Tyre Pressure for Boxster ??? (POLL)

[MNBoxster]

Originally Posted by bmussatti

Jim, good science lesson. Thanks

I have a few questions:

1) In this formula; p2 = p1 * (t2 + 459.69) / (t1 + 459.69) where does it take into account the "vessel" holding the air is rubber? Do all "vessels" follow this rule?

Hi, it assumes that the expansion of the Rubber Tire is minimal, which it is.

2) In your example about 30 psi, at 0 degrees F, then you arrive to a place that is 100 degreess F...how do you get to the tires being now 200 degrees?

100°F is the Ambient Temperature, the other 100°F is due to the driving/speed frictional heat buildup created by the Tire itself. Now, Temperatures do not compound, if you have 1 Qt. of 100° water and add it to another Qt. of 100° water you do not get 200° water, but for the Air, the friction and flexing of the Tire raise it's temperature X° above Ambient Temp which is 100°, so you end up with 200° inside the Tire, if the Ambient Temp were less, the added heat remaining the same, you'd end up with less overall temp.

3) Jim, in this example; "If you check your pressure Cold at, say 30°F and find them at 30 psi and, then again Cold at 70°F, your pressure would be 32 psi because.:

p2 = 30 * (70 + 459.69) / (30 + 459.69) = 32 psi, or a rise of 2 psi"

So if your tires we at 30 psi when it was 30 degress F outside, and then drove up to Duluth and it was -10 degrees F...the tire pressure would be 28?? My personal driving experience in the Midwest does not show this to be true. My tires would have lost more than just 2 psi!

The Formula works, check your math. Given the scenario you describe, you'd have 30*0.9183156 (489.69 / 449.69) which equals 27.549 psi.

Help me out!

Hi,

If you experienced a different drop in pressure, I suspect you were either mistaken, or some other variable was not considered such as a leaky valve or Rims (alloy Rims are notorious for leaking or seepage. This is a pretty widely used formula by Racing Teams and Tire Companies...

Happy Motoring!... Jim'99