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Friday, September 29, 2017

Who to believe on tire inflation?

I was reading an RV forum and found a number of folks offering their interpretation on what the proper inflation is for tires in RV application. Some insisted the only correct inflation pressure was what was shown on the "Tire Placard" or "Vehicle Certification label"
Here is my reply:

We have two separate issues to consider when selecting the minimum cold tire inflation pressure in RV application.

A vehicle MFG has DOT requirements they need to follow. They are to specify the minimum inflation needed in the tires they provide on the new RV. That inflation is required to be sufficient to support the stated GAWR divided by the number of tires on that axle. Basically this means the axle rating is used to establish tire inflation based on the published load & inflation tables from the tire company.

The owner of the RV needs to confirm the actual loading for their RV and should confirm they are not exceeding the GVWR or either GAWR or the individual tire load capability.

You will immediately note that the RV company is making an assumption that is just not a reflection of reality, that being that axle loads are always split exactly 50/50 side to side. While many RVs may have the load split only a couple percent off of 50/50 some have discovered one end of an axle may carry more than 1,000# more than the other. Others have learned their unbalance is closer to 40/60 and some have discovered that their tires are overloaded as soon as they fill their fuel tank before loading anything else into the RV. Obviously a responsible owner needs to do a better job than some RV companies are doing.

While some feel it is sufficient to just read the regulations about the vehicle certification label I and other tire engineers know that policy is not always the best way to get the best combination of ride, handling and tire durability.

Lets see what Goodyear says in their RV tire application web page.
"Correct tire inflation is a key component in tire care. The recommended maximum inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the load on each individual tire.
Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard

Maybe you don't think Goodyear knows more than some RV owners. What does Michelin say in their RV documents Here on pg 4:
"The amount of inflation pressure required in each tire depends on the weight of the fully loaded vehicle, to include passengers, cargo load, fuel, and water."
" To determine proper inflation pressure, Michelin recommends weighing each wheel position of the recreational vehicle individually. Weighing each axle end individually will give a clear
indication of how the weight of the recreational vehicle is distributed across the axle.

Also in their tire maintenance document on tire inflation:
"Unlike commercial light truck and truck applications, we recommend weighing motorhomes at each wheel position of the vehicle (right front corner, left front corner, etc.). This is the only way to get a clear indication of exactly how the weight of the motorhome is distributed. Otherwise, one wheel position may be significantly overloaded even though the G.A.W.R. has not been exceeded. Tire inflation for a given axle should be set according to the pressure corresponding to the higher of the two end loads."

Others are allowed to have and express their opinions and their interpretation of how the regulations are to be applied, but it was my job as a tire engineer to understand and work within FMVSS regulations for some 40 years.

I will leave it up to you to decide if the only acceptable inflation is what is on the vehicle certification label or that inflation is learned by reviewing what major tire companies say in their  published guides.
Retired Desi

1 comment:

  1. What you're saying makes sense. My question is, how do you go about getting individual weights? I don't believe my bathroom scales will go quite high enough.


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