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Modelling and analysis of suspension systems 145
Fig. 4.10 The combination of anti-roll geometry to give a single ‘roll centre’
between left and right wheels is clearly nonsensical when the loading is strongly
asymmetric – even when the asymmetry is less extreme than the racing
Ford Falcon shown here.
vehicle and mitigates sudden, aggressive steer inputs. Vehicles that do not
have this type of geometry, notably those with trailing arm rear suspensions,
are unable to benefit from these effects.
For both longitudinal and lateral load transfers there is no conceptual reason
why either the anti-pitch angle or anti-roll angle may not be negative.
Motorcycles, for example, have a negative anti-pitch angle equal to the
steer axis rake when they are fitted with conventional telescopic forks. This
has the disadvantage of requiring extra performance from the suspension
springs since they must carry more than the straightforward load transfer
one might instinctively expect.
Some practitioners attempt to calculate combined measures for both suspensions
on the same axle or indeed all the suspensions on the vehicle. For
beam axles there is some logic in combining the characteristics since the
wheels are physically joined but for independent suspensions, calculating
some combined metric is of questionable value. For example, attempting to
combine anti-roll angles across one axle in a purely geometric manner,
when their relative importance is determined by wheel loading, is clearly
nonsensical, as shown in Figure 4.10.
Multibody systems analysis allows both an understanding of the load transfers
in a rig-based environment, such as may be measured on the MIRA
Kinematics & Compliance rig (Whitehead, 1995) and also during real
driving manoeuvres. In both situations, the ability of an MBS model to
retrieve forces in each suspension member in convenient frames of reference
while working with quarter, half or full vehicle models is a powerful
tool to unscramble some of these less-than-intuitive effects with vehicle
designs.