Do not do this to your RV tires

 

Do not bleed down your RV tire pressure because you think it will “explode” if you exceed the PSI number molded on the tire sidewall.

Almost every week I see posts on one or more of the various RV forums where people say they limit the PSI in their tires. Sometimes they say why, as in “to prevent a blowout.” Sometimes they do not say why, but their reasoning behind this practice is clear to me.

It appears that many people incorrectly believe that exceeding the “PSI” number molded on the tire sidewall is going to result in tire failure and explosion.

It may help to let people know about some of the testing that tires undergo during the development phase.

First some background

The “Max Inflation” statement on your tire sidewall really is the inflation associated with the Maximum Load capacity. If you actually read your tire sidewall, you will see that is what it says. So the number of PSI is really the minimum needed to support the stated load.

It is important to understand that we are always talking about the “cold” inflation and not the inflation of a tire that is running down the highway or has been in direct sunlight or driven on in the prior couple of hours. “Cold” inflation does not mean the tire needs to be refrigerated. It means the tire is at the prevailing ambient (surrounding) temperature.

Tire pressure monitoring systems

With the introduction of aftermarket tire pressure monitoring systems (TPMS) that report both tire pressure and the temperature of the sensor, people are now being exposed to information they have no previous experience with. They see tire pressure rising as they drive down the highway. Some may see a 10% to 20% rise in pressure from the pressure they set their tires to just a half-hour earlier. This increase is a cause for concern, not because they had any working knowledge of tire operating temperature or pressure, but simply because the pressure was higher than they expected.

What not to do

A few people have decided that they need to bleed down the high pressure because they thought the pressure number of the tire sidewall was the maximum it could tolerate. Of course, the action of bleeding down the hot pressure was exactly the wrong thing to do, as that meant that the tire would no longer be operating at the pressures expected by the tire engineer when they originally designed and tested the tire specification.

Obviously, this raises the question of how much pressure a tire can tolerate before the owner should be concerned. Well, I am going to give you some numbers, but these are only examples. I cannot speak for every tire company’s process or specifications. But if we start with a few guidelines, I think you can get comfortable and, hopefully, you will believe that tire design engineers do have a reasonable idea of what they are doing.

First off, we are only going to discuss regular production street tires that have passed DOT testing. The numbers I will use would basically be seen on new tires. I cannot advise on a tire’s capability after it has been damaged or run for tens of thousands of miles. Damaged tires can fail when reinflated, as seen in this video of a 22.5-size tire.

This video was made to show that welding on a wheel with a tire still mounted can result in a tire failure. Note the pressure at failure.

 

Tell service person if you drove on under-inflated tire

If your tire suffered a puncture and you drove any distance with the tire underinflated, you may have permanently damaged the body cords. That is why you should always tell the tire service person that you drove on the under-inflated tire so they know to use a cage or other restraining safety equipment. Age and miles can reduce the strength of any tire, but we do consider this degradation when approving a design.

Another consideration is the availability of high-pressure air. Most home compressors or those that supply air to hoses made available to consumers have an upper limit of about 150 PSI. Therefore, we don’t expect any consumer to have access to or to use industrial air pressure, which can be 400 PSI, or even special inflation units as used on aircraft tires.

Basically, many tires are tested to 300% to 400% of their rated inflation, or above 200 PSI, whichever is lower. I had a number of tires that could contain over 300 PSI when new. If they had been damaged, however, the same tire might fail at less than 200 PSI. But I also never, in my 40 years as a design engineer, saw a tire fail from normal pressure increase—and I was involved in the testing and evaluation of thousands of tires.

The bottom line

The bottom line is: Please do not bleed down your hot inflation pressure. You should ONLY set and adjust inflation when the tire is cool and at ambient temperature. Doing otherwise MAY result in you having a failure days or weeks later because you were running lower pressure than what was needed for the tire load you were applying.

Also, unless you have damaged your tire, it is unlikely to “explode” simply by adding 10% more cold air pressure.

 

 

 

 

 

 

Tire Inflation. We all need to pay attention to it

 

It doesn't matter if we drive a Diesel Pusher that is 48' long or pull a 6' "Pop-Up" trailer with our car. It doesn't matter if our vehicle runs on Gas or Hydrogen or Diesel or is an EV, our vehicles currently run on tires that are inflated with "air". Even if that inflation "air" is 78% Nitrogen or 98% Argon we must use some gas in our tires. What is constant for all vehicles on the roads today is that we have tires that require inflation with some sort of "gas".

It is basic Physics that It's the "air" pressure that supports the tire load, not the tire structure, and while we all know that there are folks on the Internet, who will claim to know more about any topic than the people with years of training, I can only hope that the majority of readers of this blog will trust the actual Science of tire design and engineering rather than some random guy spouting his opinion on what level of inflation you should run.

I follow a number of RV forums and find 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 Sidewall" while others insist the "Vehicle Certification label" is the only correct level and of course there are more than a couple that have developed their own inflation tables and will be happy to tell you what their personal calculations indicate.

Here is my take on the subject:

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 must 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 industry.

The owner of the RV needs to confirm the actual loading for their RV and should confirm they are not exceeding the GVWR or 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 water and 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 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 measured 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 GAWR 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 or if some guy pontificating at the campfire is a more reliable source.