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Originally posted by zefarelly
same for the CR65 Dunlops for historic saloons, I've overheard talk of upping pressures in the wet to open the tread . . . . .in short theyre **** tyres (by todays standards) wet or dry !
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I've heard the same - seems that it depends where you are from:
American practice is to up the pressure, in the wet, to open out the tread.
European practice is to lower the pressure, in the wet, to get the temperature up.
Having noticed that Americans tend to avoid racing in the wet whenever possible, I would think twice about taking their advice on wet racing.
Lowering the pressure ups the temperature due to side wall flexing. But you are losing the temperature increase you would gain from increasing the pressure (as governed by PV/T = constant gas equation).
The Temperature in question is absolute (e.g. Centigrade plus 273) so you need a fairly large change in pressure to make a large change in temperature, assuming the volume is constant. Which suggests that flexure is more significant than pressure.
Back to the point:
We've found that the simplest solution is to adopt the mid point of both theories - e.g. run with the same pressure in wet & dry.
Just set the pressure so that you get an even contact patch (e.g. maximum contact/grip) across the tyre (where possible, some things like Bugatti suspension geometry makes that difficult!), and keep to that pressure wet or dry.
The advanced version of that is to check the pressures when the tyres are hot and to stick to that one - there will be a tiny variation due to ambient temperature!
In the end with historic racing it doesn't make a lot of difference - changing driver will have a far greater affect on speed than the tyre pressure!
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I thought the main reason for using N was to reduce the other gases in the tyre, especially water and CO2. However it doesn't eliminate the fact that the temperature and the pressure are always proportional.
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I was told that Nitrogen had less water (none hopefully) and it is in fact the expansion/contraction of the water (moisture) in air that causes tyre pressure to vary with temperature (e.g. the volume increases but since that is fixed the pressure goes up).
The expansion of air or Nitrogen themselves is relatively insignificant in the temperature range we are talking about.
My local tyre place have started offering Nitrogen for road car tyres, should avoid the need to adjust the pressure of your tyres between winter & summer.