Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the motion when a body (such as a ball, tire, or wheel) rolls on a surface. The torque to overcome this resistance and maintain steady speed on level ground (with no air resistance) is just this resistance multiplied by the radius of the wheel.
It is mainly caused by non-elastic effects, that is, not all the energy that is needed for deformation (or movement) of the wheel, roadbed, etc. is recovered when the pressure is removed. Two forms of this are hysteresis losses, see below, and permanent (plastic) deformation of the object or the surface (e.g. soil). Another cause of rolling resistance lies in the slippage between the wheel and the surface, which dissipates energy. Note that only the last one of these effects involves friction, therefore the name "rolling friction" is to some extent a misnomer.
In analogy with sliding friction, rolling resistance is often expressed as a coefficient times the normal force. This coefficient of rolling resistance is generally much smaller than the coefficient of sliding friction.
Any coasting 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. Factors that contribute to rolling resistance are the (amount of) deformation of the wheels, the deformation of the roadbed surface, and movement below the surface. Additional contributing factors include wheel diameter, speed load on wheel, surface adhesion, sliding, 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 surface it runs on. For example, a rubber tire will have higher rolling resistance on a paved road than a steel railroad wheel on a steel rail. But if one were to drive a steel wheeled vehicle on a paved road it would likely have more resistance than a rubber tire would. Also, sand on the ground will give more rolling resistance than concrete.
Read more about Rolling Resistance: Primary Cause, "Rolling Resistance" Has Different Definitions, Rolling Resistance Coefficient, Measurement, Physical Formulas, Rolling Resistance Coefficient Examples, Sound Effects, Factors That Contribute in Tires, Railroads: Components of Rolling Resistance, Comparing Rolling Resistance of Highway Vehicles and Trains
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