Thor Forums

[Beau388]

I think we can all agree that heating of the tire causes the internal pressure to increase. The heat is the result of the tire's flexing and friction against the surface (road). The greater the tire's deflection the more heat is generated. Likewise the greater the force applied to the tire in moving the vehicle (acceleration/deceleration - vertically or horizontally) will increase tire temperature. Tire flexing is greater with the load and speed. So the higher the cold pressure, the less pressure rise (delta P) for a given load.

The maximum tire pressure listed on the tire's side is not an estimate but an actual computation based on carrying the rated load at the rated speed on a standard day and not damage the tire from overheating.
Lastly most tire pressure tables use ideal gas laws for the pressure rise predictions. Usually there are no corrections for water vapor in the tire's air and that value can vary greatly. The reason that nitrogen is recommended for airing up the tires is it contains no water vapor. The water vapor problem can be explained this way. Assume you use a small potable compressor to completely air up a tire at 70 degrees and 80% humidity to 100 psi. That night the temperature drops to 25 degrees and and ice forms on the inside of the tire. The internal pressure of the tire will decrease by about about 1 psi per 5 degrees F or the gauge should read 91 psi. Checking the tire pressure (with gloves on) it reads 84 degrees because the water vapor holding up the tire before is now ice.