Tires serve as cushions for the wheels of automobiles. They give the car a good grip on slippery roads, and stop the car from sliding about when breaking or cornering. Except for racing cars, most of the tires we see have tread patterns running around the tire. These treads are the thin zigzag channels that cut around the rubber. The treads sponge up surface water and pup the water out behind as the car rolls forward. On a wet road, a tire has to move more than 5 liters of water a second to give an adequate grip.
Most cars have to function in all weathers, so their tires must be treaded. On a perfectly dry road, however, the treads are actually not needed.
Racing cars are used only during races, and these events are usually held during a dry season. That's why they don't need tires with treads. The smooth tire of a racing car ( also called a slick) gives the car a maximum possible area of contact with the road. If racing cars will have to run on wet roads, then the slicks are replaced with treaded tires.
