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364 Multibody Systems Approach to Vehicle Dynamics
COUPLER
COUPLER
Fig. 6.38 Coupled steering system model. (This material has been reproduced
from the Proceedings of the Institution of Mechanical Engineers, K2 Vol. 213
‘The modelling and simulation of vehicle handling. Part 2: vehicle modelling’,
M.V. Blundell, page 130, by permission of the Council of the Institution of
Mechanical Engineers)
modelling of these two subsystems, with only the suspension on the right
side shown, is illustrated in Figure 6.39.
The approach of using a direct ratio to couple the rotation between the
steering column and the steer angle of the road wheels is common practice
in simpler models but may have other limitations in addition to the treatment
of the ratio as linear:
(i) In the real vehicle and the linkage model the ratio between the column
rotation and the steer angle at the road wheels would vary as the vehicle
rolls and the road wheels move in bump and rebound.
(ii) For either wheel the ratio of toe out or toe in as a ratio of left or right
handwheel rotation would not be exactly symmetric.
Modelling the suspension with linkages will capture these effects. Although
this may influence the modelling of low speed turning they have little effect
for handling manoeuvres with comparatively small steer motions.
With simpler vehicle models, not including suspension linkages, the ratio
would need to be functionally dependent on the vertical movement of the
suspension and direction of handwheel rotation if the behaviour is to be
modelled. It should also be noted that compliance in the steering rack or
rotational compliance in the steering column could be incorporated if the
analysis dictates this.