"What does chnaging the pressure actually do? "
If you use teh search function to look back through the 'tyre' threads, you will find it has been discussed a lot.
Essentially the tyre works by two mechanisms controlled by pressure, the shape of the tyre and the temperature of the tyre.
The shape: The tyre is designed to put a certain contact patch on the ground. If you over inflate the tyre, the contact patch is smaller and you get less grip, if you underinflate the tyre it deforms, in the worst case the sidewall collapses and rolls the tyre off the rim.
So the tyre works best in a range of pressures that maintain the contact patch.
At the same time the tyre gets hot. F1 tyres get to 100C, enough to boil water, Kart tyres seem to operate about 60C, the temperature of the water in your central heating. Slick tyres get hot because of road friction, wet tyres by bending the rubber in the sidewalls and tread.
However, as the temperature goes up, so does the pressure in the tyre. So adjusting the pressure is a juggle to end up with a hot tyre that is still in the right pressure range for best tyre performance.
It can be a juggle because:
You need to know the temperature before you go on the track (so filling the tyres just before you go on track is a disaster). Putting air in the tyres may either heat the tyres or cool them. (If you use a pump or compressor, it heats them, if you use an air reservoir it cools them) Tyres are sensitive to the amount of water in the air in your tyre. Wet Air can effectively act like steam. Tyres are sensitive to the way you drive. If you drive smoothly your tyres stay cooler than if you are jerky. Tyres are affected by the track (hot or cold).
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