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From: fred@varian.UUCP (Fred Klink)
Newsgroups: net.bicycle
Subject: Re: Tire Pressure, etc.....
Message-ID: <369@varian.UUCP>
Date: Mon, 16-Sep-85 19:44:29 EDT
Article-I.D.: varian.369
Posted: Mon Sep 16 19:44:29 1985
Date-Received: Sun, 22-Sep-85 05:38:25 EDT
References: <5490003@acf4.UUCP>
Organization: Varian, Walnut Creek, CA
Lines: 28

> Also I have heard that in hot weather or excessive braking one should lower
> the pressure in their tires because of the air expansion in the tube do to
> heat.  Is this significant?  How much can the pressure increase due to
> air temperature or braking?  Are there any valid formulas for determining
> the increase in pressure?

Gases behave such that pv/T is constant, where p= pressure, v= volume,
T= absolute temperature (degrees K=degrees C + 273).  This is idealized, but its
close enough for this use.  Therefore p(1) v(1)/ T(1) = p(2) v (2)/ T(2).
Plugging in p(1)= 7 atm (~100 psi), T(1)= 294 K (70 F), T(2)= 311 K (100 F)
and v(1)= v(2) we get p(2)= 7.4 atm or about 104 psi.  So, if you inflate
your tires to 100 psi at 70, they will rise to about 104 psi at 100 degrees F.
Be sure to use absolute (Kelvin) temperature or the result is way off.
In any case, the rise is insignificant.  I have never worked this out before,
but I suspected it from my experience riding around here (hot part of SF
Bay Area) in summmer.

As for excessive braking, I know some people who have blown tires this way.
Given the equation above, the only explanation for this is a
result of actual heating of the rim metal and 
transfer of this heat to the inner tube where it contacts the rim metal.
The localized heating of the tube causes it to weaken and burst
rather than an overall increase in the tire's air pressure.
This should be less of a problem for sew-ups since the inner tube is 
completely isolated from the rim metal by the tire casing and rim tape.

						Fred Klink
						Varian, Walnut Creek


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