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