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Tyre characteristics and modelling 277
the curves here are plotted with assisting camber angle where the wheels
are leaning into the turn. A similar reduction in lateral force will occur
where the camber angle is reversed and the wheels lean out of the turn.
At zero degrees of slip angle the introduction of camber angle introduces
an offset from the origin, this being the camber thrust discussed earlier
occurring at a zero slip angle. The small offsets in lateral force due to conicity
and plysteer, discussed in section 5.2.3, are ignored in Figure 5.30. In the
linear range the contributions in lateral force due to slip and camber may be
added together but during the transition towards sliding it can be seen that
the additive effect of camber will reduce although the peak value of lateral
force is still increased. The maximum increase in peak lateral force will
occur at different camber angles for different wheel loads. Thus for a given
tyre on a given vehicle it is possible (Milliken and Milliken, 1995) to optimize
camber angle for a given combination of slip angle and tyre load.
5.4.10 Overturning moment
Two of the components of moment acting in the tyre contact patch have
been discussed. The generation of rolling resistance moment was described
while discussing the free rolling tyre in section 5.4.3. The self-aligning
moment arising due to slip or camber angle was discussed in sections 5.4.7
and 5.4.8. For completeness the final component of moment acting at the
tyre contact patch that requires description is the overturning moment that
would arise due to deformation in the tyre as shown in Figure 5.31. The
forces and moments as computed in the SAE reference frame are formulated
to act at P, this being the point where the wheel plane intersects the
ground plane at a point longitudinally aligned with the wheel centre.
In Figure 5.31 it can be seen that distortion of the side walls results in a lateral
shift of the contact patch, which may result from either slip angle or
Slip angle
Camber angle
Wheel plane
O
Wheel centre
O
M x = F z y
M x = F z y
y
P
Y SAE
P
y
Y SAE
F z
Z SAE
Z SAE
F z
Fig. 5.31
Generation of overturning moment in the tyre contact patch