Rolling friction

Rolling resistance, sometimes called rolling friction or rolling drag, is the resistance that occurs when a round object such as a ball or tire rolls on a flat surface. It is caused mainly by the deformation of the object, the deformation of the surface, or both. Additional contributing factors include wheel radius, forward speed, surface adhesion, and relative micro-sliding between the surfaces of contact. It depends very much on the material of the wheel or tire and the sort of ground.

For example, rubber will give a bigger rolling resistance than steel. Also, sand on the ground will give more rolling resistance than concrete. A moving wheeled vehicle will gradually slow down due to rolling resistance including that of the bearings, but a train car with steel wheels running on steel rails will roll farther than a bus of the same mass with rubber tires running on tarmac. The coefficient of rolling resistance is generally much smaller for tires or balls than the coefficient of sliding friction.

materials that flex more and bounce back slowly, such as rubber, exhibit more rolling resistance than materials that flex less, such as steel, or that bounce back more quickly, such as silica. Low rolling resistance tires typically incorporate silica in place of carbon black in their tread compounds to reduce low-frequency hysteresis without compromising traction. Several factors affect the magnitude of rolling resistance a tire generates:

• Material - different fillers and polymers in tire composition can improve traction while reducing hysteresis. The replacement of some carbon black with higher-priced silica - silane is one common way of reducing rolling resistance.
• Over inflating tires (such a bicycle tires) may not lower the overall rolling resistance as the tire may skip and hop over the road surface. Traction is sacrificed, and overall rolling friction may not be reduced as the wheel rotational speed changes and slippage increases.