Date: Tue, 29 Apr 2003 22:05:21 +0200 From: "MH" <nomailnopsamhere.no> Subject: Re: Tyre /Tire pressure at different temperatures
> Thanks for the reply - Yes I understand the thermodynamics of the=20 > pressure temperature relationship - I am not sure you really do.... ;-) > I am interested in whether a user=20 > in the Swedish arctic winter below freezing pumps their tyres to say=20 > 28psi and a user in the Arizona sun at 109deg F pumps their tyres to=20 > 34PSI=20 No, the outside temp at the moment you inflate the tyre is not relevant, = the only thing that counts is the pressure inside the tyre, indicated by = the pressure gauge.=20 However, if you are in a Swedish arctic winter and pressurise the tyres = and then take that same car to Furness Creek, Death Valley in July, the = pressure in the tyre will be higher there. How much higher? well, as CS indicated P * V / T =3D constant=20 (e.g. read about this Boyle / Gay-Lusac's law at http://www.math.montana.edu/frankw/ccp/before-calculus/function/boyle/bod= y.htm )=20 Applied to both locations - 1 =3D Sweden and 2 =3D Furnace Creek gives;=20 P1 * V1 / T1 =3D P2 * V2 / T2 example; P1 =3D tyre pressure in arctic Sweden =3D 28 psi V1 =3D volume of tyre (not relevant) T1 =3D absolute temp in arctic Sweden , -40 deg C =3D 273-40 =3D 233 K P2 =3D tyre pressure in Death Valley =3D ? V2 =3D V1 =3D Volume of tyre =3D same as in Sweden (don't care) T2 =3D absolute temp in Death Valley, say 70 deg C =3D 273+70 =3D 343 K that makes; 28 * V / 233 =3D P2 * V / 343=20 so P2 =3D 28 * 343 / 233 =3D 41.2 psi So, just arrived in Death Valley you find the pressure is way too high = and you let out some air until it is down again to 28 psi. Then you = drive the car back to arctic Sweden, there you will find that the = pressure has dropped to 28 * 233 / 343 =3D 19 psi ------------------------------------------- MH '72 97 '77 96 '78 95 '79 96 http://go.to/saab96
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