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Modelling and analysis of suspension systems 141
1500
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0
500
Front axle side force
1500
1000
500
0
Rear axle side force
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0 0.5 1 1.5 2 2.5 3
Fig. 4.7 Side forces calculated for a 0.1 rad/s ramped input to 0.01 rad
beginning at 0.3 s
velocity (Figure 4.7). Errors in that phasing generated by flexibility
(compliance) on one axle or the other can lead to an error in the vehicle
behaviour as perceived by the driver. This error may take the form of a disconcerting
delay in response to the steering if the front axle has more compliance
than the rear, or a rather more serious delay in the action of the rear
axle in constraining the body slip angle to its required value. In the latter
case, particularly for aggressive transient manoeuvres (i.e. accident avoidance)
then the rear tyre slip angle may exceed its critical value, leading to
a divergent behaviour of the vehicle – that is to say a spin.
Since for modern vehicles the isolation of road inputs is a high priority,
there is always a desire to introduce some elastomeric elements into the
vehicle between the suspension elements and the vehicle body. Multibody
system analysis tools allow the study of handling degradation due to the
introduction of such elastomers and allow the ride/refinement compromise
to be quantified before excessive experimentation is carried out on the
vehicle. MBS analysis in the right hands allows an understanding of those
design parameters that dominate refinement performance and those that
dominate handling performance. The understanding so gained allows better
conceptual design of suspension systems in order to separate clearly the
refinement and handling functionality for elastomeric elements. Modern
multi-link rear suspensions are a good example of such well-separated
systems and are a significant part of the simultaneous improvement in
both ride and handling, traditionally areas of mutual exclusivity that have
befallen modern road cars.
Even for a suspension with no elastomers, such as may be used on a competition
vehicle, some structural compliance is always present. MBS analysis
allows this compliance to be optimized and matched front to rear in
order to avoid onerous design constraints using conservative stiffness targets,
which would almost certainly incur some sort of weight disadvantage.