Winter tyre pressures

[arcamalpha]

Hi all,

I have a query about setting correct tyre pressures when the temperature is below 20C ambient.

My manual says the pressure should be 31psi at 20C. So what should I set them at now when it's ~15C?

Thanks :-)

[Magic919]

30.47 psi.

[arcamalpha]

Not quite sure if your response is tongue in cheek or not!!

Here's my thinking, and sorry if I'm over-analysing.

Tire pressure is required to maintain the right shape of the tyre. The shape of the tyre is most important when the tyres are at operating temperature, so tyre pressure should ideally be set at operating temperature. I guess the reason manufacturers give a pressure at 20C is because most people don't want to check tyre pressure at operating temp.

So at lower ambient temperatures and at any given tyre operating temperature, the pressure is also lower. Therefore the tyre is the wrong shape. If that theory is correct, the pressure actually needs increasing a touch at lower ambient temps in order to maintain the right pressure at operating temps.

Some facts: My tyres go from 31psi at 20C ambient to about 29.5psi at 12C ambient without changing anything. On a decent run, at 20C ambient the tyre pressure gets up to about 34-36 whilst they only get up to about 32psi. That also suggests I need to add more air in winter.

I'm I right or going mad?!

[sy247]

I just leave mine at 44/36 all year round

[Trev450]

I think you are over analysing a bit I would either just leave them alone or better still get yourself some winter tyres.

[anonymous-user]

Trev450 said:

I think you are over analysing a bit I would either just leave them alone or better still get yourself some winter tyres.

+1.

[HPandSpam]

For my first post perhaps I can offer a qualified opinion.

I'm living in Finland, last winter we got down to -32C.
I have a 993 C4 on 17" wheels in summer and 16" wheels with studded tyres in winter.
I inflate to the Porsche recommended pressure for 16" wheels which I guess is a standard year round pressure. Car handles perfectly.
Cheers.

[911Thrasher]

HPandSpam said:

For my first post perhaps I can offer a qualified opinion.

I'm living in Finland, last winter we got down to -32C.
I have a 993 C4 on 17" wheels in summer and 16" wheels with studded tyres in winter.
I inflate to the Porsche recommended pressure for 16" wheels which I guess is a standard year round pressure. Car handles perfectly.
Cheers.

+1 during my years living in Canada, I never over inflated the pressure on my winter/snow tyres because it was much colder.
And actually because it is colder, the same amount of air in the tyre would result in slightly lower pressure in the tyre, hence a flatter surface of contact but also increased surface of contact...hence more friction/grip

[Pope]

A thought: The reduction of ambient pressure that is associated with lower temperature would mean the same forces act on the tyre just at correspondingly lower measured figures..........


















Or is it just too late to be on PH?

[thegoose]

Given the effort to get hot pressures right when racing I'd say it's important to concentrate on the pressure when at operating temp.
Before first going out on track you have to use your best guess at cold pressures, which comes from experience of running the car (tyre type/set-up etc) but if you're parked so that one side of the car is in the sun and the other is in the shade you have to take this into account too.

If you normally get to 34 then aim for this year round, which will indeed mean increasing cold pressures, perhaps to be the same at 12C as the normal 20C setting.

[arcamalpha]

Cheers guys.

Having thought about this a bit more (and asking some proper physicists!) I was forgetting that volume also plays a part in this (as pV=nRT). That's why it takes more air to achieve a given pressure at lower ambient temps.

So short answer appears to be that pressure (for hot or cold tyres) needs to be the same all year round. Just that when ambient temps are lower, air little air is needed to achieve that same pressure.

[thegoose]

arcamalpha said:

Cheers guys.
So short answer appears to be that pressure (for hot or cold tyres) needs to be the same all year round. Just that when ambient temps are lower, air little air is needed to achieve that same pressure.

So that means that you must adjust the pressures at different times of year - such that at 12C ambient you will set them to the same cold pressure as at 20C ambient, just as you thought?

[arcamalpha]

thegoose said:

So that means that you must adjust the pressures at different times of year - such that at 12C ambient you will set them to the same cold pressure as at 20C ambient, just as you thought?

Yup - that's it.

I think you're earlier post was spot on - the operating pressures are what's key. For me, that's 33/36psi F/R.

[LeoSayer]

thegoose said:

So that means that you must adjust the pressures at different times of year - such that at 12C ambient you will set them to the same cold pressure as at 20C ambient, just as you thought?

If that's the case then why does the manual bother to mention 20c?

[chris7676]

Magic919 said:

30.47 psi.

Quote of the day

[Rockster]

arcamalpha said:

Hi all,

I have a query about setting correct tyre pressures when the temperature is below 20C ambient.

My manual says the pressure should be 31psi at 20C. So what should I set them at now when it's ~15C?

Thanks :-)

There is no temp and pressure callout for the tire pressures for either of my Porsches other than the tire pressures are given for a 'cold' tire.

So I just make sure that tire is cold (at ambient) temperature when setting the tire pressure.

This has worked just fine for my from near 0F to over 100F over the 9+ years I've owned Porsches.

Sincerely,

Rockster.