How does the tread affect the grip of the tire?

Answer 1

it affects it greatly. For racing, the best tyres to use are called "slicks". This means that there is no tread on the tyre. The more tyre you have, the more grip you have. Road cars, however, need to have tread because the tread helps to grip in gravel and water. The tyre for normal road use is also designed so that it can get rid of the water that gets caught in the tread. As well, inflation of the tyre affects the surface area if under-inflated; the tread area collapses, even if you don't see much change with the naked eye. It makes a "mushy" less effective grip. If over-inflated, the tread has more pressure exerted on it from within, which widens the tread and lessens the grip on road surfaces.