Car tyres are not just cushions for the wheels. They are there to give the car a good grip on slippery roads, and stop it sliding about when braking or cornering.
The tread pattern running all round the tyre has thin cuts (known as sipes) in the rubber to sponge up surface water, and zigzag channels to pump the water out behind as the car rolls forward. On a wet road, a tyre has to move more than 1 gallon (5 litres) of water a second to give an adequate grip.
On a perfectly dry road, the treads are not needed. A smooth tyre gives the greatest possible area of contact with the road. But if the smooth tyres are used in wet weather, the film of water on the road builds up in front of them and underneath them and actually lifts them and off the road surface – this is known as aquaplaning. When aquaplaning occurs, the driver loses control.
Most cars have to function in all weathers, so must have tyre treads, but racing cars make comparatively few outings a year. If the track is dry, they run on smooth tyres, called slicks, to get the best grip on the roads. The extra wide tyres and wheels give more grip that the average cars. In wet weather, however, the slicks have to be changed for treaded tyres.
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