Quote:
Originally Posted by CapriRacer
First, this deals with the Ideal Gas Law - which references absolute zero temperature and absolute pressure. So the answer depends on how hot you start and what elevation you are at. You'll get a different answer if you start at 30°F than 100°F as well as a difference at sea level or at 10,000 feet.
It also depends on the starting inflation pressure. You'll get a different answer if you are starting with 30 psi than if you start with 100 psi.
Having said that, if you have a 30 psi tire at 70°F at sea level, and the temperature rises to 80°F, the increase in pressure is 0.84 psi.
On the other hand if you start out with 65 psi (like on some LT tires), the pressure increases 1.50 psi.
I've worked with other tire engineers and have worked out a "Rule of Thumb" - 2% for every 10°F. It's close enough and easy to remember!
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Thank you for confirming, Barry. My bigger brain(my gut!) always suspected that 1psi per 10°F was only a generalization. Yes, I think with my belly, because prenatal alcohol and smoking destroyed half of what’s upstairs by the time I was born.
My takeaways, and confirmations, of what you just explained:
1. Delta in air pressure per given temperature change varies depending on existing air pressure. The lower the pressure, the less the change. The higher, the more the change.
I can attest to this - bleeding down a 35psi tire for two seconds, with my particular gauge, reduces pressure only around 1psi. But: bleeding down a 60psi spare ‘donut’ for two seconds, using that same gauge, reduces pressure 4-5psi! So I do understand now that an equivalent temperature change will affect the higher pressured tire more drastically than will affect the lower pressured tire.
2. At altitude, I know the cold pressure I set near sea level will increase a bit with every 1,000 feet up I go into the mountains. Because there is less atmospheric pressure imposed on the tire up there.
So at sea level, a 10°F increase in temperature would increase that 35psi tire to about 35.9psi? At, say, 4,000 feet up, that same temperature increase might increase that 35psi to…. 36-37psi?
Interesting stuff to know, and it helps me to estimate how high a pressure I should set it for based upon the altitude or temperature conditions where the tire is being driven to.