"Anyone point me in the direction of a good explanation of tyre pressure and the right / wrongs !"
We have done this several times, if you search for "tyre pressures" you may find several useful threads.
Basically tyres get grip through three mechanisms.
The tyre itself has mechanical grip, the friction of rubber on tarmac and chemical or adhesive grip, where heat makes the rubber stickier.
The third mechanism is the size, shape and consistency of the contact patch on the road.
The shape of the contact patch is determined by the tyre pressure. Too high and tyre 'domes' and reduces the size of the patch, too low and the tyre wriggles and the patch may be distorted.
(Tyre pressure and rim size also dictates the way the tyre works. The sidewalls on a slick tyre are so soft that they merely hold the pressure in, not the kart up.)
Tyre temperature is controlled by the amount of work done to the tyre when driven and the amount of energy retained in the tyre as a result of that work. A smooth driver may make the tyre do less work and hence stay cooler.
As tyre temperature increases, the adhesive grip increases but also the tyre pressure increases. They tyre pressure increases in proportion to the temperature in Kelvin. (P1/T1= P2/T2)
So, if you go out with a pressure of 1 bar and a tyre Temperature of 10C, (283K) and the tyre temperature increases by 10 percent ( by 28K to 311K) then the tyre pressure will increase by 10 percent too.
Most kart tyres seem to get maximum grip around 60C (233K), but no-one admits to knowing the best temperature for any particular tyre.
So clearly a tyre has both a pressure at which it works best (contact patch and movement of the kart on the sidewalls) and a temeperature at which it works best.
The idea is to juggle the pressures so that the tyres both reach their best temperature and their best pressure. Really good drivers can alter their driving style to put more or less energy into the tyre to modify the temperature too.
The problem is reputed to be more difficult for cadets because the tyres are hard (so require more energy to heat up) but cadets are light so it's difficult to get the energy in. One juggles the gain in adhesive grip by a higher temperature on a small contact patch caused by high pressure against the gain in mechanical grip from having the tyres respond 'properly' at the correct pressures.
Apparently in F1 the manufacturers specify the tyre will perform at between 1 and 1.2 bar and a change of around 0.03 bar ( about 0.5 psi) can make the difference between an adequate and an excellent performance.
Given that a track temperature now may be around 7C and in the summer may get to 35C or more, you can see that pressures will get lower as the days get hotter.
You can also see that driver style, the track, and the speed or amount of traffic can have an effect. As someone else has pointed out, you may be getting consistency because the tyres just stop getting any warmer and the driver can rely on the tyres response.
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Here We Go Again : Tyre Pressures??? by 'Roblin' (23 Mar 2009 @ 12:24)
