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Friday, August 10, 2018

Do inner duals fail more often? If so, why?

Read this on an RV Forum thread:

"Isn't it weird how it always seems to be the inner tire that goes bad or blows? Seemed to always be my experience (bad luck) when I was driving semi's for a living."

While I have never seen a documented study, I can certainly believe this observation can be made by many. There is Science behind the "Why" this may be true.

For those that have reviewed the post on Tire Covers that explains the effect of temperature on tires, you have learned that it is the higher temperature that accelerates the tire "aging" process. If you look at the temperature readings on sets of dual tires you will see that when the tires are inflated to equivalent pressures the temperature of the inner dual on Motorhomes will usually show as a bit hotter. The difference isn't a lot but the effects of that difference I believe are cumulative.

It is also true that older tires are more likely to fail due to the degradation of the rubber flexibility and strength.

Please do not take this observation and assume you need to start adjusting the inflation in your duals to run more air in the inner tire. Doing this could end up resulting in a shift in loading between the pair of tires to place more load on the inner tire, and we know that increased load results in increased operating temperature.

Tire operating temperature develops from complex actions of flexing of the belts and of the lower sidewall which are the two hottest locations on a tire. You might be able to lower the temperature in one location while increasing the temperature in a different part of the tire.

The best practice I can suggest is that you:
1. Confirm the tires in each pair of duals are a "Matched Set" (See THIS older post).
2. Ensure you know the actual load on each set of duals not just the total axle load.
3. Use the Load and Inflation tables to learn the MINIMUM Cold Inflation Pressure for the heavier loaded axle end.
4. Add a 10% inflation to that minimum number to establish your CIP.
5. Inflate all tires on the axle to the same CIP. (matching the inflation within +/- a couple psi is good enough.
6. Run a TPMS to monitor your pressure whenever driving.

Friday, August 3, 2018

Can you change tire size on your RV?

There seems to be a bit of confusion when it comes to selecting tires to replace the size/type that came as Original Equipment on your RV. What you can do and what you should do are not always the same thing.

Here are some statements collected and posted by a knowledgeable person who frequently responds to questions on some RV Forums.

"Goodyear: Never fit tires to a vehicle that have less load carrying capacity than required by the Original Equipment Manufacturer.


Cooper: The new tires must have a load carrying capacity equal to or greater than the maximum load carrying capacity specified on the tire placard on the vehicle.

Toyo: Any replacement tire must be of a size and load range that will offer equal or higher load carrying capacity compared to the original equipment (OE) tire on the vehicle

I am not aware of any "legal" requirement that specifies what the RV owner is required to do. While it is a legal requirement from DOT that the vehicle manufacturer must select and specify tires and the inflation necessary to support the stated Gross Axle Weight Rating, as far as I know, this legal requirement does not apply to individual owners of vehicles.

HOWEVER, I doubt that you will find any company or responsible individual willing to state that it is good practice to select replacement tires that do not have a stated load capacity that is equal to or greater than the load capacity of the original tires.

All tires sold for use on public highways have a stated maximum load capacity at a given inflation pressure molded onto both sidewalls. This fact, along with the information in published Load/Inflation tables, makes it relatively easy to find tires with the needed load capacity.

There are a number of reasons to consider an alternate size tire (availability, cost, brand reputation, etc.) but in every case you should only select new tires that can support at least as much load as the OE tires.

The above is based on an assumption that the load on your tires is split equally 50/50 side to side on each axle. Since this is seldom the case, this is an extra bit of information you should consider when shopping for new tires.

I have numerous posts on this blog where I outline the importance of confirming the actual load on each end of each axle. This is because it is possible to have the load unbalanced to the point that one tire may be overloaded even if the total tire load capacity for all the tires on an axle numerically exceeds the total axle load measured on a truck scale.