Adjusting Tire Pressue for Tempertature

[Daryll]

So if I want to have exactly 30 LBS in my tires when they are "cold". My internal garage is 55 degrees but the outside temperature is 25 degrees. The car has been sitting overnight in the garage. How much more than 30 lbs do I put in so that when the car is outside in "operating temperature", the cold pressure, before driving, will be 30 lbs?

I've experimented with this a little and it SEEMS that the answer is 2.5 pounds. Meaning put 32.5lbs into the cold but garage car would be the same as 30lbs if the car was out all night in 25 degrees.

Does this sound right?

 

[xBa380]

I think you are making it more complex then it needs to be... A pound or two will not make much difference, I would honestly just over-inflate to 32 or 33PSI and be happy with it. Most tires are fine up to 44PSI or so anyway, safely.

 

[Quattro Pete]

Originally Posted By: Daryll
I've experimented with this a little and it SEEMS that the answer is 2.5 pounds. Meaning put 32.5lbs into the cold but garage car would be the same as 30lbs if the car was out all night in 25 degrees.

Does this sound right?

The rule of thumb is 1 PSI difference for every 10 degree F change in temp, so if you have 32.5 PSI at 55F, you'll have 29.5 PSI at 25F.

 

[AstroTurf]

Generally speaking it is about 1 psi per 10 degrees of temperature change.

Yes, You are on the right track.

A psi difference of a pound or two will make a difference in performance and tire wear.

 

[Daryll]

Thanks.

 

[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,

 

[Daryll]

Excellent!

 

[HTSS_TR]

You don't adjust tire pressure everyday, you need to adjust pressure once a month. If the low of Dec is 0F and the temp in garage is 50F, I would add 5-6 PSI above placard to compensate for the possible 0F.

 

[AstroTurf]

Originally Posted By: HTSS_TR
You don't adjust tire pressure everyday, you need to adjust pressure once a month. If the low of Dec is 0F and the temp in garage is 50F, I would add 5-6 PSI above placard to compensate for the possible 0F.


Why not? Who wrote the hard and fast rule about once a month?

I say check them as often as you like. I know that when I was testing various pressures, I would check them several times a day.

Jim

 

[Daryll]

I actually check my tires twice a month but admit that the variances I find generally are small enough to support that once per month is enough. And on my LS460 pressures are on my dash and agree with my $50 digital pressure gauge.

 

[m37charlie]

Measure pressure and temperature at "cold ambient". Measure tire pressure at "hot ambient" after driving 100 miles in mid-day.
Take the the ratio of (hot ambient pressure + 1 bar) divided by (cold ambient temp + 1 bar). That ratio equals the ratio of tire internal temp divided by tire cold ambient temp - in absolute, of course.
Useful if you have a CTIS system and want to use it to determine max tire temps while running.

Charlie

 

[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.

 

[odie]

Put 36 psi in the snows for both cars . Checked them about 11 in the morning after a night in the low 20s . It was probably around mid 30s when monitoring the tires . The tires on the shaded side of the car read 33.5 , while the tires in the sun showed 35.5 . So , 36 > 37 psi seems ideal for those swings in temps .

 

[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.

 

[johnsmellsalot]

I just park the car outside the garage overnight and check the tire pressure in the morning.

 

[mechtech2]

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.


3 lbs at MOST from cold sitting to post driving? Are you kidding?

 

[Quattro Pete]

Originally Posted By: mechtech2
3 lbs at MOST from cold sitting to post driving? Are you kidding?

3 PSI means the tires heated themselves up 30 degrees above their cold temperature. In my experience, that's about right unless you went to the track or drove like a maniac.

 

[CapriRacer]

Originally Posted By: mechtech2
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.


3 lbs at MOST from cold sitting to post driving? Are you kidding?


You obviously have a different opinion, but we don't know what it is. Could you please state it?

Thanks

 

[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.

 

[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.