The force required to cause initial movement against friction is greater than the force required to maintain uniform motion. The sliding force of friction is directly proportional to the force pressing the surfaces together – this force is slightly less than the static friction force. Belt drive systems rely on friction between the belt and the pulley for their ability to transmit power. The friction force derives from the force exerted on the pulleys by the difference in tension between the tight and slack sides of the belt, T₁ – T₂. Tyres rely on friction between the tyre tread and the driving surface to provide the grip for driving purposes. The coefficient of friction for tyres is affected by the rubber compounds used in the tyre construction, the tread pattern, the tyre dimensions the type and condition of the driving surface and the speed of the vehicle.