The Michelin site refers to driving up a mountain for skiing so by very cold they are talking, say, -30° C. Boyle's Law indicates that a temperature change of 30° is going to have the effect of 10% decrease in pressure (30/300), i.e. approx 3psi.
BIG
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The Michelin site refers to driving up a mountain for skiing so by very cold they are talking say -30° C. Boyle's Law indicates that a temperature change of 30° is going to have the effect of 10% decrease in pressure (30/300) i.e. approx 3psi.
Surely that doesn't make a difference if you check the tyres when they are cold (ie. -30C)?
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Surely that doesn't make a difference if you check the tyres when they are cold (ie. -30C)?
I was thinking the same, so the answer is to check pressures regularly.
Or in this case, do as I say, not as I do.
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Boyle's Law indicates that a temperature change of 30° is going to have the effect of 10% decrease in pressure (30/300) i.e. approx 3psi.
Actually, Boyle's Law states that the volume of a given mass of gas at constant temperature is inversely proportional to its pressure, i.e. P x V is constant.
Charles's Law states that the volume of a given mass of gas at constant pressure is directly proportional to its absolute temperature (temperature in kelvins), i.e. V/T is constant.
Combine the two and you get PV/T is constant. Assuming V is constant then P/T is constant.
tinyurl.com/9xfc7l explains it in more detail.
Edited by L'escargot on 30/12/2008 at 13:21
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Should we not reduce the pressure slightly to get a better grip on icy roads?
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Should we not reduce the pressure slightly to get a better grip on icy roads?
Thats torn it, a whole new can of worms opened up now..;)
I checked my car tyres yesterday and found them to be about 3 to 4 psi down from 6 weeks ago (sshh, i really do check them weekly), can only put it down to the cold.
I use an old fashioned pen sliding type of checker, haven't found a good quality digital one yet, no doubt one of the BR'rs could point me in the right direction, reading the sliding scale always involves me and reading glasses falling on the floor x 4 x 2 cars = 8.;)
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Oh good, it's not just me then!
I have two of the pen type guages and one on my footpump. Not only do the tyres on the ( three ) cars all get checked every week but I have been known to check them with both guages and cross reference that with the reading on tthe footpump. They all, happily, give a very similar result. My oldest pen guage has been around forever and still works fine but you have to keep an eye on these things........
Also have one of those tread depth thingies which gets used a bit too often too. Have stopped short of buying a backup one of those. Screenwash checked weekly too. In fact just been to Halfords and bought another 15 litres of the stuff. It was on offer in my defence!
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Of course these things can be difficult to gauge. Oops !
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Oops !
Plumbing the depths in these deflationary times..
i'll get me coat..
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I always inflate my tyres to one lb/sq inch over the manufacturers recommended pressure.
I figured that tyre pressures will always drop with use - at least this way they're nearer to the recommended pressure for longer
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I always inflate my tyres to one lb/sq inch over the manufacturers recommended pressure.
Perhaps the manufacturer's recommended pressure allows for a certain amount of pressure drop?
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Perhaps the manufacturer's recommended pressure allows for a certain amount of pressure drop?
I supose the only way to tell if the pressures have been correct is at the end of the tyre's life and it's worn evenly.
Unfortunately it's too late then!
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Given the AMBIENT temperature range in this country we are talking a maximum maximum of about 30C in the summer and a maximum minimum of around -10C in the winter. [Yes, the winter temperature can feel colder with wind chill I know!]
So we have a range of about 40C to contend with.
If anyone has checked their cold tyres recently, the ambient temperature will have been anything from about 5C to 15C
Given that the lowest temperatures I've seen on the BBC weather charts so far is about -5 then the AMBIENT tyre temperature variation is going to be around 20C at most.
If Michelin reckon on a 3 psi drop at -30C then with the variation we've had so far I'd be surprised if anyone's tyre pressures are adrift by more than 1 or 2 psi which could be more than covered by the +/- accuracy of their tyre pressure gauge.
So, yes, check your tyre pressures, but there's really no need to re-check if the temperature suddenly drops by 10C.
As for running lower pressures on icy roads - no, don't do that.
The best tyres for icy roads are those with a very different rubber compound which remains much more flexible at very low temperatures and with a tread that has a lot of sipes [very small gooves] that grip better - Vredestein are renowned for winter tyres, but there are others made by Michelin and other tyre companies for this purpose.
And if you need to cope with snow as well, then you fit narrower wheels and tyres as wide low profiles can be next to useless as they snowplough resulting in you going straight on instead of turning when you want to!
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For any given tyre there is a unique & specific amount of air loss due to manufacturing tolerances etc. Hence the 4 tyres on a car that are exactly measured and have equal pressure can and do lose pressure at differing rates.
The most significant times for sudden tyre pressure drop is from summer to the first cool burst in autumn and then again a cold winter snap as we have now, pressure drops due the volume of air inside a tyre contracting. Tyre pressures then increase when warmer weather arrives in spring & summer due to the air inside the tyre expanding.
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Boyle's Law indicates that a temperature change of 30° is going to have the effect of 10% decrease
Boyle is pressure and volume. Gay-Lussac is the man for pressure and temperature. The pressure is proportional to the temperature in (absolute) Kelvin, so a 30 degree change will be roughly 10% as BIG suggests.
The pressure though should also be measured in absolute terms, so a typical 30psi is really nearer 45psi absolute assuming that what tyre gauges actually measure is the differential pressure, not absolute pressure. So 10% reduction in degrees K would drop the pressure in a tyre inflated to 30psi by 4.5 psi.
My observations fit this theory pretty well. (I'm fussy about tyre pressures when towing and check them frequently).
In practice this is a bit ideal - volume will change a bit, and the actual difference in running temperature might be less than the difference in ambient.
E&OE - it's 40 years since I did physics, and I don't think we did tyres ;-)
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Well, for a 20 degree fall in temperature I make the pressure drop to be 7%. I can't see why you have to use absolute pressure though, it should work regardless.
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I can't see why you have to use absolute pressure though
Think about it a bit more David!
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