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post #9 of (permalink) Old 12-19-2007, 10:36 AM
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Originally Posted by curtis97322 View Post
I think the question is - if I filled the tires when it was 80 degrees out and want to ride when it's 40, do I have to adjust for the temperature difference?


While you should check your tire pressure before every ride, the pressure difference is negligable with the range of temperatures a sane person would ride in.
Quick chemistry lesson...

PV=NRT (T is in kelvins) - P is pressure, V is volume, N is number of molecules (in moles) and T is temperature in Kelvins. Simple algebra will change the formula to P=T/V

30 degrees farenheit = 272 kelvin
100 degrees farenheit = 310 kelvin

I don't feel like working the math, but at 32 psi that gives a pressure drop of about 4 psi (many gauges are only accurate to 2 or 3) and most people don't ride in that wide a temperature range.

All that to say - it's a good idea to check your tire pressure before each ride, but the change in pressure due to temperature is generally negligible. (and yes, the tires will heat up some during a ride making same difference even more negligible).

Ok... gotta run to work...
Using 100F @ 35psi, 10L for vol, .93 moles, calucating for 30F, all other variables remain the same yields 30.6 psi, that is a significant change, worth correcting.


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