Tyre Pressures

[corgi]

(stupid question time)

Won't actually driving change the temperature of the tyre anyway and change the resultant pressure anyway? Maybe only a few percentage points, but roughly the same amount you are debating here?

(/stupid question time)

Not stupid at all...

Yes, as the tyre heats up the pressure will rise... for a given (fixed) volume the pressure will rise in proportion to the rise in (absolute) temperature... The temperature increases owing to energy exerted on the tyre, most of the temperature increase come from sidewall flex...

[corgi]

This is the logic underlying the use of nitrogen for filling tyres, isn't  it?

Allegedly, although air is approx 75% nitrogen... the main issue with air is water vapour...

[fredgroves]

(stupid question time)

Won't actually driving change the temperature of the tyre anyway and change the resultant pressure anyway? Maybe only a few percentage points, but roughly the same amount you are debating here?

(/stupid question time)

Not stupid at all...

Yes, as the tyre heats up the pressure will rise... for a given (fixed) volume the pressure will rise in proportion to the rise in (absolute) temperature... The temperature increases owing to energy exerted on the tyre, most of the temperature increase come from sidewall flex...

So given two different ambient starting temperatures (and the vehicle having been left over night) say 10 degrees and 20 degrees you would expect to see two different tyre pressure readings even if previously they had been filled the same and you hadn't lost any air?

If you then went out and thrashed around, would the tyres actually get to the same temperature anyway and therefore wouldn't the effective pressure and the resultant contact patch (which is what we are trying to achieve) be the same (roughly)??

[mcmaddy]

Tpms doesn't work on our cars like the electronic tpms. If all 4 tyres either lose roughly the same psi or gain roughly the same psi through temperature or driving it won't tell you on the MFD. It only checks through the abs sensor the rotational speed of the tyres. Useless unless it's for punctures which is what it's really designed for 

[itavaltalainen]

You have more fluctuation in tyre (and air in tyre) temperature from driving than a mere 10 degrees in outside temperature (think not just heat from tyre wall flexing, friction from road surface, BRAKES).

The other thing is that the tyre is not a vessel with constant volume, i.e. the volume will change with pressure, temperature.... and even more variables (centrifugal force, lateral forces, ....).

I used to live in Finland and even from -20 to +30 you would see little change in pressures (if you bothered to check the pair not in use).