Too much pressure increase

Reading an RV forum thread on TPMS usage. One comment jumped out at me.

"I have a serious concern with the G rated tires on my 5th wheel trailer. When I run the cold max pressure at 110 PSI, I get TPMS readings up to 134 PSI rolling down the road. Lately, I have been running 95 PSI cold and am getting 115-119 PSI rolling. The tires and rims are rated for 110 PSI, I can live with 120 PSI, but the 134 PSI concerns me."


I replied that inflation ratings for wheels are based on a "cold" pressure. Increase in pressure due to operation is considered by manufacturers and I would not be concerned as long as the proper cold inflation is used along with appropriate limits on load and speed.
Tire wording "Max Pressure" can be confusing but in reality the pressure stated on the tire sidewall is the cold pressure needed to support the stated load and that load is the maximum load the tire should be subjected to. I advised the owner that he should NOT underinflate his tires and plan on operation temperature to increase the pressure. "Cold" pressure is the only pressure you should be concerned with.

Tire pressure increases by about 2% for each increase in temperature of 10F. If you don't remember the Science from High School you can read THIS post.. If you are seeing a 21% increase in pressure (110 > 134) that means you are seeing about a 100F increase in internal tire temperature, which I would consider a bit excessive for normal tire operation.

If you are seeing a 25% increase in pressure (95 > 119) then you are getting a 125F increase in temperature which indicates you are working the tires even harder . This extra "work" that is generating a greater temperature increase is not good for long term tire life. You are "aging" the tire rubber faster. Some might want to review this "Key Point" of tire life as covered in THIS post.

I cover these points on Temperature, Inflation and Aging in various posts on my RV tire blog.

Your temperature increase indicates you are possibly overloading your tires and also possibly driving faster than desirable for your tire loading.

You need to confirm your pressure is 110 psi AND that your gauge is giving an accurate reading at that level.

The poster then responded:

"Today drove 250 miles and my 95 PSI tires were running 115-119 PSI and the tire temps were at 20F above out side temp, 50 outside and 71 tire readings. I still contend that 134 PSI is way to dangerous for tires to run on 110 rated tires."

So I responded back:
 Few people realize that the pressure increase as a function of temperature is based on well established and confirmable Physics and that a TPMS is not reading the actual tire temperature  but is actually reading the temperature of the brass valve stem and the metal base of the TPMS itself, which is being cooled by outside air.

Air is a very good insulator and if you think about it, you have a small column of air running up the inside of the valve stem which makes it difficult for the heat to travel up the center of the stem and past the valve core itself all the while the valve is moving rapidly around being cooled by the outside air.

I am aware of laboratory tests that goes against what "common sense" might indicate. That being that the air inside a tire is not uniform in temperature but it is always cooler than the hot spots of a tire and it is the hot spots that can result in tire failing if hot enough for long enough.

I have no doubt that the TPMS was indicating only 20F above the cool 50F outside air temperature. If you are still concerned about the hot pressure of 134 on tires that have a cold pressure rating of 110 for its max load capacity rating.

As a tire design engineer with 40 years experience, I trust the science of the "Gas Law" and knowledge that air is an insulator and metal conducts heat from a hot source to a cooler one.
I don't know what to advise other than to decrease the operating load and speed and to confirm your hand gauge is accurate and to always inflate the tire when cold to 110psi, as continued operation at current load and speeds will certainly result in pressure reading that are above the cold pressure of 110 psi.

Quick post on Dual tire air balance systems

Read a thread on an RV Forum on connecting two tires in dual position, with hoses to "balance" the inflation pressure. There were some claims made about performance improvements but IMO the conditions necessary to deliver those improvements are pretty extreme and improvements are tenuous.

The information on the function of this pressure balance equipment also ignores the potential for damage to the RV when one tire loses air but the driver is not immediately informed of the loss. There is even talk of serious air loss in excess of 25% of the pressure needed, only resulting in faster wear.

I found no mention of operating a tire that low possibly resulting in a catastrophic failure (coming apart and potentially doing thousands of dollars' worth of damage to the RV).

I can't understand why some would think that lowering the hot tire pressure (decreasing load capacity) would ever be a good thing to do.

Someone will probably point out that some of the systems that connect the air between a set of duals limit the amount of air transferred, but that still leaves a problem.

If you have two tires set to 80 psi and one gets a puncture and starts to leak, the "good" tire bleeds off air to the leaking tire until 5 or10 psi has been transferred. Now both tires are at 70 to 75 psi and are just starting to be overloaded.

But the leak continues and the tire going flat transfers its load to the "good" tire, resulting in the "good "tire seeing ever-increased the level of overload.

This can continue until one tire is flat and one tire is 100% overload. Now you are driving down the road with no knowledge you have a flat and a tire at 200% of rated load.

If you are lucky, you stop and discover the problem before the "good" tire fails, too, leaving you with two scrap tires.

This system does make airing up tires easier in that you only have one valve for the pair of tires, but I'm not sure if that "benefit" is worth the cost.

##RVT825