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Date: Tue, 29 Apr 2003 09:39:11 +0000 (UTC)
From: "Derek Wright" <nospam.drwrighnopsamlobal.net>
Subject: Re: Tyre /Tire pressure at different temperatures


Thanks for the reply - Yes I understand the thermodynamics of the pressure temperature relationship - I am interested in whether a user in the Swedish arctic winter below freezing pumps their tyres to say 28psi and a user in the Arizona sun at 109deg F pumps their tyres to 34PSI On Mon, 28 Apr 2003 21:42:50 UTC, "charles stoyer" <interpexnopsamhlink.net> wrote: > I always assumed that "cold" meant ambient temperature; that is, tires not > heated from running (or from sun). > > PV=nRT, where T is in absolute, where 0 is as cold as it gets. So for a > tire, P/T is constant unless you add or remove air or change the size of the > tire. Thus, P1/T1=P2/T2 or P2/P1=T2/T1. Absolute zero is roughly -460F > or -273C > > So 68F is about 528R (R is "Rankine", which is an absolute temp scale with > degrees the same size as F), so that a change in 10F which is also 10R would > change the temperature ratio by about 1/53 or about 2%. > > So at sea level, a tire with 30 psi in it actually has about 45 psi because > the atmospheric pressure is about 15 psi. a 2% increase is about 1 psi. > > For Celsius/Kelvin temps, 20C is 293 K. > > Charles. > > "Derek Wright" <nospam.drwrighnopsamlobal.net> wrote in message > news:PzFBjCTkhOvp-pn2-dBSbHh6hZeNInopsamlhost... > > Reading the manual for the Saab it states that the tyre pressure for > > cold tyres is 32 psi (for my combination of tyres and wheels) > > > > The temperature of the cold tyre being 20C or 68 F, it also states > > that the pressure of the tyre will change by 2psi (or 0.1 bar) for > > each increase or decrease of 10F > > > > The question is - do you take this into account when checking the > > pressure of the tyres in excessevily hot or cold temperatures. > > > > -- -- Derek Wright drwrighnopsamlobal.net In the UK

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