Date: Tue, 29 Apr 2003 12:12:56 +0100 From: "Andy Hookins" <andrew.hookinsnopsamamnopsamodata.co.uk> Subject: Re: Tyre /Tire pressure at different temperatures
The physics are pretty complex, the compliance of the tyre's sidewalls will change with temperature, 'sports' tyres will have stiffer sidewalls than 'comfort' tyres etc.. But basically you can consider the tyre to be a spring with a mass - it is part of the the whole damped suspension system. The spring rate is affected by a number of factors, but it will be pretty much fixed for a given air pressure. Of course the air mass within the tyre for a given pressure will change with temp. - but this is insignficant compared to the mass of the tyre, rim and all the other 'unsprung' (wrt to the coil springs) components. As you drive then the tyres will heat up due to friction (rolling resistance) but 'hot/cold' temperature difference will be about the same regardless of whether you are in a hot or cold climate since the heat loss mechanism is the same. Since we exist at a much warmer temperature than absolute temperature then the 'hot/cold' pressure differential will also be about the same in hot or cold terrestrial climates. My interpretation is that you aim to maintain the 'cold' tyre pressure regardless of whatever the prevailing ambient temperature may be. AndyH '94 9000 CDE "Derek Wright" <nospam.drwrighnopsamlobal.net> wrote in message news:PzFBjCTkhOvp-pn2-P7ZEPKdKvUjznopsamlhost... > 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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