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146 Multibody Systems Approach to Vehicle Dynamics
4.1.4 Compliant wheel plane control
Hand-in-glove with an understanding of the load transmission paths and
time delays associated with the activity manoeuvring the vehicle in the
ground plane comes an understanding of the resulting motion of the wheel
plane with respect to the ground. From the treatment of tyres that follows
in Chapter 5, and as described briefly in the introductory chapter, it is clear
that the angles at which the tyres are presented to the road are of crucial
importance in modifying the forces generated by the tyres and hence the
resulting motion of the vehicle.
There are some subtle and intricate effects present in real vehicle systems
that defy simplistic comprehension and evaluation. For example, the deformation
of anti-roll bars reorients the links with which they are connected to
the moving suspension members and may introduce forces that ‘steer’ the
wheel plane with respect to the vehicle. This may not have been considered
at the time the suspension was schemed conceptually but yet may modify
the behaviour of the vehicle in practice.
Multibody systems analysis allows the reconstruction of rig-based measurements
for such wheel plane compliant behaviour before prototype
vehicles exist. It also allows a systematic and well-controlled comparison
of behaviour with different levels of compliance in order to establish the
influences of the different aspects of wheel plane compliance on vehicle
behaviour. Typically such studies carry across several revisions of a model
within a market segment and are thus of some strategic benefit.
4.1.5 Kinematic wheel plane control
Suspension arrangements typically consist of some connection of linkages
to a device for holding the bearings in which the wheel actually turns. The
interaction of those individual elements comprising the linkage means that
the wheel plane typically undergoes some sort of translation and rotation as
the suspension articulates.
This motion of the wheel plane is traditionally defined with respect to the
vehicle body, although for a moving vehicle it is the angles and velocities
with respect to the road that are of import. However, using the vehicle body
gives a convenient frame of reference and so the following descriptions
will do so:
(i) Toe change (steer) with suspension articulation gives a lateral force and
yaw moment by directly adding to or subtracting from the tyre slip angle
and is readily understood. Some care is needed when discussing this characteristic
since there are a number of possibilities for definition; steer may
be defined as a right-hand positive rotation about the vehicle vertical axis
or it may be defined as a handed rotation, different on left and right sides of
the vehicle (‘toe-out’) or even as a term relating the behaviour to the yaw
moment it induces on the vehicle (‘roll understeer’).
(ii) Camber change acts to mitigate the angle with which the tyre is presented
to the road due to the roll of the body with respect to road surface.
For typical passenger vehicles, the vehicles roll ‘out’ of turns, with the
inside edge of the vehicle platform being further away from the road than