Tire pressure vs temperature

Geo-TH,In · Nov 30, 2018

I find it interesting to watch tire pressure when driving. Start out with cold tires and in a few miles the pressure increases. Park a car so the sun heats up one side and tire pressure goes up.

I very familiar with the ideal gas laws and how temp and pressure are related.

Here is a physics question? How much does the temp have to change to change tire pressure 1 psi?

JD Farmer · Nov 30, 2018

6-12 degrees.

Mutt and Jeff · Nov 30, 2018

really
how does that work
you know what the pressure is in the tire as you drive?
wow

Geo-TH,In · Nov 30, 2018

TPS, tire pressure sensors give you real time pressure. I'm surprised how close they are to a tire pressure gauge.

ASEguy · Nov 30, 2018

10 degrees

FBH44 · Nov 30, 2018

Well, it is not that simple. Atmospheric air is not an ideal gas, and it does not follow exactly the ideal gas law equation. You also are ignoring expansion and contraction of the volume of the tire; the volume changes with pressure. Ideal gas law applies to constant volume. PV=nRT.

Ray IN · Nov 30, 2018

Ok here is a little harder question. Does the pressure in a wagon tire increase a measurable amount by loading the wagon, such as a gravity wagon loaded with grain?

matthies · Nov 30, 2018

If they weren't close to a tire pressure gauge, how the heck would you air your tires up to keep the computer happy?? Not awake yet??

Determined · Nov 30, 2018

The tires on my quad run at 3-4 psi in the summer.
If the pressure were to drop 1 psi for every 10 degree change in temperature why are the tires not sucked flat to the rims when the temp drops to 0.
Assuming they were originally filled to 3 psi when it was 80 degree they should be reading negative 5 psi when the temp drops to 0 based on the 10 degree to 1 psi suggested.

gene bender · Nov 30, 2018

40 psi whether wagon is mt or full. PSI is just what is says load has nothing to do with PSI

Russ from MN · Nov 30, 2018

Do you find that the tire pressure sensors agree with your gauges? I aired up the car tires yesterday as we are heading to AZ tomorrow, and it seems I have to ge at least 2-3 lb higher on my inflator to get the recommended amount on the dash. And I go by the door post data, not the side of the tire!

gotmyfarm! · Nov 30, 2018

I, too, was watching the pressure in my tires yesterday. I have a new enough truck now, that lets me see the tires' pressures in real time as I drive down the road. Last summer, the tires were at 78 PSI cold. Now that it is 10-30 degrees F everyday, the pressure had dropped to upper 50s-60 PSI!!! Wow, what a difference! So I filled the tires back up to 68 PSI, and figure that's a cold reading considering the gas station is only 2 blocks away. Then I started driving to the hardware store 20 miles away, and I watched the tires go up to 74 PSI. Now, the temp outside was not changing in that 20 miles, so what made the PSI shoot up, the friction heat from the wheels turning against the road surface? I don't care for all these gadgets put into new vehicles, but being able to watch the PSI go up and down is interesting. I had never kept an eye on it in summer, but if the tires were filled to nearly max cold PSI, I wonder how much above recommended cold PSI they went on those hot 90 degree days? Don't want a tire to explode!

dpendzic · Nov 30, 2018

Just use the universal gas law for the specific volume of your tire--PV=NRT

JMOR · Nov 30, 2018

(quoted from post at 10:40:35 11/30/18) I, too, was watching the pressure in my tires yesterday. I have a new enough truck now, that lets me see the tires' pressures in real time as I drive down the road. Last summer, the tires were at 78 PSI cold. Now that it is 10-30 degrees F everyday, the pressure had dropped to upper 50s-60 PSI!!! Wow, what a difference! So I filled the tires back up to 68 PSI, and figure that's a cold reading considering the gas station is only 2 blocks away. Then I started driving to the hardware store 20 miles away, and I watched the tires go up to 74 PSI. Now, the temp outside was not changing in that 20 miles, so what made the PSI shoot up, the friction heat from the wheels turning against the road surface? I don't care for all these gadgets put into new vehicles, but being able to watch the PSI go up and down is interesting. I had never kept an eye on it in summer, but if the tires were filled to nearly max cold PSI, I wonder how much above recommended cold PSI they went on those hot 90 degree days? Don't want a tire to explode!

es. Feel tire at beginning of extended drive and at end of drive. Temp rises.

Dave Sherburne, NY · Nov 30, 2018

O' Great, something more for me to worry about.

GEO-TH, In · Nov 30, 2018

Yes spot on.

docmirror · Nov 30, 2018

(quoted from post at 07:08:16 11/30/18) 40 psi whether wagon is mt or full. PSI is just what is says load has nothing to do with PSI

Ectually, it will go up slightly. The tire is not a fixed volume. Adding weight to the trailer will decrease the volume of the tire just a bit. All other factors in PV = NRT being equal, the V will decrease, and setting NRT fixed means the P(pressure) go up slightly to compensate for a smaller volume. If the tire were made of 'steel'(or other in-compressible media), then the pressure would remain the same.

JMOR · Nov 30, 2018

Man with one watch knows what time it is, but man with two watches is never sure

docmirror · Nov 30, 2018

No, I'm quite sure.

www.ntppool.org

IowaJohn · Nov 30, 2018

I've always understood that 10 degrees change in temp is a pound change in tire pressure.

BigPapa53 · Nov 30, 2018

Manufacturers set tire pressures in anticipation of a 10% pressure increase from a cold tire to one at operating temperature so, if your cold pressure is 50psi, at operating temp it should be right at 55psi. The temp increase is more from flex in the carcass than road friction. This is also why max load is always at max pressure. A tires carcass will flex more under a heavier load so pressure needs to be increased to add rigidity. This is also why a tire that is flat or severely underinflated will come apart and even catch fire. It's from the increased friction in the carcass.

This is why on your wagon tires, Ray, pressure will increase as temp increases as it flexes more while rolling. The tires will be much cooler unloaded versus loaded because they don't flex as much.

-Scott

carpenter · Nov 30, 2018

I understand that the fact that tires get bigger when cold has a greater impact on pressure than the air temp in the tire. I have no opinion yet if this could be true.

showcrop · Nov 30, 2018

BigPaPa is correct there is very little rolling friction between the tire and the road surface except from the "squirm" if they are under-inflated the heat comes from the flex.

Glen SW Wi · Nov 30, 2018

Now that we are in the cold season my low pressure indicator came on. Had to put air in all four tires to get the sensors to stop sending the warning. Took about 5 pounds in each tire. Don't know how much air they put into the new tires and I never got the warning until the temp dropped below freezing. No change in load.

504 · Nov 30, 2018

In tire repair class, they claimed 1 psi for every 7deg of change.

BigPapa53 · Nov 30, 2018

Just as the pressure increases as the tires heat up, the pressure decreases when the tires get colder when the weather gets cold. That’s why you have to add air when it gets cold.

I worked at a shop a few years ago that did a LOT of tire business. Every year at the first cold spell we were over run with people calling about their TPS light being on. The owner put this on the sign in front of the shop:

[b:68498027d5]“TPS light on? Come in today and get your summer air changed!”[/b:68498027d5]

It was a joke, of course, we didn’t charge anybody anything, but quite a few folks took it seriously, at least until we explained it to them.

-Scott

W_B · Nov 30, 2018

(quoted from post at 13:07:13 11/30/18) really
how does that work? you know what the pressure is in the tire as you drive?
wow

Welcome to the 21st century.

Deutz Lover · Nov 30, 2018

PV=nRT Ideal Gas Law

Bolye's law.

P=pressure
V=volume
n=Number of moles of a gas
R=Ideal gas constant
T=temperature
Not all gases behave as ideal gases

Tire pressure vs temperature

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