Roger Marble on TPMS and tire temp considerations

[Bill]

All -

This is an article written by Roger Marble, the Tire Guru, who is a member of this forum. I found it in a recent issue of RV Travel. Especially in light of his comments on TPMS devices, I thought it was important enough to add to the Library. He has given me his permission to copy it here to the forum.

Bill

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Tire Temperature always seems to be a hot topic

From a couple of RV forum threads.

Hot tire temperature is important but the problem is in learning what the actual temperature is.

The average temperature is not a good indicator and that is about all you can get by reading the Contained Air Temperature. While many seem to believe that their external TPM is giving them a reading of the CAT, I can tell you that the TPM will always read significantly cooler than the CAT and that the CAT is always cooler than the hottest and most critical location in the tire.

Rubber is a good insulator and does not transfer heat from the hot spots to the cooler locations very well. As a result you can have a tire failure from extreme heat in one location while another location only inches away can be perfectly OK.

I read that many TPMS have set the high temp warning point at about 158F. That is all well and good if you are measuring the actual CAT but I do not know how an external TPM will sense that temperature especially if it is out in the cooling air stream on a metal stem or at the end of an extension hose

I would always be more concerned about the pressure measurement as that is not going to be affected by cooling of the valve stem or extension hose. We know that for our purposes pressure does follow the "Ideal Gas Law" and it doesn't matter if you use air or Nitrogen the results are essentially the same unless you are measuring your pressure to the 0.1 psi.

So knowing that tire pressure will change by about 2% for each change of 10F in temperature of CAT we can get a reasonable estimate of the CAT and will note that out TPM is showing a lower temperature change than indicated by the pressure change.

I would not use the temperature reading from a TPM to set tire pressure. If I got a high temperature reading but the pressure reading was reasonable I would suspect a mechanical problem such as bearing or brake issue.

BOTTOM LINE

A motorhome properly loaded with the inflation set based on actual measured load on the tire and consulting the load Inflation tables will not result in an over-heated tire. NOTE Trailers should use different method to set pressure due to the Interply Shear issue of trailer axle alignment and should set the cold inflation to the tire sidewall pressure.

If the above isn't enough detail for you, you can review the other posts that have Temperature as part of the topic.