During the tire motion new rubber material enter continously into the contact zone and is deformed,to spring back to its initial shape when leaving it .To produce this deformation it is necessary to spend some energy which is not completely recovered at the end of contact zone due to the internal damping of the rubber material .
This energy dissipation is what causes rolling resistance. It is then clear that it increases with increasing deformation and mainly with decreasing elastic return.The distribution of contact pressure which at standstill was symmetrical with respect to the centre of the contact zone , become unsymmetrical when the tire is rolling and the resultant Fz move forword (Pic.1) producing a torque My = –Fz*dx whith respect to the rotation axis. Rolling resistance is due to this torque . For practical purposes , rolling resistance is usually expressed as
Fr = f*Fz
where the rolling resistance coefficient f must be determined experimentally. Coefficient f depends on many parameter as the tire travelling speed V , inflation pressure p , vertical force Fz ,the size of tire and contact zone ,the structure and rubber material of tire,working temperature and road conditions .(see to energy tire)
Pic. 1 Force in rolling tire
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