Talk Morgan Forums Community The Malvern Arms tyre pressure loss

I run my tyres at 22 psi.
This weekend was quite a bit colder and i found all four tyres had dropped to 19.5 psi.
What is the thinking because there is clearly the same amount of air in them and when going along i assume the tyre temp will go up to working levels.
In the meanwhile i pumped them up.

Were we not paying attention in O Level physics, Paul?

Boyle's Law, P.V/T = Constant. As temperature drops, so does pressure in a constrained vessel (ie constant volume).

I find 22 psi a bit low myself. In any case the rule is to set the tyre pressure cold. Unless you are in a particularly cold zone I would imagine that the temperature difference between cold and "warm" running tyres would be relatively constant as the tyres would never achieve the same temperature that they would on a hot day.

Are you using the same pressure gauge that you checked the tyre pressure with recently.

I think a drop of two or three psi is usual in cold weather.

Tim has nailed it......................

P1xV1/T1=P2xV2/T2
Transposing.......
P1xV1xT2/T1xV2=P2
V1=V2 so cancel each other.
P2=( P1xT2)/T1
Reduce your variables by using same gauge.
Measure when cold not after a run so the ambient temperature is the variable.
You can then decide on cold/warm weather settings if you wish. F1 teams are very meticulous on pressures based on air/track temps but normal roadgoing is not critical.

Fun bit.......
Use Malvern air?

When I lived in Bavaria where -25 was a regular winter temp then a 5 psi plus drop was possible.

It is also true that reduced tyre pressure will give better grip in some circumstances. It is certainly better if you are stuck in sand and some say reduced pressure will give you better grip in snow. You need to be able to pump the tyres up again when you get on a clear road so it is not always a viable option.

It's the combined gas law not Boyle's Law that you are referring to. A 20C temperature drop would only reduce his tyre pressures by 1.5 psi but likely the pressure guage isnt wildly accurate anyway

It would appear that I wasn't paying sufficient attention in my O level physics lessons either.