Talk Morgan Forums Morgan Sports Cars Morgan Aero 8, 2012< Plus 8, SuperSports, Coupe & Aeromax Max Wheels and Tyre Pressure

Tarka has sulked in the Garage since early November...a combination of me travelling and foul weather.
When put to rest the tyres were all 23psi.
I checked today and they were all 1 to 1.5psi down.
I wonder, is this typical?

Anyway, all now back at 23, fluids checked and Optimate disconnected.
All I need is a few dry days, I'll do without the sun!

Difference in temp?

+1 - temperature drops and pressure drops.

Boyle's Law and Charles' Law.

That is right. It is often said that you shouldn't inflate odd tyres on a car when the tyres are warm.

makes sense.

Does anybody know how tyre pressures set at altitude (say 2000 to 2500 metres) change on descent to sea level (or vice-versa)?

It depends on where and how you set the tyre pressure, and what the temperature difference is. Most pressure gauges measure difference to ambient, not absolute pressure.

If you set the pressure at 2 bar at 2500m, this mean the pressure in the tyre is 2 atmospheres above the ambient pressure at that height, which is around 11psi, so you'll have 22psi in your tyres. Set it at 2 bar at sea level, and you'll have 28psi. If your gauge is calibrated to sea level (which it almost certainly will be)and you use it at 2500m, you'll think you have put in 28 psi but actually you'll only have 22psi.

Come down to sea level and (excluding the effects of temperature) you will still have 22psi in your tyres and the gauge will show this, but of course ambient air pressure will now be 14 psi so if you want 2 bar in your tyres you'll need to pump them up to 28psi.

But since PV/T = Constant, if the temperature has gone up, then so will the pressure, since car tyres can be considered to be reasonably inelastic.

So if you've gone from say 0C to say +10C as you come down the mountain, your tyre pressures will have increased by approx 2psi, partially offsetting the decrease - at least, that's what I always understood, pressure increases around 1psi for every 5C increase in temp.

I'm no expert.............

Boyle's law P1V1=P2V2 means that increased volume = decreased pressure.

But logically, 24psi is 24psi whether set at sea level or at altitude. But the thing is - 24psi set at sea level will be less at altitude according to Boyle's law. And vice versa.

So the pressure will decrease as the volume inside the tyre increases as atmospheric pressure decreases with altitude.

So in the same way that a weather balloon expands with altitude until it pops, I would expect a tyre to increase in volume with altitude.

Set your spare pressure at sea level, try to keep the spare at the same temp and recheck the pressure at altitude. Or vice versa. I don't think it will make much difference.

Warning! - this might be rubbish.

I think you might be overestimating the elasticity of a car tyre, Dave

A watched kettle never Boyle's Law.

Watt?