"whats the correct way to set tyre pressures?"
The correct way to set tyre pressures is to calculate what the effect is going to be when they are running on the track.
The tyre manufacturer sets a 'best pressure' and 'best temperature' for each type of tyre because the best pressure means teh tyre is a certain shape, maximising 'mechanical grip' and the best temperature maximises 'chemical grip'.
So, if the best figures are 1.2 bar and 63 Celsius, then you need to know how hot the tyres are going to get from your driving.
Fortunately there is a connection between tyre heat and tyre pressure, the cold temperature divided by the cold pressure is the same as the hot temperature divided by the hot pressure (wher the temperatures are measured in Kelvin ....Celsius+273).
So if before you go out the tyre temperature is 10C (283K) and 1 bar and when you come in the pressure is 1.2bar (an increase of 20%) then the temperature will have increased by 20% too (283 *20%=56.6 degrees)to a temperature of 66.6 C. nearly the ideal.
(obviously you have to measure the pressure as soon as you stop, otherwise the tyres cool off and drop pressure).
If you come in and the pressure is only 1.15 bar, (ie 4% low) then you needed to increase the original tyre pressure by 4%. (ie from 1 to 1.04 bar)
(or from 14.5 psi to 15.08 psi)
(But just to be complicated, you need to adjust for the difference between the original tyre temperature and the current one, which is why it is better to wait a bit until the tyre has cooled down before putting the new air in. Always assuming that the new air is the same temperature as the air you put in originally. (eg: if you put air in at dawn from a cold tank, it's different from air that has been cooking in 25degree heat all day.)
Itpro's rule of thumb solution works because the average increase to a best temperature for the average driver and the average cold tyre and average cold pressure is about 0.2 bar (3 psi).
If your cold tyres run at 30psi (2 bar) then the figures will be different, but the principle the same.
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tyre pressures .set hot or cold ? by 'johnyboy' (19 Apr 2011 @ 19:44)
