What temperature for CIP "cold inflation pressure - Part two

The following is based on a couple posts I made on an RV forum but wanted to share with the rest of my readers

I had posted my comments on the thread from my Aug 25 post on CIP in the hopes of clarifying questions about adjusting inflation for various changes in ambient temperature, but there have been comments that indicate the need for more details when talking about tire temperature so here is a bit more info on the topic.

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We really have two different but related temperatures to consider.

One set of temperatures is the external set. This would be the "Ambient" temperature of the air around us and the temperature of the road surface.
 The other and much more important temperature is the temperature of the tire structure.

Now with the correct instruments, it is possible to measure the temperature of the tire structure. This is done at test tracks and at race tracks using a small needle probe that sticks into the tire rubber 1/8" to 1/2" deep depending on the tire type & size and gives a reading the engineers use to make adjustments to race car suspension or to tire pressures. Since good & accurate tire "Needle Pyrometers" cost a couple hundred bucks, few individuals have them. Some think a simple IR gun from Harbor Freight is OK but all that does is give the approximate surface temperature of a tire which is always cooler than the important internal structural temperature. Rubber is an insulator so heat does not move through a tire structure from hot to cool very well so you can mislead yourself if you only use surface temperature. The IR gun is fine for conductive materials such as metal in a hub or brake drum.

Heat is generated at the molecular level not from belts rubbing against each other. This heat moves slowly from the hottest location (belt edges) to cooler surfaces. There is the surface of the air chamber and the surface on the outside of the tire.

The air on the inside of the tire gets warmed from the heat transfer from the inside surface of the tire. This air is also cooled by transfer to the metal wheel and the turbulence inside the tire means the temperature of the inflation gas (air) is fairly uniform.

The inside temperature is related to the outside temperature in that cooler outside temperature allows faster heat transfer from inside to outside so the inside temperature will be a little lower when the outside is lower but not in a 1:1 relationship.

Tire pressure will increase as described by Boyle's Law PV=NRT. I did the math proof in my blog post of March 13 2014 .

So after all that we are left with the facts that:

1. A tire generates heat based on deflection (tire inflation & load) and rate of deflection (speed)
2. More deflection due to more load or less inflation or both, results in more heat in the tire structure.
3. More speed means greater generation of heat. This is one reason tires have speed ratings.
4. If you lower the CIP you will generate more heat and this will hurt the rubber and eventually weaken it.
5. With sufficiently low inflation or sufficiently weakened rubber the heat will increase at a faster rate than it can flow out of the tire structure and can go into "run-away" increase which can result in a sidewall flex failure or "Blowout" due to shorter term low inflation or possibly a belt separation due to long term heating and weakening of the belt rubber.

I hope this clarifies and answers any additional questions.

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