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Old 08-22-2015, 02:07 PM   #1
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What temperature for "cold inflation pressure?

I occasionally see posts on the need to do an adjustment of your Cold Inflation Pressure ( CIP )when the temperature is not at some "standard".

Many times the "standard" is stated to be 70F or 68F. Neither of these are correct.

From a tire design standpoint CIP means when a tire has not been warmed up either by being in direct Sunlight or from having been run.
We are not talking about some chemistry lab experiment but real life. This is defined by the Tire & Rim Association, the organization that published the standards book for tire dimensions and recommended Load & Inflation for all kinds of tires, wheels and valves.
These standards are primarily intended to provide "interchangeability" as we want to be sure that every 15" tire fits properly on a 15" wheel of the appropriate type. Or that the valve will properly seal against air leaks by having the hole in the wheel of the correct diameter.

Now you don't have to get all wound up with temperature probes or IR guns to confirm a tire has not been warmed up. Just follow the guideline that when checking or adjusting tire pressure to your CIP, the tire should not have been driven more than 2 miles in the previous 2 hours, AND that the tire has not been in direct sunlight or otherwise artificially warmed up in the previous 2 hours.
"Cold" really means when the tire is at the temperature of the surrounding air or what is called "Ambient" temperature.

So unless you are taking your RV to the Antarctic or to the Sahara desert you can use the above as a simple guideline. Even if I were planning a trip from the top of Pikes Peak to Phoenix in a single one day drive I would not worry about making adjustments based on expected temperature. Tires are designed to have a large tolerance for pressure increase due to variations in the surrounding temperature.

Remember the rule of thumb is that tire pressure will only change about 2% for every change of 10F so even going from 30F to 100F might only result in a pressure change of 14%. Now you would probably need to adjust the CIP the next morning before setting out but again you would set the CIP when the tire is in the shade and not driven on for a couple of hours.
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Old 08-22-2015, 07:08 PM   #2
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Thanks!
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Old 08-31-2015, 07:44 PM   #3
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I recently read an A/S blog where the writer said he inflates to less than the recommended pressure because he had three blowouts in very hot weather when the tires were inflated as recommended by the placard.

Must be wrong, but I had read (or assumed) that the opposite would be true: underflation would cause failure in very hot conditions.

Whose right?
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Old 09-01-2015, 07:46 AM   #4
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Quote:
Originally Posted by ITowSilver View Post
I recently read an A/S blog where the writer said he inflates to less than the recommended pressure because he had three blowouts in very hot weather when the tires were inflated as recommended by the placard.

Must be wrong, but I had read (or assumed) that the opposite would be true: underflation would cause failure in very hot conditions.

Who's right?
The guy is totally off base.

Higher inflation = lower operating temperatures = less risk of a durability related tire failure

I have to assume he is thinking that the higher pressures exceeded the burst pressure - and that is totally false. Tire burst pressures are way, way higher than operating pressures = no danger of a burst due strictly to pressure.

Road puncture on the other hand is a totally different story. They can happen at any pressure.
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Old 09-01-2015, 08:33 AM   #5
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Many Thanks

I appreciate your taking the time to reassure me on this.

Sam
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Old 09-01-2015, 08:53 AM   #6
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CapriRacer beat me to it with the correct answer.
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Old 09-01-2015, 09:16 AM   #7
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Quote:
Originally Posted by CapriRacer View Post
The guy is totally off base.

Higher inflation = lower operating temperatures = less risk of a durability related tire failure

I have to assume he is thinking that the higher pressures exceeded the burst pressure - and that is totally false. Tire burst pressures are way, way higher than operating pressures = no danger of a burst due strictly to pressure.

Road puncture on the other hand is a totally different story. They can happen at any pressure.
I am not sure why you say "this guy is totally off base"?
Your post said the same thing Tireman9's did, just different wording.
Were did he say to run a tire at a lower temp than sidewall rating?
When I read his post, I understand he was talking only about the "cold inflation pressure" rating on the side wall of a tire. Just explaining what "cold inflation pressure" is from the tire industry engineers.
I could be wrong but perhaps you should read it again.

-Dennis
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Old 09-01-2015, 09:23 AM   #8
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I thought tireman and Capri racer were agreeing, from what I read.
Guess I'm missing something?


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Old 09-02-2015, 06:51 AM   #9
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Quote:
Originally Posted by batman View Post
I am not sure why you say "this guy is totally off base"?
Your post said the same thing Tireman9's did, just different wording.
Were did he say to run a tire at a lower temp than sidewall rating?
When I read his post, I understand he was talking only about the "cold inflation pressure" rating on the side wall of a tire. Just explaining what "cold inflation pressure" is from the tire industry engineers.
I could be wrong but perhaps you should read it again.

-Dennis
Just to clarify, I was answering this question:

Quote:
Originally Posted by ITowSilver View Post
I recently read an A/S blog where the writer said he inflates to less than the recommended pressure because he had three blowouts in very hot weather when the tires were inflated as recommended by the placard.

Must be wrong, but I had read (or assumed) that the opposite would be true: underflation would cause failure in very hot conditions.

Who's right?
The person in question was REDUCING the inflation pressure - and that is the wrong direction to reduce the risk of a "blowout"

I really, really don't like the term "blowout" - and the reason is exemplified by this situation. Many people think that the burst pressure is exceeded and that is what is causing the failure, when the burst pressure is NOT being exceeded at all and it is the material properties of the rubber that is changing due to heat generation.

To further clarify, I was NOT responding to the OP (Original Post).
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Old 09-02-2015, 08:34 AM   #10
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I don't understand what is so hard about airing up tires ,my marathons have 17000 miles on them, before we leave on a trip I check them, 65 lbs, and go,, if it is 95 or 45 degrees , makes no difference, low pressure causes flexing which makes heat...
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Old 09-02-2015, 11:32 AM   #11
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Originally Posted by tjdonahoe View Post
I don't understand what is so hard about airing up tires ,my marathons have 17000 miles on them, before we leave on a trip I check them, 65 lbs, and go,, if it is 95 or 45 degrees , makes no difference, low pressure causes flexing which makes heat...
That is the correct thing to do.
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Old 09-02-2015, 11:59 AM   #12
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Reason for thread

I started the thread in the hopes of clarifying questions about adjusting inflation for various changes in ambient temperature.

ITowSilver added a comment/question about what the CIP should be in relation to expected hot road temperatures.

We really have two different but related temperatures.

One is the external temperature of the air around us and the temperature of the road surface. The other 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 needle probe that sticks into the tire rubber and gives a reading the engineers use to make adjustments to race car suspension or to tire pressures. Since good & accurate tire "Pyrometers" cost a couple hundred bucks so 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 so you can mislead yourself if you only use surface temperature.

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 possibly to a lesser extent to cooler areas on the internal structure of the tire but in general the turbulence inside the tire means 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 if you are interested in reading it.

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 of both results in more heat in the tire structure.
3. more speed means greater generation of heat
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 ins a sidewall flex failure due to shorter term low inflation or possibly a belt separation due to long term heating and weakening of the belt rubber.

Any questions?

Note this and my first post will end up on my blog.
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Old 09-02-2015, 04:02 PM   #13
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Quote:
Originally Posted by CapriRacer View Post
Just to clarify, I was answering this question:



The person in question was REDUCING the inflation pressure - and that is the wrong direction to reduce the risk of a "blowout"

I really, really don't like the term "blowout" - and the reason is exemplified by this situation. Many people think that the burst pressure is exceeded and that is what is causing the failure, when the burst pressure is NOT being exceeded at all and it is the material properties of the rubber that is changing due to heat generation.

To further clarify, I was NOT responding to the OP (Original Post).
Sorry Capriracer. I guess I am the one who should have read your post again Thank you for graciously clarifying.

-Dennis
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Old 09-02-2015, 04:06 PM   #14
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Quote:
Originally Posted by Tireman9 View Post
I started the thread in the hopes of clarifying questions about adjusting inflation for various changes in ambient temperature.

ITowSilver added a comment/question about what the CIP should be in relation to expected hot road temperatures.

We really have two different but related temperatures.

One is the external temperature of the air around us and the temperature of the road surface. The other 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 needle probe that sticks into the tire rubber and gives a reading the engineers use to make adjustments to race car suspension or to tire pressures. Since good & accurate tire "Pyrometers" cost a couple hundred bucks so 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 so you can mislead yourself if you only use surface temperature.

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 possibly to a lesser extent to cooler areas on the internal structure of the tire but in general the turbulence inside the tire means 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 if you are interested in reading it.

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 of both results in more heat in the tire structure.
3. more speed means greater generation of heat
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 ins a sidewall flex failure due to shorter term low inflation or possibly a belt separation due to long term heating and weakening of the belt rubber.

Any questions?

Note this and my first post will end up on my blog.
Nope, I have been clear with all your posts and appreciate your time and patience. Even as an industry professional, I have picked up several valuable "nuggets" in the couple years you have been posting.
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