ADUK Archive

I know we should all check the tyre pressures when the tyres are cold, but a few questions that occured to me over the weekend:-
i. What is the increase in pressure from cold to hot (after say a 20 mile drive)
ii. What difference does a change in ambient temp make to tyre pressures make when "cold) (e.g 5 degrees C to 20 degrees C) - I am sure my tyres incressed presure from being checked outside at 5 degrees to when I checked them at 15 degrees

My (renault) handbook says pressures should be increased by about 3psi if not cold.

It does make a difference, if I could remember my physics from almost 30 years ago, I could quote it. It does vary in direct proportion to the change in temperature IIRC.

I have seen pressures vary by 3-5psi when one side of the car has been sat in the sun. Also between late summer and winter, or winter-spring tyres seem to vary by 2-3psi, even though they otherwise do not change.

Renny wrote:It does make a difference, if I could remember my physics from almost 30 years ago, I could quote it. It does vary in direct proportion to the change in temperature IIRC.

I thought it was Boyle's law (my Physics is pretty rusty too!) but it was in fact http://en.wikipedia.org/wiki/Gay-Lussac%27s_Law

Chris

Renny wrote:It does make a difference, if I could remember my physics from almost 30 years ago, I could quote it. It does vary in direct proportion to the change in temperature IIRC.

I have seen pressures vary by 3-5psi when one side of the car has been sat in the sun. Also between late summer and winter, or winter-spring tyres seem to vary by 2-3psi, even though they otherwise do not change.

Tyre pressures might vary in proportion to the change in absolute temperature, but not the temperature as simply indicated by a thermometer.

The pressure is not going to double if the temperature goes from 10 deg. C to 20 deg. C.

Best wishes all,
Dave.

The pressure is not going to double if the temperature goes from 10 deg. C to 20 deg. C.

That's because the law works on the Kelvin scale.
If you checked your pressure in the summer, at 20 deg. C., and it was 26psi, and it was now freezing, the pressure would be about 24.2psi - assuming no other loss.

SLine wrote:

The pressure is not going to double if the temperature goes from 10 deg. C to 20 deg. C.

That's because the law works on the Kelvin scale.
If you checked your pressure in the summer, at 20 deg. C., and it was 26psi, and it was now freezing, the pressure would be about 24.2psi - assuming no other loss.

<grabs calculator>

273.16 / 293.16 x 26 = 24.226

OK, I won't argue.

Best wishes all,
Dave.

I knew someone would know the formula

I remembered it was Kelvin, but forgot the formula

Not forgetting, of course, that the tyre will have expanded with the heat too, so the volume will be bigger, and the pressure increase correspondingly less...

Mr Cholmondeley-Warner wrote:Not forgetting, of course, that the tyre will have expanded with the heat too, so the volume will be bigger, and the pressure increase correspondingly less...

I'd disagree...

The shape may change due to the increased pressure, but the volume will still be the same. That is unless it has structurally failed

You don't think materials expand as they get hotter?

Mr Cholmondeley-Warner wrote:You don't think materials expand as they get hotter?

Have you seen the steel/nylon belts round a tyre carcass? They can defeorm, but not expand (unless they have structurally failed). Either way the change in volume will be proportionatly small.

Renny wrote:

Mr Cholmondeley-Warner wrote:You don't think materials expand as they get hotter?

Have you seen the steel/nylon belts round a tyre carcass? They can defeorm, but not expand (unless they have structurally failed). Either way the change in volume will be proportionatly small.

OK, so you don't think steel expands as it gets hotter, either, then?

Yes of course it will be a small change (but then, so was the temperature induced pressure change)

Mr Cholmondeley-Warner wrote:

Renny wrote:

Mr Cholmondeley-Warner wrote:You don't think materials expand as they get hotter?

Have you seen the steel/nylon belts round a tyre carcass? They can defeorm, but not expand (unless they have structurally failed). Either way the change in volume will be proportionatly small.

OK, so you don't think steel expands as it gets hotter, either, then?

Yes of course it will be a small change (but then, so was the temperature induced pressure change)

I'm inclined to think the effect of volume change caused by expansion of the tyre casing will be much less than the effect of the temperature rise on the air itself.

Best wishes all,
Dave.

You're undoubtedly right, I was just pointing out that it's a contributory factor to the pressure change. Since we were getting all technical about gas laws and Kelvin temperatures, I thought it should be pointed out that the gas is not contained in the same volume all the time