Setting Cold Pressures - Soaking Tyres in the Sun

[defuser]

Hi Guys,

I am a bit confused as to the correct order of setting cold tyre pressures for racing (GT) which involves soaking tyres under the sun.

So once I get the tyres from the tyre manufacturer, do I...

a) Bleed them to my desired cold pressure, soak them under the sun, and then bleed again to my desired cold pressure and then put them on the car and run

or

b) directly soak them under the sun, and then bleed them (only once) to my desired cold pressures before putting them on the car to run ?

Thanking in advance !

[terryobeirne]

this sounds like an exercise in silliness. does the sun ever come out in some places??

your chosen tyre supplier will suggest optimum operational temps and possibly pressures. you need to experiment to get that.

most of us build up a database for each car and driver and quickly work out what cold starting pressures will get us to where we want to be within a few laps. you always have to consider track temperatures which of course are a function of the season. we race from Zero to 40C ambient temp and track temps can get well above 40 and very occasionally into the 70's

not many race categories permit tyre warmers , but of course, putting them in the sun will have some effect, albeit small.

[ydd]

@defuser - appreciate your question is a few months old but maybe you or others are still struggling with setting your tyre pressures?

The issue can be that even though you know the hot pressure target i.e. the pressure you want to be running out on track, you can only set tyre pressures cold.

In your case it looks like you are also struggling with the real world problem of the sun warming up the tyres and thus changing the pressures over when you set them cold. Unfortunately this can cause confusion because your references are now all over the place - for example, if you have worked out a cold pressure to track delta for each tyre, and the tyres have been sat in the sun with a different temperature, will the delta be the same or less? If less by how much? Etc.

Tyre pressures are so fundamental to car balance and performance yet so many people basically guess them - even if it is an educated guess.

What would be better would be to have absolute certainty around setting your tyre pressures. So that you were in complete control. That you knew exactly how to achieve whatever tyre pressure you wanted at each wheel, when ever you were out on track.

One way to approach this is to use a scaling factor and then correct for ambient and track temp variations (rule of thumb being 0.1 psi / degree).

If you've not heard of the scaling factor method, then I've written it out in some detail on my blog (link below), but in effect you are using a ratio of hot to cold pressures, to then very accurately establish a method for precisely setting your cold pressures.

It is super simple once you start doing it but I don't hear of many people approaching things this way so thought I'd share.

Best of luck

https://www.yourdatadriven.com/how-t...ly-every-time/

[ydd]

Originally Posted by defuser

Hi Guys,

I am a bit confused as to the correct order of setting cold tyre pressures for racing (GT) which involves soaking tyres under the sun.

So once I get the tyres from the tyre manufacturer, do I...

a) Bleed them to my desired cold pressure, soak them under the sun, and then bleed again to my desired cold pressure and then put them on the car and run

or

b) directly soak them under the sun, and then bleed them (only once) to my desired cold pressures before putting them on the car to run ?

Thanking in advance !

I wrote quite a long reply to this and linked it to an article on my blog, but as I'm new here I don't think I am aloud to link yet!

In essence, what I think you are after is to be able to know for sure that the pressures you set beforehand will enable the car to achieve perfect hot tyre pressures when out on track?

The issue is that the sun soak heats the tyre in an unpredictable way. Any change in ambient and track temperature will also have some effect here.

You therefore need to establish a solid before and after status immediately before and then status after the car has been on track.

Then I'd suggest using a combination of a scaling factor approach, plus a pressure delta, to correct things if they are not where you want them to be.

How much does the sun soak affect the cold pressures?