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Monday, February 27, 2023

Why a safety margin on tire inflation is important

 If you have read any of my posts here at or on my blog or on any individual post on the various RV forums I follow, you have probably seen me suggest you add a “margin” (safety factor aka reserve load to some folks) to either the load capacity or the minimum inflation recommendation, or even to both. You may wonder what are my reasons. I have seen some people try to chip away at my load margin or the minimum inflation I suggest.

Reason for adding extra margins

I do have a reason for adding these extra margins. It has to do with the basic nature of the rubber and other materials used in tire manufacturing. Adding margins to other items such as bridges, buildings, or even the cars and trucks used to haul a trailer or load in the bed, is also fundamental to the nature of Engineering Design.

Engineers and others in scientific fields such as chemistry, medicine or physics all know that the properties of the materials we specify have some level of variation in their properties. For tires, I know and have seen firsthand that you cannot have an absolute number for the strength of a material. Each component of a tire, as seen above, be it rubber, steel, polyester or others, has a set of properties depending on which specific material I select. If we take a sample of material, be it steel or natural rubber (NR) and ethylene–propylene diene monomer (EPDM), we are presented with stated strengths. Even what we call “steel” is available in more than 3,500 different “grades”, each with different properties and levels of strength ranging from 30,000 psi to 72,000 psi.

When designing a tire we can basically choose from one type for the wire used to make the “bead” (the part that keeps the tire on the wheel when inflated—see above) and a different type of steel used in the belts. But then we can also select from a variety of wire cables which is a configuration of individual steel strands. Each cable has its own set of properties of strength or flexibility or even the ability of rubber to adhere to the cable.

As you can see, there is an almost infinite variety of combinations of materials a tire design engineer can select from. There are other things that can also be selected such as the temperature and time to “cure” the rubber, which can also affect the strength and durability of a tire.

All tires for U.S. highway use must pass DOT tests

Now all tires sold for highway use in the U.S. must be certified by the tire manufacturer to be able to pass specific DOT tests. So passing those tests sort of established a minimum “strength.” Tire companies can select to exceed those minimums but generally to exceed the minimums we would need to use more expensive materials or constructions. Since the tire companies want to stay in business, they also need to keep the costs of making a tire in mind. It is also well-documented that if you take a sample of steel cord and pull on it till it breaks, you never get a single result but will get a range of numbers. The same variation is observed in the strength of rubber and polyester, as used in tires. Now to ensure we produce tires that are acceptable to the public and pass the DOT tests, tire companies also have their own internal minimum performance standards. BUT I do not think that all companies design or make tires that perform identically for every type of test possible.

It is also important to remember that the DOT requires that ALL tires be capable of passing the tests—not some or an average or even most. But 100% of the tires made must be capable of passing each and every DOT test. To ensure that all new tires are capable of passing the tests, tire companies use statistical analysis of test variation in an effort to be confident that production tires will pass the DOT testing.

Finally, we come to you, the user. We know that a significant percentage of RV users do not set or even maintain tire inflation necessary to meet published minimums. We also know that many have no idea of the actual load they have on their tires. While some may keep driving speed below 65 mph, some will actually boast of towing at well over 75 or even 80 mph.

A tire’s strength decreases with use and time

In previous posts here and on my blog I have covered the fact that after a tire is used and as time moves on, its ultimate strength decreases. In an effort to decrease this variation, I have been advising that people not run at the lowest possible inflation for the load on their tires. The lower the inflation you run, the more heat is generated, which in turn lowers the ultimate strength of the rubber.

Degradation of rubber strength is not an on-off switch but a continuous process. The more you drive at higher heat, the more strength is “consumed.” The more pot-holes you hit, the more damage you do to the tire structure. This, in turn, can result in a decrease in the maximum strength of your tires.

Load & Inflation tables give you a guide for the MINIMUM inflation for a tire loaded to the stated number and operated at a given speed.

More speed means more heat.

Lower inflation means more heat.

More load means more heat.

And it is HEAT that ultimately can result in a tire coming apart.

However, if you run more inflation than what the tables show, that will decrease the heat.


In a future post, I will cover what I feel is the improper use of the word “defect.”

Monday, February 20, 2023

How much air pressure is too much in RV tires?

 With the introduction of Tire Pressure Monitoring Systems (TPMS), many RV owners are presented with what to them is new information on the status of their tire pressure and temperature.

We can start off with a question:

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.

The inflation ratings for wheels are based on a “cold” pressure. An increase in pressure due to operation is considered by wheel 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. In reality, the pressure stated on the tire sidewall is the cold pressure needed to support the stated load. 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 seeing the operation temperature to result in an increase in the pressure sufficient to support the load. The “cold” pressure is the only pressure you should be concerned with.

Tire pressure increases with increase in temperature

Tire pressure increases by about 2% for each increase in temperature of 10° F. If you don’t remember the science from high school, you can read THIS post from my blog on Tire Safety. If you are seeing a 21% increase in pressure (110 > 134 psi), that means you are seeing about a 100° F increase in internal tire temperature. I would consider that increase a bit much for normal tire operation. In my 50 years of designing, testing, and working with tires, I only remember measuring a temperature rise of 100° F in Indy race car applications.

If you are seeing a 25% increase in pressure (95 > 119), then you are getting a 125° F increase in temperature. This indicates you are working the tires very hard. 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 the “Key Point” of tire life and how increased temperature can shorten life as covered in THIS post.

Possibly overloading tires and/or driving too fast

Your temperature increase indicates you are possibly overloading your tires and also possibly driving faster than desirable for your tire loading. It is recommended in tire company data books that your operating speed for any tire in RV application be no greater than 75 mph. The “Speed Rating” is just a short-term rating and should never be considered acceptable in day-to-day RV applications.

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

This was the reply:

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

So I responded:

Few people realize that the pressure increase as a function of temperature is based on well-established and confirm able physics. 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 good insulator

Air is a very good insulator. 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. This is all while the valve is moving rapidly around being cooled by the outside air.

I am aware of laboratory tests that go 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. It is the hot spots that can result in a tire failing if hot enough for long enough.

I have no doubt that the TPMS was indicating only 20° F above the cool 50° F outside air temperature. You should not be concerned about the hot pressure of 134 psi on tires that have a cold pressure rating of 110 psi for its max load capacity rating, that is, as long as you have confirmed the actual load you are placing on your tires is no greater than 90% of the load capacity shown in the Load & Inflation tables for your tire. By “confirmed” I mean with actual scale readings for each axle when the RV is fully loaded

As a tire design engineer with 50 years’ experience, I trust the science of the “Gas Law” and the 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. Also, always inflate the tire when cold to 110 psi, as continued operation at current load and speeds will certainly result in pressure readings that are well above the cold pressure of 110 psi.