822 Mechanical properties

To help the reader understand some of the terms used to define the mechanical properties of rubber the following brief definitions are given:

Material resilience This is the ability for a solid substance to rebound or spring back to its original dimensions after being distorted by a force. A material which has a high resilience generally has poor road grip as it tends to spring away from the ground contact area as the wheel rolls forward.

Material plasticity This is the ability for a solid material to deform without returning to its original shape when the applied force is removed. A material which has a large amount of plasticity promotes good road grip as each layer of material tends to cling to the road surface as the wheel rolls.

Material hysteresis This is the sluggish response of a distorted material taking up its original form so that some of the energy put into deforming the carcass, side walls and tread of a tyre at the contact patch region will still not be released when the tyre has completed one revolution and the next distortion period commences. As the cycle of events continues, more and more energy will be absorbed by the tyre, causing its temperature to rise. If this heat is not dissipated by the surrounding air, the inner tyre fabric will eventually become fatigued and therefore break away from the rubber encasing it, thus destroying the tyre. For effective tyre grip a high hysteresis material is necessary so that the distorted rubber in contact with the ground does not immediately spring away from the surface but is inclined to mould and cling to the contour of the road surface.

Material fatigue This is the ability of the tyre structure to resist the effects of repeated flexing without fracture, particularly with operating temperatures which may reach something of the order of 100°C for a heavy duty tyre although temperatures of80-85°C are more common.

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