Braking - Friction at Work


The deceleration of vehicles is performed by the braking systems. There are three brake systems on most vehicles: standard; parking or service; and auxiliary. This article will examine the various types and features of brakes used in emergency and personal vehicles.

Drum brakes and disc brakes are the two types of standard brakes. Drum brakes have brake shoes, which are wedge or cam operated. These shoes press against the rotating drum assembly on the axle creating friction and slowing the axle's revolutions. Disc brakes, meanwhile, utilize pads that engage a rotating disc assembly on the axle, slowing the axle. Braking systems were originally all hydraulically operated, then heavier vehicles went to air operated brakes, and now electronically operated brakes are being introduced.

Electronic brake systems are sometimes referred to as "brake-by-wire" or "stability control" since they work in conjunction with the Anti-lock Braking System. The advantage of electronic braking is that it is faster, and increases the vehicle's stability by applying a combination of appropriate wheel braking while de-powering the engine to prevent loss of control.

Emergency vehicles that have independent front suspension will also have increased braking capability. This is a result of larger front wheel brakes that are incorporated in the system.

Antilock Braking Systems (ABS) are very popular on personal and emergency service vehicles. ABS should not be confused with auxiliary braking systems. The purpose of Antilock Braking Systems is to enable the driver to maintain vehicle control and avoid skidding while stopping in the shortest possible distance on a slippery surface. ABS prevent the wheels from locking, keeping the tires turning so there is friction with the road surface, allowing directional control.

ABS essentially perform automatically and better than what formerly was accomplished by "pumping" hydraulic brakes. ABS allow steering, improve control, and enhance stability during braking on wet, icy, or loose gravel surfaces. One manufacturer's system works with auxiliary braking devices by sending pulses through the devices. It should be noted that ABS are fully compatible with both types of air brake systems, drum and disc brakes, and with all types of auxiliary or secondary braking devices. NFPA 1901 Standard for Automotive Fire Apparatus requires an all-wheel ABS if such a system is available from the chassis manufacturer. This should also apply to ambulances and all other emergency services vehicles.

Automatic Traction Control (ATC) can also be integrated into the ABS. The ATC uses the signals from the wheel sensors to determine if the drive wheels are spinning because excessive power is being furnished.

Auxiliary braking systems are an added feature that assist in stopping heavier vehicles and reduce brake wear and heat buildup that can cause brake fade. NFPA 1901 also requires an auxiliary braking system on all vehicles with a Gross Vehicle Weight Rating (GVWR) of 36,000 pounds or more. The Standard also recommends their use on apparatus down to 31,000 pounds GVWR. Type IV ambulances should also consider auxiliary braking. These systems are necessary because emergency service vehicles make repeated stops from high speeds that cause rapid brake lining wear and brake fade that can lead to collisions. Auxiliary braking systems are also recommended for apparatus that respond on long or steep grades, in congested areas with repeated stops, or to a high number of incidents.

There are four types of auxiliary braking systems: driveline retarders; engine brakes (for large displacement diesel engines); exhaust brakes (for smaller diesel engines); and transmission retarders.

Driveline retarders work by slowing the rotation of the driveshaft. They are generally electro-magnetic devices that can be installed directly to the front of the rear axle or at a mid driveline mounting. Hydraulic driveline retarders are also available.

Engine brakes work by altering valve timing under deceleration by opening the exhaust valves at Top Dead Center of the stroke for an instant, releasing cylinder compression and eliminating the expansion stroke. In essence, the engine is converted to an air pump as each cylinder compresses and releases. The noise produced is simply that of the released compression pressure. Although there are other manufacturers, these are commonly referred to as "Jake Brakes."

Exhaust brakes consist of a butterfly valve that closes, restricting the flow of exhaust gasses and creating a back pressure on the engine. This slows the engine revolutions per minute (rpms). Some brands are not effective at speeds (rpms) less than 20 miles per hour. Pacbrake* has an exhaust brake that works by a variable orifice size, closing up as the rpms decrease, resulting in a more effective retarder than the fixed orifice design. Exhaust brakes are lighter and less expensive than engine brakes, but not as effective.

Transmission retarders are an integral part of the automatic transmission. Transmission retarders may be of two types, the input type that operates at the input section of the transmission (between the torque converter housing and the main housing), or the output retarder that operates at the tail shaft of the transmission. The output retarder is the most prevalent on emergency vehicles. These systems work by using the transmission fluid to power an opposing direction wheel on the drive shaft within the transmission.

Secondary braking systems can increase the brake lining life by as much as three to five times the normal service period, depending upon the specific design application and use. This can result in considerable savings that can pay for the expense of installing the system. In addition to increasing brake life, these systems reduce other wear factors such as heat damage to brake drums and discs and tire wear. Automatic transmission and driveline retarders are especially effective. Some auxiliary braking devices should be shut off when the apparatus is responding on slippery surfaces. Follow the manufacturer's recommendations for proper operation.


  • NFPA 1901, Standard on Automotive Fire Apparatus
  • Bruce Sebring, Pacbrake Co., Surrey, B.C.
  • Chuck Wittenberg, Merritor-Wabbco, Troy, MI

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