide the documentation
required by that standard, including identification of all ESC system
hardware and an explanation of the system operational characteristics.
We also request additional information about the ESC system including
manufacturer make and model, telltale(s), pertinent owner's manual
excerpts and suggested malfunction scenarios. All of the requested
information allows NHTSA to verify that the ESC system meets the
definitional and operational requirements that cannot necessarily be
verified during the performance test. Furthermore, this information
aids the test engineers with execution and completion of the compliance
test.

D. ESC Disablement

    The agency has also considered whether to allow a control for the
ESC to be disabled by the driver; however, heavy vehicles currently
equipped with ESC systems do not include on/off controls for ESC that
would allow a driver to deactivate or adjust the ESC system. Given the
lack of on/off switches on heavy vehicles equipped with ESC, we do not
propose to allow an on/off switch for ESC systems in this NPRM.
Nevertheless, we seek comment on the need to allow an on/off switch.
Such comments should address why manufacturers might need this
flexibility and how manufacturers would implement a switch in light of
the ABS requirements for truck tractors and large buses.

E. ESC Malfunction Detection, Telltale, and Activation Indicator

1. ESC Malfunction Detection
    This proposed rule would require that vehicles be equipped with an
indicator lamp, mounted in front of and in clear view of the driver,
which is activated whenever there is a malfunction that affects the
generation or transmission of control or response signals in the
vehicle's ESC system. Heavy vehicles presently equipped with ESC
generally do not have a dedicated ESC malfunction lamp. Instead, they
share that function with the mandatory ABS malfunction indicator lamp
or the traction control activation lamp. The agency proposes requiring
a separate ESC malfunction lamp because it would alert the driver to
the malfunction condition of the ESC and would help to ensure that the
malfunction is corrected at the earliest opportunity.
    We believe that there are safety benefits associated with such a
warning. An ESC malfunction indicator warns the driver in the event of
an ESC system malfunction so that the system can be repaired. ESC
system activations on a heavy vehicle will be infrequent events in
panic situations, and drivers should not experience the activation of a
stability control system during the normal operation of the vehicle.
Because most steering maneuvers performed during the normal operation
of a heavy vehicle are not severe enough to activate the ESC system, a
vehicle may be operated for long periods without an ESC activation
event. Without such a malfunction indicator, a driver might have no way
of knowing that an ESC system is malfunctioning until a loss of control
or rollover event occurs. For example, the agency received a complaint
recently in which a heavy truck had an inoperative ESC system, but the
driver was unaware of the malfunction, primarily due to the lack of a
malfunction indicator lamp. The agency believes that such a warning is
important to ensure that the driver could have the malfunction
corrected at the earliest opportunity in order to continue to realize
the system's safety benefits.
    The ESC malfunction telltale would be required to remain
illuminated continuously as long as the malfunction exists whenever the
ignition locking system is in the ``On'' (``Run'') position. The ESC
malfunction telltale must extinguish after the malfunction has been
corrected. These proposed requirements are identical to the
requirements established in the light vehicle ESC standard, FMVSS No.
126, and help to ensure that the system provides a warning indication
in the event of a malfunction.
    Because many malfunctions cannot be detected when the vehicle is
stationary, this NPRM includes a test that would allow the engine to be
running and the vehicle to be in motion as part of the diagnostic
evaluation. We are aware that some malfunctions are not time-based, but
instead require comparisons of sensor outputs generated when the
vehicle is driven. Hence, some malfunctions would require certain
driving motions to make the ESC system's malfunction detection
possible. We believe that an ESC malfunction should be detected within
a reasonable time of starting to drive. As a result, we propose that
the malfunction telltale illuminate within two minutes after attaining
a test speed of 48 km/h (30 mph) so that the parts of a system's
malfunction detection capability that depend on vehicle motion can
operate. This two-minute period is identical to the period included in
the test procedure in FMVSS No. 126 for ESC malfunction detection.
    We anticipate that FMCSA will issue a companion proposal to NHTSA's
proposal to require ESC on truck tractors and large buses, which would
require that the ESC system on a commercial vehicle be maintained in a
fully operating condition. In addition, we expect that the roadside
inspection procedures developed for commercial vehicle ESC systems
would be facilitated by the ESC malfunction telltale and the format
that is required to indicate whether or not the system is operational.
2. ESC Malfunction Telltale
    The ESC malfunction lamp requirement in this NPRM states that each
truck tractor and large bus must be equipped with a telltale that
provides a warning to the driver when one or more malfunctions that
affect the generation of control or response signals in the vehicle's
electronic stability control system is detected. Specifically, the ESC
malfunction telltale will be required to

be mounted in the driver's compartment in front of and in clear view of
the driver and be identified by the symbol shown for ``ESC Malfunction
Telltale'' or the specified words or abbreviations listed in Table 1 of
FMVSS No. 101, Controls and displays. FMVSS No. 101 includes a
requirement for the telltale symbol, or abbreviation, and the color
required for the indicator lamp to show a malfunction in the ESC
system.
    The agency believes that the symbol used to identify ESC
malfunction should be standardized with the symbol used on light
vehicles. The symbol established in FMVSS No. 126 is the International
Organization for Standardization (ISO) ESC symbol, designated J.14 in
ISO Standard 2575. The symbol shows the rear of a vehicle trailed by a
pair of ``S'' shaped skid marks, shown below in Figure 5. The agency
found that the ISO J.14 symbol and close variations were the symbols
used by the greatest number of vehicle manufacturers that used an ESC
symbol before the requirement was established. Furthermore, FMVSS No.
126 allows, as an option, the use of the text ``ESC'' in place of the
telltale symbol. This same option is being proposed.
[GRAPHIC] [TIFF OMITTED] TP23MY12.005

    The color of the ESC malfunction telltale specified in Table 1 of
FMVSS No. 101 for light vehicles equipped with ESC is yellow, which is
the color used to communicate to the driver the condition of a
malfunctioning vehicle system that does not require immediate
correction. The agency chose to associate indication of an ESC system
malfunction with a yellow telltale color as a warning to the driver
because we believe that it communicates the level of urgency with which
the driver must seek to remedy the malfunction of the ESC system.
    For this proposed rule, we believe that the ESC malfunction
telltale and color designation developed for light vehicles would be
appropriate for use on heavy vehicles. Accordingly, the agency proposes
that the ESC malfunction telltale symbol and color requirements of
FMVSS No. 101 be proposed for use on truck tractors and buses, and that
the abbreviation ``ESC'' should be allowed as an option instead of the
symbol.
    In addition to the ESC malfunction telltale being used to warn the
driver of a malfunction in the ESC, the telltale is also used as a
check of lamp function during vehicle start-up. We believe that the ESC
malfunction telltale should be activated as a check of lamp function
either when the ignition locking system is turned to the ``On''
(``Run'') position whether or not the engine is running. This function
provides drivers with the information needed to ensure that the ESC
system is operational before the vehicle is driven. It also provides
Federal and State inspectors with the means to determine the
operational status of the ESC system during a roadside safety
inspection.
    Accordingly, this NPRM proposes that the ESC malfunction telltale
must be activated as a check of lamp function either when the ignition
locking system is turned to the ``On'' (``Run'') position when the
engine is not running or when the ignition locking system is in a
position between the ``On'' (``Run'') and ``Start,'' which is
designated by the manufacturer as a check position.
3. ESC Activation Indicator
    The agency is requesting comment on whether there is a safety need
for an ESC activation indicator. In the light vehicle ESC rulemaking,
the agency considered the safety need for an ESC activation indicator
to alert the driver during an emergency situation that the ESC is
activating. NHTSA conducted a study using the National Advanced Driving
Simulator (NADS), which included experiments to gain insight into the
various possibilities regarding ESC activation indicators. The study
compared the performance of 200 participants in driving maneuvers on a
wet pavement, and used road departures and eye glances to the
instrument panel as measures of driver performance. The significant
finding was that the drivers who received various ESC activation
indicators did not perform better than drivers who were given no
indicator. That finding formed the basis for the agency's decision not
to require an ESC activation indicator for light vehicles.

F. Performance Requirements and Compliance Testing

    The agency's research initially focused on a variety of maneuvers
which we could use to evaluate the roll stability performance and the
yaw stability performance of truck tractors and large buses. Several of
these maneuvers were also tested by industry and some of them are
allowed for use in testing for compliance to the UN ECE stability
control regulation. The agency's goal was to develop one or more
maneuvers that showed the most promise as repeatable and reproducible
roll and yaw performance tests for which objective pass/fail criteria
could be developed.
    As the research program progressed, the data indicated that the
ramp steer maneuver to evaluate roll stability performance and the sine
with dwell maneuver to evaluate yaw stability performance were the most
promising. The slowly increasing steer maneuver was developed to
normalize testing conditions for each vehicle so that the level of
stringency for each test vehicle would be similar. The agency also
found that the SIS maneuver could also be used to evaluate the engine
torque reduction capability of a vehicle's ESC system, which is
important because engine torque reduction may bring a vehicle under
control before brakes are applied. After further testing, the agency
was able to develop test parameters for the SWD maneuver so that both
roll stability and the yaw stability could be evaluated using a single
maneuver and loading condition. This development eliminated the need
for the ramp steer maneuver to evaluate roll stability performance.
    Therefore, based on testing at VRTC and the results from industry-
provided test data, two stability proposed performance tests have been
chosen to

evaluate ESC systems on truck tractors and large buses--the SIS test
and the SWD test.
    The agency also considered the ECE performance tests for heavy
vehicle stability control systems, which are included in the brake
systems regulation, ECE Regulation 13. The performance test for a heavy
vehicle with a directional control function includes meeting the
requirements in one of eight tests allowed for compliance. The eight
tests are as follows: Reducing radius test (which is identical to the
decreasing radius test discussed above), step steer input test, sine
with dwell, J-turn, mu-split lane change, double lane change, reversed
steering test or ``fish hook'' test, and asymmetrical one period sine
steer or pulse steer input test. No test procedure or pass/fail
criteria are included in ECE Regulation 13, but it is left to the
discretion of the Type Approval testing authority in agreement with the
vehicle manufacturer to show that the system is functional.
    The issue of whether the U.S. should adopt the stability control
requirements similar to those in ECE Regulation 13 is addressed in the
context of whether a definitional requirement specifying required
equipment along with a performance test that does not include a test
procedure or pass/fail criteria would be considered sufficiently
objective for a safety standard. The agency considered several of the
eight ECE tests that we believed showed the most promise for
repeatability and reproducibility, and decided to focus on the SWD
test, which is one of the eight tests allowed for compliance testing to
ECE Regulation 13. However, in light of the requirement in the Motor
Vehicle Safety Act that FMVSSs be stated in objective terms, NHTSA is
required to develop objective performance criteria for the SWD test to
be set forth in the regulatory text.
1. Characterization Test--SIS
    The agency is proposing to conduct compliance testing
characterization using a slowly increasing steer to determine the
steering wheel angle needed to achieve 0.5g of lateral acceleration at
30 mph and also to evaluate the capability of the ESC system to reduce
engine torque. The SIS maneuver has been used for many years by the
agency and the industry to determine the unique dynamic characteristics
of a vehicle. This maneuver allows the agency to determine the
relationship between the steering wheel angle and lateral acceleration
for a vehicle, which varies due to different steering gear ratios,
different suspension systems, and wheelbase and other dimensions, among
other things. To normalize the severity of the SWD maneuver that
follows, each vehicle is tested based on its steering wheel angle
determined in the SIS maneuver. The agency is proposing a 0.5g lateral
acceleration target because our test results indicated that a truck
tractor or large bus is highly likely to experience instability at that
level of lateral acceleration. Even though the vast majority of truck
tractors are typical 6x4 tractors, there are other configurations, such
as those with 2-axle or 4-axle configurations and buses, which would
require a different steering wheel angle to normalize the test
conditions for each different vehicle.
    To perform the SIS maneuver, the tractor or bus is driven at a
constant speed of 30 mph, and then the steering controller increases
the steering wheel angle at a slow, continuous rate of 13.5 degrees per
second. The steering wheel angle is increased linearly from zero to 270
degrees and then held constant for one second, after which the maneuver
concludes. The vehicle is subjected to two series of runs, one using
clockwise steering and the other using counterclockwise steering, with
three tests performed for each test series. During each test run, ESC
system activation must be confirmed. If ESC system activation does not
occur during the maneuver, then the commanded steering wheel angle is
increased by 270-degree increments up to the vehicle's maximum
allowable steering angle until ESC activation is confirmed.
    From the SIS tests, the value ``A'' is determined. ``A'' is the
steering wheel angle, in degrees, that is estimated to produce a
lateral acceleration of 0.5g for that vehicle. Using linear regression
on the lateral acceleration data recorded between 0.05g and 0.3g for
each of the six valid SIS tests, a linear extrapolation is used to
calculate a steering wheel angle where the lateral acceleration would
be 0.5g. If ESC system activation occurs prior to the vehicle
experiencing lateral acceleration of 0.3g, then the data used during
the linear regression will be that data recorded between 0.05g and the
lateral acceleration measured at the time of ESC system activation. The
six values derived from the linear regression are then averaged and
rounded to the nearest 0.1 degree to produce the final quantity, ``A,''
used during the SWD maneuver.
    As part of the SIS characterization test, the engine torque
reduction test is also conducted. As mentioned above, during each of
the six completed SIS maneuvers, ESC activation is confirmed by
verifying that the system automatically attempts to reduce engine
torque. To confirm ESC activation, engine torque output and driver
requested torque data are collected from the vehicle's J1939
communication data link and compared. During the initial stages of each
maneuver, the rate of change over time of engine torque output and
driver requested torque will be consistent. Upon ESC activation, the
ESC system activation causes a commanded engine torque reduction, even
though the driver requests increased torque by attempting to accelerate
the vehicle to maintain the required constant speed. Therefore, the
rate of change over time of engine torque output and driver requested
torque will diverge.
    For each of the six SIS test runs, the commanded engine torque and
the driver requested torque signals must diverge at least 10 percent
1.5 seconds after the beginning of ESC system activation. This test
demonstrates that the ESC system has the capability to reduce engine
torque, as required in the functional definition.
    The metric used to measure the engine torque reduction performance
is stated in terms of the difference in percent between the actual
engine torque output and driver requested torque input just after ESC
activation. The pass-fail criterion that the agency proposes for this
test is that the stability control system must be able to reduce engine
torque output by a minimum of 10 percent from the torque output
requested by the driver, which will be measured 1.5 seconds after the
time when the ESC activated. The vehicles that the agency tested were
all able to meet this proposed performance level.
2. Roll and Yaw Stability Test--SWD
    The objective of the sine with dwell test is to subject a vehicle
to a maneuver that will cause both roll and yaw instabilities and to
verify that the ESC system activates to mitigate those instabilities.
The SWD test is based on a single cycle of a sinusoidal steering input.
For testing, we are proposing to use a frequency of 0.5 Hz (\1/2\ cycle
per second or 1 cycle in 2 seconds) was used with a pause or dwell of
1.0 second after completion of the third quarter-cycle of the sinusoid.
We chose a 0.5 Hz frequency because it produces the most consistently
high severity on the majority of the vehicles tested by the agency.
Hence, the total time for the steering maneuver is three seconds.
    Conceptually, the steering profile of this maneuver is similar to
that expected to be used by real drivers during some crash avoidance
maneuvers. As the agency found in the

light vehicle ESC research program, the severity of the SWD maneuver
makes it a rigorous test while maintaining steering rates within the
capabilities of human drivers. We believe that the maneuver is severe
enough to produce rollover or vehicle loss-of-control without a
functioning ESC system on the vehicle.
    For a truck tractor, the SWD test would be conducted with the truck
tractor coupled to an unbraked control trailer and loaded with ballast
directly over the kingpin. The combination vehicle would be loaded to
80 percent of the tractor's GVWR. Testing indicates that this is
sufficient load on the tractor to enable the tractor's stability
control mass estimation program to provide full tractor braking
intervention during the SWD maneuver. The ballast is placed low on the
trailer to minimize the likelihood of actual trailer rollover, and the
trailer is equipped with outriggers in case the ESC system does not
function properly to prevent the trailer from rolling over.
    For a bus, the vehicle is loaded with a 68-kilogram (150-pound)
water dummy in each of the vehicle's designated seating positions,
which would bring the vehicle's weight to less than its GVWR. No
ballast is placed in the cargo hold beneath the passenger compartment
so that the desired CG height of the test load can be attained.
    The SWD test would be conducted at a speed of 72 km/h (45 mph). An
automated steering machine would be used to initiate the steering
maneuver. Each vehicle is subjected to two series of test runs. One
series uses counterclockwise steering for the first half-cycle, and the
other series uses clockwise steering for the first half-cycle. The
steering amplitude for the initial run of each series is 0.3A, where A
is the steering wheel angle determined from the SIS maneuvers discussed
in section V.F.1 above. In each of the successive test runs, the
steering amplitude would be increased by increments of 0.1A until a
steering amplitude of 1.3A or 400 degrees, whichever is less, is
achieved. Upon completion of the two series of test runs, post-
processing of the yaw rate and lateral acceleration data to determine
the lateral acceleration ratio, yaw rate ratio, and lateral
displacement, as discussed below.
(a) Roll Stability Performance
    The LAR is a performance metric developed to evaluate the ability
of a vehicle's ESC system to prevent rollovers. Lateral acceleration is
measured on a bus or a tractor and corrected for the vehicle's roll
angle. As a performance metric, the corrected lateral acceleration
value is normalized by dividing it by the maximum lateral acceleration
that was determined at any time between 1.0 seconds after the beginning
of steering and the completion of steering.
    Conceptually, stability control system intervention will reduce
lateral acceleration of the vehicle during a crash avoidance steering
maneuver. This intervention increases the roll stability of the vehicle
by reducing the vehicle speed, which results in a reduction in the
lateral acceleration, Ay, because Ay = V\2\/R,
where V is the vehicle speed, and R is the radius of curvature of
vehicle path. However, lateral acceleration was found to be less
favorable than a ``normalized'' calculation, lateral acceleration
ratio, developed from the vehicle's lateral acceleration measured
during the maneuver because the lateral acceleration alone does not
account for different stability thresholds among different vehicles.
The agency believes that LAR has the most potential for an accurate
measure of an ESC system to prevent rollovers. From the agency's
testing, we have noted that LAR differentiates vehicles equipped with
stability control systems as well as the potential determine and
quantify roll instability. Lateral acceleration ratio is calculated by
dividing the vehicle's lateral acceleration, corrected for roll angle,
at a specified time after the completion of steer (COS) by the peak
corrected lateral acceleration experienced during the second half of
the sine maneuver (including the dwell period). The LAR at two time
intervals after completion of steer is calculated to determine the
change in lateral acceleration from the peak lateral acceleration. A
reduction or decay in the lateral acceleration ratio at specified
intervals after completion of steer is an indication that the stability
control system has intervened to reduce the likelihood of vehicle
rollover. The lateral acceleration ratio, LAR, is determined as
follows:
[GRAPHIC] [TIFF OMITTED] TP23MY12.006

    Where A--y Veh (COS + 0.75 sec, + 1.5 sec,) is the
corrected for roll lateral acceleration value at the specified time
after the completion of steer, and Max Ay is the peak
corrected lateral acceleration measured during the second half of the
sine maneuver (including the dwell period), i.e., from time 1.0 second
after the beginning of steer to the completion of steer.
    In developing the performance requirements for light vehicle ESC
systems, several commenters requested that the agency include a
definition for the term ``lateral acceleration'' and define a method
for determining the lateral acceleration at the vehicle's center of
gravity. In FMVSS No. 126, the agency uses the definition from SAE
J670e, Vehicle Dynamics Terminology, which states, ``Lateral
Acceleration means the component of the vector acceleration of a point
in the vehicle perpendicular to the vehicle x axis (longitudinal) and
parallel to the road plane.'' This definition was carried over,
effectively unchanged, to the more recent revision of SAE's Vehicle
Dynamics Terminology, SAE J670--200801. The agency is proposing to use
the same definition of lateral acceleration for this standard as was
used in FMVSS No. 126.
    The agency's research also looked at wheel lift measurement as a
possible performance measure. Wheel lift is the most intuitive
performance measure we considered because wheel lift precedes all
rollovers. Wheel lift is considered to be lift that is two inches or
greater, which occurs for any wheel of the vehicle, including the
control trailer for the tractor during a test. One challenge with using
wheel lift is that it does not necessarily indicate that rollover is
imminent. For example, certain vehicle suspension designs are likely to
cause wheel lift during severe cornering maneuvers, and also non-
uniform test surfaces can cause brief instances of wheel lift.
    Therefore, the agency proposes evaluating vehicle roll stability
performance by calculating the LAR at 0.75 seconds and at 1.5 seconds
after the completion of steer. The two performance criteria are
described below:
     From data collected from each SWD maneuver executed, a
vehicle equipped with a stability control system must have a LAR of 30
percent or less 0.75 seconds after completion of steer. This

LAR will be calculated from the vehicle's lateral acceleration,
corrected for roll angle, at its center of gravity position.
     From data collected from each SWD maneuver executed, a
vehicle equipped with stability control must have a LAR of 10 percent
or less at 1.5 seconds after completion of steer. This LAR will be
calculated from the vehicle's lateral acceleration, corrected for roll
angle, at its center of gravity position.
    The performance criteria mean that 0.75 seconds after the
completion of the steering input, the corrected lateral acceleration
must not exceed 30 percent of the maximum lateral acceleration recorded
during the steering maneuver, and at 1.5 seconds after the completion
of the steering input, the lateral acceleration must not exceed 10
percent of the maximum lateral acceleration recorded during the
steering maneuver. The agency believes that these criteria represent an
appropriate stability threshold. NHTSA's research indicates that an ESC
system's ability to maintain an LAR above these criteria would provide
an acceptable probability that the vehicle would remain stable and that
a level of LAR above these criteria would result in a high probability
of the vehicle becoming unstable.
(b) Yaw Stability Performance
    The yaw rate ratio is a performance metric used to evaluate the
ability of a vehicle's ESC system to prevent yaw instability. The YRR
expresses the lateral stability criteria for the sine with dwell test
to measure how quickly the vehicle stops turning, or rotating about its
vertical axis, after the steering wheel is returned to the straight-
ahead position. A vehicle that continues to turn or rotate about its
vertical axis for an extended period after the steering wheel has been
returned to a straight-ahead position is most likely experiencing
oversteer, which is what ESC is designed to prevent. The lateral
stability criterion, expressed in terms of YRR, is the percent of peak
yaw rate that is present at designated times after completion of steer.
    The yaw rate ratio, YRR, is determined as follows:
    [GRAPHIC] [TIFF OMITTED] TP23MY12.007

    Where [Psi]Vehicle (COS + 0.75 sec, + 1.5 sec) is yaw
rate value at a specified time after the completion of steer, and Max
[Psi]Vehicle is the maximum yaw rate measured during the
second half of the sine maneuver including the dwell period from time
1.0 second after the beginning of steer until the completion of steer
during each maneuver.
    This performance metric is identical to the metric used in the
light vehicle ESC system performance requirement in FMVSS No. 126. We
believe that this metric is equally applicable to truck tractors and
large buses, though it is calculated at different time intervals after
the completion of steer.
    Therefore, the agency proposes to evaluate yaw stability
performance by calculating the YRR at 0.75 seconds and at 1.5 seconds
after the completion of steer. The two performance criteria are
described below:
     From data collected from each 45-mph SWD maneuver
executed, a vehicle equipped with a stability control system must have
a YRR of 40 percent or less 0.75 seconds after completion of steer.
     From data collected from each 45-mph SWD maneuver
executed, a vehicle equipped with stability control must have a YRR of
15 percent or less at 1.5 seconds after completion of steer.
    The performance criteria mean that 0.75 seconds after the
completion of the steering, the yaw rate must not exceed 40 percent of
the peak yaw rate recorded during the second half of the sine maneuver
including the dwell period, and at 1.5 seconds after the completion of
the steering input, the yaw rate must not exceed 15 percent of the peak
yaw rate recorded. The agency believes that these criteria represent an
appropriate stability threshold. NHTSA's research indicates that an ESC
system's ability to maintain an YRR above these criteria would provide
an acceptable probability that the vehicle would remain stable and that
a level of YRR above these criteria would result in a high probability
of the vehicle becoming unstable.
(c) Lateral Displacement
    Lateral displacement is a performance metric used to evaluate the
responsiveness of a vehicle, which relates to its ability to steer
around objects. Stability control intervention has the potential to
significantly increase the stability of the vehicle in which it is
installed. However, we believe that these improvements in vehicle
stability should not come at the expense of poor lateral displacement
in response to the driver's steering input.
    A hypothetical way to pass a stability control performance test
would be to make either the vehicle or its stability control system
intervene simply by making the vehicle poorly responsive to the speed
and steering inputs required by the test. An extreme example of this
potential lack of responsiveness would occur if an ESC system locked
both front wheels as the driver begins a severe avoidance maneuver that
might lead to vehicle rollover. Front wheel lockup would create an
understeer condition in the vehicle, which would result in the vehicle
plowing straight ahead and colliding with an object the driver was
trying to avoid. It is very likely that front wheel lockup would reduce
the roll instability of the vehicle since the lateral acceleration
would be reduced. This is clearly, however, not a desirable compromise.
    Because a vehicle that simply responds poorly to steering commands
may be able to meet the proposed stability criteria, a minimum
responsiveness criterion is also proposed for the SWD test. Using a
lateral displacement metric to measure responsiveness ensures that the
vehicle responds to an initial steering input to avoid an obstacle.
This metric was chosen because it is objective, easy to measure, has
good discriminatory capability, and has a direct relation to obstacle
avoidance.
    The proposed lateral displacement criterion is that a truck tractor
equipped with stability control must have a lateral displacement of 7
feet or more at 1.5 seconds from the beginning of steer, measured
during the sine with dwell maneuver. For a bus, the proposed
performance criterion is a lateral displacement of 5 feet or more at
1.5 seconds after the beginning of steer. The lateral displacement
criteria is less for a bus because a large bus has a longer wheelbase
than a truck tractor and higher steering ratio, which makes it less
responsive than a truck tractor. The value will be calculated from the
double integral with respect to time of the measurement of the
corrected for roll lateral acceleration at the vehicle center of
gravity, as expressed by the formula:
Lateral Displacement = [int][int]AyCG dt

    Where: AyCG is the corrected for roll lateral
acceleration at the center of gravity height of the vehicle

    This is the same performance metric used in FMVSS No. 126.
Furthermore, the vehicle would be required pass this requirement during
the every execution the SWD maneuver where the steering wheel angle is
0.7A or greater.
3. Alternative Test Maneuvers Considered
    We have considered other test maneuvers besides the sine with dwell
test. The SWD maneuver was tentatively selected over the other
maneuvers discussed above and below because our research demonstrates
that it has the most optimal set of characteristics, including the
severity of the test, repeatability and reproducibility of results, and
the ability to address rollover, lateral stability, and responsiveness.
    The agency's research initially focused on developing the ramp
steer maneuver to evaluate the roll stability performance and the sine
with dwell maneuver to evaluate the yaw stability performance. However,
after additional testing, we were able to develop test parameters for
the sine with dwell maneuver so that both roll stability and yaw
stability could be evaluated using a single loading condition and test
maneuver. The sine with dwell maneuver has typically been used to
evaluate only the yaw instability of a vehicle. The agency has
previously used a lightly loaded vehicle weight condition for such
evaluations where the lightly loaded condition and the resulting lower
CG height were much more likely to cause vehicle directional loss-of-
control as opposed to rollover. In the light vehicle ESC standard, the
sine with dwell maneuver is used to evaluate only yaw instability, not
roll instability, with the vehicle loaded to LLVW only but not to GVWR.
Given the different dynamics of heavy vehicles when compared to light
vehicles, NHTSA evaluated several loading conditions and found that a
loading condition which equals 80 percent of the tractor's GVWR enables
us to evaluate roll instability as well as yaw instability.
    The number of tests that would be needed to cover all likely
vehicle operational conditions for varying vehicle designs is
potentially large, and many tests (particularly those using low
friction surfaces) may not be sufficiently repeatable for an objective
performance requirement. Our testing indicates that the SWD maneuver is
sufficiently severe to ensure that nearly all vehicles without ESC
would not be able to comply with the proposed performance requirements.
For example, the vehicles we tested without ESC either had wheel lift
or spun out during the SWD maneuver. Hence, a vehicle that avoids loss
of control according to our objective lateral acceleration and yaw rate
decay definitions demonstrates that it has an ESC system typical of
today's technology and would have safety benefits.
    In addition to our test results, the agency thoroughly evaluated
the test vehicles and test data submitted by EMA and others to the
agency. EMA provided information on one tractor that appeared to
satisfy the agency's proposed SWD performance criteria without a
stability control system. After careful review of this data, we do not
believe this fact means the test has no value.\43\ It is possible that
there are currently truck tractors or large buses sold today that are
exceptionally yaw stable, even in a severe maneuver such as a double
lane change, which the SWD maneuver is designed to simulate. When
evaluating light vehicles, the agency noted that there was a very small
number of vehicles that were stable enough without a stability control
system to pass our performance criteria without an ESC system.
Therefore, the existence of vehicles that could pass the proposed SWD
test without a stability control system simply indicates that it would
take many tests to cover all potential instability scenarios across
varying vehicle designs in order to design a perfect test regime, as
discussed earlier. Such a complex test regime would require excessive
costs to manufacturers to ensure compliance and excessive costs to the
agency to determine and enforce compliance.
---------------------------------------------------------------------------

    \43\ As discussed earlier, EMA's testing of Vehicle J used a
control trailer with a wider track width and a lower deck and used
ballast that resulted in a lower vehicle center of gravity than used
by NHTSA's researchers. Each of these differences caused EMA's
combination vehicle to be more stable than NHTSA's during testing.
---------------------------------------------------------------------------

    We recognize that manufacturers may wish to base their
certification of compliance with this proposed standard on their
vehicles' performance in NHTSA's proposed test maneuvers. If
manufacturers intend to conduct the maneuvers proposed by the agency,
they may need to make additional investments in their facilities or
have their certification testing performed at a contractor's facility.
However, we believe some manufacturers may have already made these
investments, and others would make similar investments as they develop
and validate ESC systems for their vehicles. This is based on our
understanding of the maneuvers used by the heavy-vehicle industry for
ESC system development and validation, some of which include variations
of the agency's proposed maneuver.
    We also recognize that, over time, manufacturers will be able to
develop other methods for certifying compliance with the proposed
standard. For example, manufacturers can develop computer models or
simulations to demonstrate ESC system performance. However, we
recognize that these alternative methods may not be suitable for
atypical vehicles that are custom-built for customers. We seek comment
on the issues surrounding manufacturers' certification of compliance
including the assumptions made regarding manufacturers' current and
future test facilities, the methods used by manufacturers to validate
ESC system performance, the ability of manufacturers to use other
methods (such as computer modeling, simulation, or alternative test
maneuvers) to certify compliance, the cost of certification, and the
issues surrounding certification of atypical truck tractors.
    Below, we discuss the alternative test maneuvers that were
considered and what we considered to be acceptable performance criteria
for each test. We also discuss why we are choosing the SWD maneuver for
compliance testing in lieu of each of these maneuvers. We invite
comment on each of these test maneuvers, including whether they should
be used instead of, or along with, the proposed compliance test
maneuvers.
(a) Characterization Maneuver
    While NHTSA has conducted extensive testing using the SIS maneuver,
we believe that alternative methods may be used to determine the
steering wheel angle needed to achieve 0.5g of lateral acceleration at
30 mph. For example, a test based on the SAE J266 circle test may yield
a similar steering wheel angle without requiring the track space
necessary to conduct the SIS maneuver. The steering wheel angle that
produces 0.5g of lateral acceleration at 30 mph may be above the ESC
system's activation threshold for some vehicles, making it impractical
to conduct a direct measurement of the steering wheel angle. The agency
seeks comment on the feasibility of an alternative characterization
test based upon the SAE J266 circle test.
(b) Roll Stability Test Maneuvers
    To evaluate roll instability, we have considered two alternative
roll stability test maneuvers--the J-turn and the ramp steer maneuver.
The two tests are similar in that both maneuvers require the tested
vehicle to be driven at a

constant speed and then the vehicle is turned in one direction for a
certain period of time. The test speed and the severity of the turn are
designed to cause a test vehicle to approach or exceed its roll
stability threshold such that, without a stability control system, the
vehicle would exhibit signs of roll instability. Both tests would be
performed with the tractor loaded to its GVWR. Furthermore, we would
not expect a vehicle that could pass one test to fail the other.
    The most notable difference between the J-turn and the RSM
maneuvers is that the J-turn is a path-following maneuver. That is, it
is performed on a fixed path curve. In contrast, the RSM maneuver is a
non-path-following maneuver that is performed with a fixed steering
wheel input. For example, during the agency's and EMA's testing, the J-
turn maneuver was performed on a 150-foot radius curve. In contrast,
the RSM is performed based on a steering wheel angle derived from the
SIS test. We would expect that, with the RSM, the radius of the curve
would be close to the fixed radius used in the J-turn maneuver.
However, in the RSM, the driver would not have to make adjustments and
corrections to steering to maintain the fixed path.
    When comparing the J-turn to the RSM, the agency considers the RSM
to be a preferable test maneuver because the RSM maneuver can be
performed with an automated steering wheel controller. Because the J-
turn is a path-following maneuver, a test driver must constantly make
adjustments to the steering input for the vehicle to remain in the lane
throughout the test maneuver. Moreover, driver variability could be
introduced from test to test based upon minor variations in the timing
of the initial steering input and the position of the test vehicle in
the lane.
    In addition, the RSM appears to be more consistent because it
involves a fixed steering wheel angle rather than a fixed path. There
is negligible variability based on the timing of the initial steering
input because the test is designed to begin at the initiation of
steering input, rather than the vehicle's position on a track.
Moreover, an automated steering wheel controller can more precisely
maintain the required steering wheel input than a driver can.
Therefore, we tentatively conclude that the RSM is more consistent and
more repeatable than the J-turn, which is critical for agency
compliance testing purposes.
    Notwithstanding the above observations, we recognize that many
manufacturers perform NHTSA's compliance tests in order to certify that
their vehicles comply with NHTSA's safety standards. We also recognize
that, over time, manufacturers are likely to use other methods such as
simulation, modeling, etc., to determine compliance with Federal Motor
Vehicle Safety Standards. In this regard, we observe that, because the
J-turn and the ramp steer maneuvers are so similar, manufacturers may
be able to determine compliance with a stability control standard by
using the J-turn maneuver even if the agency ultimately decides to use
the RSM for compliance testing. Thus, if a manufacturer sought to
certify compliance based upon performance testing, a manufacturer would
not necessarily need to perform compliance testing with an automated
steering controller.
    In considering the RSM test conditions, the agency looked to its
test data and the data submitted by EMA. Data analysis indicated that
the RSM test performed from at an initial speed of 30 mph is sufficient
to demonstrate effective stability control performance for truck
tractors. At GVWR, the tested buses were observed to have different
speed thresholds at which wheel lift occurred and stability control
initially activated. Without stability control, buses were observed to
produce wheel lift between 35 and 39 mph in the RSM, compared to
tractors, which ranged from 28 to 30 mph. Large bus stability control
systems initially activated at speeds greater than 30 mph in the RSM,
which was higher than the 26 mph observed with tractors. In light of
these differences, an initial speed of 36 mph was selected for buses to
ensure an appropriate level of test severity and that stability control
would intervene.
    Another issue in conducting the RSM is whether to use fixed rate
steering or to steer at a rate such that the full steering input is
reached in a fixed time. Using fixed rate steering, the steering wheel
is turned a 175 degrees per second until the desired steering wheel
angle is reached. If a vehicle with a lower steering wheel angle input,
such as a short wheelbase 4x2 tractor, is tested using this steering
method, the desired steering wheel angle would be reached relatively
quickly after the initial steering input. In contrast, for a longer
wheelbase truck or a large bus, the desired steering wheel angle would
be reached relatively slowly after the initial steering input. This
results in a more severe test for vehicles with a lower steering wheel
angle because the predicted lateral acceleration of 0.5g would be
reached more quickly than for vehicles with a higher steering wheel
angle. In an extreme case with an exceptionally large steering wheel
angle, such as a bus with a long wheelbase the system may activate
before the full steering wheel is input.
    Using a fixed-time steering input, we would program the steering
wheel controller to reach the desired steering wheel angle in exactly
1.5 seconds using a constant steering rate, which was derived from the
manually steered 150-foot J-turn maneuver. Using this steering method
would prevent the RSM results from varying with steering wheel angle
input. We are requesting comment as to whether fixed-rate steering or
fixed-time steering is a preferable manner for conducting the RSM.
    The RSM would use a similar, but not identical lateral acceleration
ratio performance metric to evaluate roll stability. As with the SWD
maneuver, the LAR used in the RSM would indicate that the stability
control system is applying selective braking to lower lateral
acceleration experienced during the steering maneuver. In the SWD
maneuver, the LAR is the ratio of the lateral acceleration at a fixed
point in time to the peak lateral acceleration during the period from
one second after the beginning of steer to the completion of steer. In
contrast, the LAR metric we would use for the RSM would be the ratio of
the lateral acceleration at a fixed point in time to the lateral
acceleration at the end of ramp input, which is the moment at which the
steering wheel angle reaches the target steering wheel angle for the
test. Also, in contrast to the SWD maneuver, the LAR measurements for
the RSM would be taken at a time when the steering wheel is still
turned. This means that, although the SWD maneuver is a more dynamic
steering maneuver, the LAR criteria for the RSM would be greater than
the LAR criteria for the SWD maneuver.
    The performance criteria for the RSM would depend on whether fixed-
rate steering or fixed-time steering input is used. For truck tractors
and large buses using fixed-time steering input, we would expect that
the LAR would be less than 1.05 two seconds after the end of ramp input
and less than 0.8 three seconds after the end of ramp input. For truck
tractors tested using fixed-rate steering inputs, we would expect that
the LAR would be less than 1.1 two seconds after the end of ramp input
(the point in time at which the target steering wheel angle is reached)
and less than 0.9 three seconds after the end of ramp input. For buses
using fixed-rate steering, we would expect that the LAR would be less
than 1.0 two seconds after the end of ramp input and less than 0.7
three seconds after the end of ramp input. The performance criteria for
large

buses would be lower because, as we stated above, when using fixed-rate
steering input, the longer wheelbases of buses cause the maneuver to be
less dynamic.
    In a March 2012 submission, which was revised with additional
details in April 2012, EMA suggested that NHTSA use different test
speeds and performance criteria for the J-turn maneuver.\44\ EMA
suggested that a test speed that is 30 percent greater than the minimum
speed at which the ESC system intervenes with engine, engine brake, or
service brake control. Instead of measuring LAR, EMA suggested that,
during three out of four runs, the vehicle would be required to
decelerate at a minimum deceleration rate. NHTSA has conducted testing
on variations of this EMA maneuver, and we plan to conduct further
testing. We request comments on EMA's suggested test procedure and
performance criteria for the J-turn maneuver.
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    \44\ Docket No. NHTSA-2010-0034-0032; Docket No. NHTSA-2010-
0034-0040.
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    Based on our testing to date, the agency tentatively concludes that
the RSM is a preferable test to the J-run to demonstrate a stability
control system's ability to prevent roll instability. However, as
discussed in greater detail below, in order to reduce the number of
compliance tests that the agency and those manufacturers who choose to
demonstrate compliance by conducting the agency's performance tests
must perform, the agency proposes using on test maneuver, the SWD, to
demonstrate both roll and yaw stability performance. Although we are
proposing to use the SWD maneuver for evaluating roll stability, we
request comment on issues related to the RSM and J-turn tests,
including test conditions, steering input method, and performance
criteria.
(c) Yaw Stability Test Maneuvers
    After evaluating several maneuvers on different surfaces, the
agency was unable to develop any alterative performance-based dynamic
yaw test maneuvers that were repeatable enough for compliance testing
purposes. Bendix described two maneuvers intended to evaluate the yaw
stability of tractors.\45\ However, neither of these test maneuvers was
developed to a level that would make them suitable for the agency to
consider using as yaw performance tests.
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    \45\ These tests are discussed in section IV.E.3. See Docket No.
NHTSA-2010-0034-0037 and Docket No. NHTSA-2010-0034-0038.
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    In July 2009, EMA provided research information on several yaw
stability test maneuvers.\46\ One of these maneuvers was the SWD on dry
pavement that is similar to what is proposed in this notice. The second
maneuver was an SWD maneuver conducted on wet Jennite. The third
maneuver was a ramp with dwell maneuver on wet Jennite.\47\ EMA did not
provide any test data on the last two maneuvers. Thus, we considered
them to be concepts rather than fully developed maneuvers that we could
consider using for yaw stability testing.
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    \46\ Docket No. NHTSA-2010-0034-0035.
    \47\ This ramp with dwell maneuver is the same one identified by
Bendix referenced in the prior paragraph and in section IV.E.3.
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    We received no other alternative yaw performance tests from
industry until EMA's submission of Vehicle J data in late 2010.\48\ EMA
suggested using a wet Jennite drive through test maneuver demonstrated
yaw performance in a curve on a low friction surface. The maneuver is
based upon a maneuver the agency currently conducts on heavy vehicles
to verify stability and control of antilock braking systems while
braking in a curve. As part of the test, a vehicle is driven into a
500-foot radius curve with a low-friction wet Jennite surface at
increasing speeds to determine the maximum drive-through speed at which
the driver can keep the vehicle within a 12-foot lane. As with the J-
turn, we are concerned about the repeatability of this test maneuver
because of variability in the wet Jennite test surface and the driver's
difficulty in maintaining a constant speed and steering input in the
curve.
---------------------------------------------------------------------------

    \48\ Docket No. NHTSA-2010-0034-0022; Docket No. NHTSA-2010-
0034-0023.
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    In a March 2012 submission, which was revised with additional
details in April 2012, EMA provided information about another yaw
stability test along with additional information on the J-turn
maneuver.\49\ This maneuver would simulate a single lane change on a
wet roadway surface. It would be conducted within a 4 meter (12 foot)
wide path. The roadway condition would be a wet, low friction surface
such as wet Jennite with a peak coefficient of friction of 0.5. The
other test conditions (i.e., road conditions, burnish procedure,
liftable axle position, and initial brake temperatures) would be
similar to those proposed in this NPRM. In this maneuver, the truck
would enter the path at progressively higher speeds to establish the
minimum speed at which the ESC system intervenes and applies the
tractor's brakes. The maneuver would then be repeated four times at
that speed with the vehicle remaining within the lane at all times
during the maneuver. EMA suggests, as a performance criterion, that
during at least three of the four runs, the ESC system must provide a
minimum level (presently unspecified) of differential braking. The
agency has not had an opportunity to conduct testing of this maneuver,
but we intend to do so to determine whether this is a viable
alternative yaw stability test.
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    \49\ Docket No. NHTSA-2010-0034-0032; Docket No. NHTSA-2010-
0034-0040.
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    In light of the inability to develop a different performance-based
yaw stability test, the agency is proposing to use the SWD test
maneuver to evaluate yaw stability performance. Although we are
proposing to use the SWD maneuver for evaluating yaw stability, we
request comment on other yaw stability tests that could be suitable for
performance testing and possible performance criteria for any such
test. Furthermore, we specifically request comment on all aspects of
EMA's yaw stability test discussed in its March and April 2012
submissions, including the test conditions, test procedure, and
possible performance criteria that would allow the agency to test both
trucks and buses with this maneuver.
(d) Lack of an Understeer Test
    The SWD maneuver is designed to induce both roll and yaw responses
from the vehicle being evaluated. However, the agency has no test to
evaluate how the ESC responds when understeer is induced. The technique
used by a stability control system for mitigating wheel lift, excessive
oversteer or understeer conditions is to apply unbalanced wheel braking
so as to generate moments (torques) to reduce lateral acceleration and
to correct excessive oversteer or understeer. However, for a vehicle
experiencing excessive understeer, if too much oversteering moment is
generated, the vehicle may oversteer and spin out with obvious negative
safety consequences. In addition, excessive understeer mitigation acts
like an anti-roll stability control where it momentarily increases the
lateral acceleration the vehicle can attain. Hence, too much understeer
mitigation can create safety problems in the form of vehicle spin out
or rollover.\50\
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    \50\ EMA's testing of Vehicle J on the 500-foot wet Jennite
curve shows understeer mitigation at maneuver entry speeds up to 34
mph, but at 35 mph, the vehicle could not overcome understeer. See
Docket No. NHTSA-2010-0034-0022; Docket No. NHTSA-2010-0034-0023. At
these low levels of lateral acceleration, no adverse effects
appeared to occur as a result of the understeer mitigation.
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    During the testing to develop FMVSS No. 126, the agency concluded
that understanding both what understeer mitigation can and cannot do is
complicated, and that there are certain

situations where understeer mitigation could potentially produce safety
disbenefits if not properly tuned. Therefore, the agency decided to
enforce the requirements to meet the understeer criterion included in
the ESC definition using a two-part process. First, the requirement to
meet definitional criteria ensured that all had the hardware needed to
limit vehicle understeer. Second, the agency required manufacturers to
submit engineering documentation at the request of NHTSA's Office of
Vehicle Safety Compliance to show that the system is capable of
addressing vehicle understeer.
    Based on the agency's experience from the light vehicle ESC
rulemaking and the lack of a suitable test to evaluate understeer
performance, the agency is not proposing a test for understeer to
evaluate ESC system performance for truck tractors and large buses. The
agency requests comment on this NPRM's lack of a proposed understeer
test.
4. ESC Malfunction Test
    During execution of a compliance test the agency proposes
simulating several malfunctions to ensure the system and corresponding
malfunction telltale provides the required warning to the vehicle
operator. Malfunctions are generally simulated by disconnecting the
power source to an ESC system component or disconnecting an electrical
connection to or between ESC system components. Examples of simulated
malfunctions might include the electrical disconnection of the sensor
measuring yaw rate, lateral acceleration, steering wheel angle sensor,
or wheel speed. When simulating an ESC system malfunction, the
electrical connections for the telltale lamp would not be disconnected.
Also, because a vehicle may require a driving phase to identify a
malfunction, the vehicle would be driven for at least two minutes
including at least one left and one right turning maneuver. A similar
drive time exists in the FMVSS No. 126 test procedure.
    After a malfunction has been simulated and identified by the
system, the system is restored to normal operation. The engine is
started and the malfunction telltale is checked to ensure it has
cleared.
5. Test Instrumentation and Equipment
    For the truck tractor and large bus stability control system
research program, each test vehicle was fitted with specific
instrumentation and equipment necessary to execute each test safely and
to collect necessary performance data. The compliance test program
proposed in this NPRM would use essentially the same equipment and a
subset of the instrumentation. As was done for FMVSS No. 126, the
agency proposes including in the regulatory text the basic design
parameters for the automated steering machine, outriggers, and the
control trailer because this test equipment and instrumentation can
influence test vehicle performance. However, the proposed regulatory
text does not include a list of the less critical test instrumentation
used during the compliance test. The agency's common practice has been
to provide instrumentation details, test instrumentation range,
resolution, and accuracy for all the required instrumentation in the
separate NHTSA Laboratory Test Procedure. Furthermore, the agency is
aware that manufacturers and test facilities will be interested in
knowing what instruments will be used for a compliance test program.
The following table and corresponding discussions identify the critical
equipment and instrumentation used by NHTSA's researchers and for the
most part, the same or similar is proposed for use by NHTSA's Office of
Vehicle Safety Compliance.

                                    Table 3--Critical Test Instrumentation Used for Data Collection by NHTSA Research