4569846498

256 Multibody Systems Approach to Vehicle Dynamics
Direction of sliding
Loading
Unloading
Fig. 5.10
Road surface
Loading and unloading of tyre rubber in the contact patch
be zero it can be seen from Figure 5.10 that an element of rubber in the
contact patch will be subject to continuous compressive loading and
unloading.
In the idealized situation of no friction as the tyre slides over the irregular
road surface compressive forces normal to the surface are generated as the
rubber is loaded and unloaded. Due to the hysteresis in the rubber the sum
of the loaded forces is greater than the sum of the unloaded forces resulting
in, for example here, a resultant braking force opposing the direction of
sliding.
5.3.2 Pressure distribution in the tyre contact patch
In order to understand the manner by which forces and moments are generated
in the contact patch of a rolling tyre an initial appreciation of the
stresses acting on an element of tread rubber in the contact patch is
required. Each element will be subject to a normal pressure p and a shear
stress acting in the road surface. In theory the element will not slip on the
road if p where is the coefficient of friction between the tread rubber
and the road surface.
The pressure distribution depends on tyre load and whether the tyre is stationary,
rolling, driven or braked. The pressure distribution is not uniform
and will vary both along and across the contact patch. In order to understand
the mechanics involved with the generation of forces and moments in
the contact patch some simplification of the pressure distribution will be
adopted here starting with Figure 5.11 where typical pressure distributions
in the tyre contact patch for a stationary tyre and the effects of inflation
pressure are considered.
Generally the pressure rises steeply at the front and rear of the contact
patch to a value that is approximately equal to the tyre inflation pressure.
Overinflation causes an area of higher pressure in the centre of the contact
patch while underinflation leads to an area of reduced pressure in the
centre of the patch.
When the tyre is rolling it will be shown later that pressure distribution in
the contact patch is not symmetric and is greater towards the front of the
contact patch.