Adjusting Tire Pressue for Tempertature

[Daryll]

Originally Posted By: CapriRacer
Originally Posted By: Daryll
I'm not sure what you said, but: it appears that, in the winter, my tires show pressure going up 3 lbs from sitting outside, cold, to being driven after a good warm up. 5 pounds in the summer.


My rule of thumb is that normal driving should see AT MOST a 3 psi build up. I would be concerned if I saw more and I would add pressure if I saw 5 psi.

Based on that, Daryll, I think you aren't using enough pressure.



Actually, I overinflate my tires a little so that's not the problem. Truthfully I don't remember for sure how much the increase was, in the summer, from cold to hot. I'll be sure to find this out next summer. But I am seeing 2-3 pounds, max increase now (Pittsburgh fall/winter), assuming a constant temperature outside from cold to warmed up tires.

 

[CapriRacer]

Originally Posted By: mechtech2
Who? Me?
Sure. Not only friction, but ambient road heat and direct sun heats up a tire. Hot asphalt does a number.
And quite a bit. Over the decades I note much higher pressure changes cold to hot than a MAX of 3 lbs. That is an arbitrary#.
No way is 3 a max.
Racers indeed get even more.


Let me put it like this.

Yes, sometimes you'll see more than 3 psi build up - and just to be sure, we are talking tires about passenger cars - not LT type tire where the starting pressure can be as high as 80 psi. Those would have proportionally more.

But 3 psi is my "rule of thumb" as fairly normal build up - not a max! - AND - that's why I said that 5 psi ought to be the point where the starting cold pressure should be adjusted.

BTW, anything that happens on a race track is a completely different animal - and we shouldn't discuss that that when we talk about street usage.

Originally Posted By: mechtech2
Originally Posted By: Astro14
Daryl - the guys have reached the same conclusion that I would have, in this case: consensus is accuracy.

The physics derivation is this: PV=nRT. V is fixed (the volume of the tire), n is a constant as is R in your case. So, the variable is temperature. But it's temperature in Kelvins...so...at 32 degrees F outside, you're at 273 degrees K. one degree K is the same temp variation as one degree C...so, plug in a few numbers, like your 30 PSI, and 269 (the 25 degree F day), then change 269 to 285 (your 55 Degree F garage) and you'll get about 32...for the new pressure at the higher temp...proving the rule of thumb...

I would go 3 PSI over while in the garage...close enough!

Cheers,


Except that the volume DOES change. It is not a constant.


But the volume change is so small you can ignore it. Besides, we are talking "close enough"!

 

[Daryll]

It seems that this conversation died without getting a concensus as to how much tire pressure will rise, given the same outside weather. There has to be a scientific answer somewhere.

 

[CapriRacer]

Originally Posted By: Daryll
It seems that this conversation died without getting a concensus as to how much tire pressure will rise, given the same outside weather. There has to be a scientific answer somewhere.


I am hoping that the question above is NOT the question at the beginning of this thread as they are different questions with different answers.

But the answer to the question above is: There isn't enough info supplied.

Pressure buildup depends on a lot of things. First, it takes a hour for a tire to reach an equilibrium temperature.

Second, it depends on the load and speed (given the starting pressure)

That's why there are kind of "Rules of Thumb" for normal maximums. Heck, when I drive to work, I don't get any buildup at all. The commute is too short.

 

[mechtech2]

Sure - there is a scientific answer and a general consensus.
About 3 lbs over.

 

[DaHen]

Originally Posted By: mechtech2

Sure - there is a scientific answer and a general consensus.
About 3 lbs over.


Sounds good to me, I'll eccept that.

 

[Astro14]

Originally Posted By: mechtech2
[Except that the volume DOES change. It is not a constant.


Yeah, I know...it changes a tiny bit as the tire changes shape, but for a correctly inflated tire, is it really statistically significant? I mean, when you really start to get into the engineering precision, the math becomes very complex.

The simple physics equation was to show how the rule was derived, based on science, and likely to a 95% accuracy...which is enough to explain the "why" behind the recommendations...

Edit: I see that CapriRacer answered this point as well...I am a science geek, he's the tire engineer...I defer to his experience...

 

[mechtech2]

This aspect [volume change] should not be stated as a non variable. That is all.

 

[Coprolite]

Originally Posted By: mechtech2
This aspect [volume change] should not be stated as a non variable. That is all.


When the impact of the slight change in a variable can not be measured, engineers will state that it is assumed to be constant.

You can think of it from a different angle. Can you calculate the change in tire volume by measuring the pressure at two different temperatures?

 

[Jim Allen]

Mechtech- What Capri is saying is that temp and pressure rise is an indicator of rolling resistance, which is an indicator of low pressure. In setting an "ideal" pressure, you would look for a minimal temperature rise as one of the checks.

I think an arbitrary 3 psi rule of thumb is a little low too but it could be based on experience with the stuff he runs. I see somewhat higher numbers in the aggressive truck tires I have fooled with extensively. Road surface will have something to do with it too, as well as tread compound and tread pattern. I have run into situations where the tire was at it's maximum rated pressure but I still had more temp rise than I preferred (which indicated tome a need for a higher load rating, obviously)

 

[Daryll]

In my Lexus LS460, which is the only vehicle I have which has a digital tire pressure display on the dash, I am seeing about 2.5 pounds of increase from cold to hot tires, assuming the weather stays about the same. This is with the tires properly inflated to the Lexus indicated 33 pounds per tire.

I didn't pay close attention in the summer, but will next summer. But from what I remember, the difference was higher...say 4-5 pounds. But I think some of that can be attributed to a higher temperature range in Pittsburgh from morning to mid-day. So 3 lbs is probably a good rule of thumb, assuming the weather/temperature is the same.

 

[Bruce T]

I've seen a 3-4 lb difference between cold and hot tires following a summer highway drive, but never 5 lbs. This was measured using a Buxton Engineering gauge rated to +/- 0.25 lb, so it wasn't reading low due to a lack of accuracy. I consider CapriRacer to be a tire authority.

 

[Bruce T]

Ah, as Daryll mentions, the time of day was a factor for me, too. I first measured the cold tires before sunrise, then checked the hot tires at 4pm. This makes the 3 lb variation sound even more plausible, since the outside temps probably rose 20°F from 80°F to 100°F.