Due to internal damping of the tire material, not all of the energy used to deform the tire as it rolls is recovered when the tread elements exit the contact patch and return to their original shape. Also, the distribution of the normal load on the tire becomes asymmetric when the tire is rotating. This is because the spring elements are not purely elastic but instead have viscous dissipation which prevents the force used to compress the springs from being recovered in the rear half of the contact patch [8]. This shifts the normal load distribution towards the forward portion of the contact patch. The loss of energy can be represented as a force on the tires which opposes the motion of the vehicle. This force, called the tire rolling resistance, can be represented by