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MG MGF Technical - tyre pressure

hi all i know this sounds silly checked my tyres last night it was showing 24 psi should have been 26.
and the rear was 26 not 28. can you lose a slight air when they are cold or is it me
can the valves be checked #

thanks
darren

Hi,

Well, you don't lose air because it's cold, but the pressure does drop, as in all gases, relative to the absolute temp. I can't remember all the figures, but it ends up at about 1.6 psi per 10 deg C (I think). Whether your pressure loss is due to temp all depends on what the temp was when you last checked the pressure.

Can the valves be checked? Yes, take off the cap and squirt some water with a little washing up liquid in it on 'em, and look for bubbles. It's better when the valves are positioned between 4 and 8 o'clock. It's all hi tech here.

Regards, Kes.
Kes

Hi Darren - not a silly question at all. Trying to remember my physics from long ago - the relationship between the volume, pressure and temperature of a gas is governed by Boyle's law. Applying this to an air filled tyre - the volume of gas inside the tyre is fixed, so the only things that can vary are the pressure and temperature. If the temperature decreases then the gas will want to contract and occupy a smaller volume, but the volume is fixed (the tyre) and so the only variable that can alter is the pressure, which will reduce... and vice versa.

Don't we apply the same thinking to the F's hydrolastic suspension when we note that the ride height can be reduced when it's cold.

Don't know about checking the valves (I'm theoretical rather than practical), but it seems strange that the pressure drop is a consistent 2 psi in all tyres. If the valves were going wouldn't you expect a less consistent pressure drop

Bill.
wjf farnsworth

Did you use the same pressure guage each time? Garage airlines are supposed to be checked and certified but vary by a few psi from garage to garage and from day to day.
JohnP

Bill:
Boyle's Law - p x V = constant
Charles' Law - V / T = constant
If volume is constant, the one you want is the
Pressure-Temperature Law - p / T = constant
But rubber is an elastic material with quite a high coefficient of thermal expansion so it'll get complicated but you probably need
the Universal Gas Law - p x V /T = constant.
Steve Madden

Steve, thanks for the revision - it's 38 years since I studied physics (A-level failed)!!!

Bill.
wjf farnsworth

This thread was discussed between 27/01/2005 and 28/01/2005

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