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Friday, April 23, 2021

Internet "Experts" have Dennis confused. Maybe I can help

 Dennis said:

Okay ... I've been RV'ng in Class-A's for 40 plus years and thought I had this figured out, but the "experts" have emerged to confuse me, once again. So, what is the definitive answer to the question of tire pressure ... should it be the coach manufacturer's recommendation on the placard in most every RV, or the tire manufacturer's inflation recommendation? I know for sure, the "cold pressure" stamped on the tire is NOT the recommended pressure. Please advise.
Dennis, I completely understand your frustration.
You need to remember that there are three entities that are trying to answer three different questions when it comes to tire inflation.
First the tire company.    They make tires that must meet various tire industry standards for tire dimensions and load capacity. The tire company must also certify to DOT that the tires they make are capable of passing a number of different strength and durability standards if they want to sell tires for use on US roads. These requirements are why you end up with the numbers molded on the tire sidewall. These include the Maximum load capacity for both Single and Dual-position. They must also identify the minimum inflation needed to support both the single and dual-position loads. We need to remember that the tire company does not know which RV or truck the tire will be mounted on. The tire company also doesn't know if the tire will be on the Front, in Dual on the drive, or even if it will be on a TAG axle so they can not give a single inflation number that meets all these requirements

Second, the RV Mfg has to meet some different DOT regulations. The primary one is that the Certification Label aka tire placard must specify inflation for the tire that would be sufficient to support the stated GAWR. Example: If the GAWR for a front axle was 6,000# then the tire they select must be capable of supporting 3,000# and the RV company must tell you, the owner, the MINIMUM inflation needed for the tire to be able to carry 3,000#. Since each axle probably has a different GAWR that is why you may have different inflations on the Certification label for each axle.

Third If the RV company wants to be able to meet RVIA standards the tire must be capable of supporting 110% of the axle load so in our example that means 3,300#. Here things can start to get messy. The inflation needed to support 3,300# is almost certainly different so the Tire placard would need to state the inflation for 3,300#. It is OK for the inflation on the placard to be higher than what DOT requires but it can not be any lower.

Finally, we get to you, the owner.
Option A. is to simply inflate to the number on the tire sidewall. This, in some cases, is significantly higher than the tire needs so you may get a hard ride and in extreme cases more rapid center wear.
Option B is to follow the placard inflation which on many Motorhomes is lower than the inflation number on the tire. So the owner gets confused.
Option C is what I and other tire engineers recommend. We have solid reasons for saying that the Rv owner needs to get on a truck scale, learn the actual load on the tires, and then learn the minimum inflation needed to support the actual load on the tires. We know this is the best for the tire because the data shows that a majority of RV have a tire or axle in overload and this is a major reason for tire problems in the RV world.
Option D This is a better version of Option C. This is even better but is not as easy as Option C. The data shows that almost no RV has the axle load split 50/50 side to side so simply taking the truck stop scale reading and dividing by two can be misleading. Some RVs have been found to be as much as 1,000# out of balance side to side. Option D means you need to learn the individual load on each tire position when the RV is loaded to its heaviest. This is almost impossible at regular truck stop scales. To learn the individual tire position load you need to get a company like RVSEF or Escapees to use their scales to get the individual weights. It is also possible, with a little work to find a platform scale at a gravel yard or some lumber or cement block companies have large scales. Here ere is a worksheet to help with the numbers. Once you learn the actual load on the heavy end of each axle you would use that weight to consult the tire Load and inflation tables to learn the MINIMUM inflation for all the tires on that axle. We also recommend that if possible you add 10% to the table inflation to give you a nice margin. Just do not exceed the max inflation rating for your wheels.

I hope this helps clear up your confusion.

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