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Wednesday, September 2, 2015

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.

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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  1. Having a monitor system allowed me to see the temp go up to in excess of 160 f.
    Goodyear said damage needs more than 200f to occur. Pressure also rose from 85lbs to over 100 on many occasions.

    1. I am concerned that there may be a problem. Ire you sure your inflation is greater than the minimum needed to support the measured load for each tire? What speeds are you traveling? If your TPMS is external mount it is also giving temperatures that are lower than the actual temperature of the tire structure.

  2. I'm hoping that tire manufacturers know of this heat increase as speeds increase and have built in 'protections' against blow outs (at least somewhat!).

    1. Yes they do but the real question is do operators know the max speed the tire is rated for in RV application (could be 62 or as high as 75 depending on size).
      The Max speed is not an average but a limit. Travel faster and permanent, cumulative damage and degradation is being done to the tires.
      If your tires are rated for 65 as most ST type are then I would not consider 64 as providing sufficient margin. I run my Class-C LT type tires with a +10 to +15psi inflation margin and while in theory could travel at 75 I set the cruse at 62 with only occasional 65 to 67 seen. Don't think I have ever exceeded 70. Even on my cross country drive Ohio to Oregon last Summer.
      Even so I had one tire develop stress cracks at 7 years so ended up needing to do a replacement.

  3. Roger certainly impressed us with his knowledge of tires and temperatures but did nothing to assist the average RVer in being safer.

    1. Sorry, Tom and Tee, but I have to disagree. Perhaps you missed Roger's first part on this topic:
      That contained more general information regarding safe tire pressure and how and when to check. Part two of his article gave more specifics, for those who were inquiring. "Knowledge is power" -- and in this case, I think the more knowledge, the safer the RVer. Thanks for writing. Have a great day! Diane at

  4. A question not a comment: When I check tire pressures in the morning it is common that the tires on the sunny side are a couple pounds higher than the ones on the shady side. Do I adjust so that both sides are the same or should I assume that the tires will even out as I drive and not do anything if the warmer tires are at proper pressure? If neither side is up to pressure, how should I adjust them?

    1. You should not adjust your pressure down when it has been warmed.
      You are seeing the result of the warming of the tire due to being in the Sun. This is why it is said to set your pressure when the tires have not been driven or been in direct Sunlight for the last couple of hours.
      In your case I think you can check the pressure in the evening when the Sun is on the other side of the RV.


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