Tire pressure versus ambient temperature

On my last trip I measured the tire pressure twice before starting.

1. In the garage when the outside temperature is 95 degrees at noon. TM was in the garage overnight. My guess is the temperature was 75. Pressure was 50.

2. Pulled the TM out of the garage, drove around the block once and parked it for an hour. Perssure went up 8 degrees so it was now at 58.

Since out in the sun seemed to more reasonably represent ambient temperature than parked in the garage overnight I let some air out.

Now the really interesting question. At 5pm the outside temperature was 108 degrees. I can't really measure the tire pressure because I just drove for 4 hours and the tires were obviously hot. Should I have let some air out of the tires? because the ambient temperature had risen 13 degrees?

Suppose you start out with properly inflated tires with the outside temperature around 50 in the mountains early in the morning and then drive to Death Valley in July where the temperature is 125. What should you do? It seems to me that you should let some air out of the tires but how much? When travelling, how long should you park to let the tires cool before measuring the pressure? There must be a rule of thumb to deal with drastic temperature changes. My simple test shows that a temperature increase of 20 degrees (garage to street) increases the tire pressure by 8 pounds (assuming the trip around the block with an hour to cool down had no effect on pressure).

[Denny_A]

Originally Posted by PopBeavers

On my last trip I measured the tire pressure twice before starting.

1. In the garage when the outside temperature is 95 degrees at noon. TM was in the garage overnight. My guess is the temperature was 75. Pressure was 50.

2. Pulled the TM out of the garage, drove around the block once and parked it for an hour. Perssure went up 8 degrees so it was now at 58.

Since out in the sun seemed to more reasonably represent ambient temperature than parked in the garage overnight I let some air out.

Now the really interesting question. At 5pm the outside temperature was 108 degrees. I can't really measure the tire pressure because I just drove for 4 hours and the tires were obviously hot. Should I have let some air out of the tires? because the ambient temperature had risen 13 degrees?

Suppose you start out with properly inflated tires with the outside temperature around 50 in the mountains early in the morning and then drive to Death Valley in July where the temperature is 125. What should you do? It seems to me that you should let some air out of the tires but how much? When travelling, how long should you park to let the tires cool before measuring the pressure? There must be a rule of thumb to deal with drastic temperature changes. My simple test shows that a temperature increase of 20 degrees (garage to street) increases the tire pressure by 8 pounds (assuming the trip around the block with an hour to cool down had no effect on pressure).

.........one PSI per 10 deg F increase of Outside Air Temp (OAT). Therefore, if it had been pressurized to 50 psi at 75 deg F, it should rise to 52 psi at 95 deg F. However, a black tire exposed to the sun can accumulate a lot of additional heat due to thermal inSOLation.

The 'round the block' driving shouldn't have made too much difference, but let's say it raised the temp 1 deg F. Then the tire pressure would have been 53 psi. An hour later, outside, in the sun the pressure is 58 PSI. Assuming the original Cold air temp was 75 deg F, then the sun heating the tire added another 50 deg. F to the air temp in the tire.

What to do?: When the tire is cold, set the pressure to 50 psi. If 50 deg F, set it to 50 psi. If 100 deg F, set it to 50 psi. If you know what the cold temp was when last adjustment to pressure made, then when traveling into death Valley (per your example), remove 1 psi for each 10 deg F greater temp. I.e.; (125-50)/10 = 7.5 psi reduction. Though the tires will be hot from travel, the cold temp is now 125 deg F. [Of course, by the time the tires dissipate the heat generated whilst towing, the local OAT will have dropped. Even Death Valley cools at night]

Denny_A

[fcatwo]

I wonder how much pressure it would take to actually cause a problem and what the problem might be. I know people who pump paper-thin racing bicycle tires up to 140lbs in the morning and ride them all day without giving it another thought. I suspect Lance does that.

I've also seen tires (and tubes) that failed from their riders keeping their rim brakes on for extended periods while descending mountains on tandem bicycles - one was in our group and was about 10 min ahead of us in Switzerland last summer. It appeard to have been near the melting point when it failed. The plastic strip that covers the inside bottom of the rim had melted to the point that the spoke ends were showing.

Just one small addition to your trip from the mountians down to death valley, how much will the air pressure have changed going from 8000' above sea level to 280' below sea level? When one dives (if I remember right) one adds one atmosphere for the first 35' below sea level and one for each half of 35 after that. (could be wrong on that but lets just say it is right). Then the question becomes:
Remembering we started with 50 psi @ 75 deg. at 8000' and decend to death valley where the temp. is 125 and we are -280' at what point will the tires explode or implode. Will it be the heat or the air pressure? Will we have exploded or imploded before the tires so we could care or not? Just wondering .

[rotor_wash]

You are nearly correct with the 1 atmosphere of pressure; however that applies to ONLY underwater. Standard pressure decreases approximately 1" Hg (mercury) per 1000' above sea level increase given a "standard day". The change from slightly below sea level at Death Valley to 8000' pressure altitude is a change from 30.21" Hg to ~22.21" Hg. At 18,000' pressure altitude, you are roughly above 1/2 of the atmosphere.
Temperature is another matter with ~3.5 degrees Fahrenheit (2deg C) change per 1000 '. 125F at Death Valley becomes ~96F at 8000'.
I'll continue to put the correct tire pressure when recorded cold - regardless of the temp/altitude. I'll leave Boyles Law to the 'Rocket Surgeons' out there.
R_W

[Bill]

Don't do any fiddling. "Cold" doesn't mean "it has been in the icebox", it means "it hasn't been driven yet today". Instead, consult the tire manufacturers' advice, such as may be found at
http://www.goodyear.com/rv/tirecare/inflation.htm
They specifically say
"Tire pressure should be checked cold, or before you have driven that day, as tire pressure ratings have been designed with typical running heat/pressure build-up in mind."
and
"Don't bleed air from warm tires to reduce pressure buildup."

If they are inflated to 50 psi at the beginning of the day, then you need do nothing more.

Bill