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Modelling and assembly of the full vehicle 363
Motion on the steering
system is ‘locked’ during an
initial static analysis
Downward motion of vehicle
body and steering rack relative
to suspension during static
equilibrium
Connection of tie rod
causes the front wheels
to toe out
Fig. 6.37 Toe change in front wheels at static equilibrium for simple models.
(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 129, by permission of
the Council of the Institution of Mechanical Engineers)
This has a pulling effect, or pushing according to the rack position, on the
tie rod that causes the front wheels to steer during the initial static analysis.
The solution to this is to establish the relationship between the steering column
rotation and the steer change in the front wheels and to model this as
a direct ratio using two coupler statements to link the rotation between the
steering column and each of the front wheel joints as shown in Figure 6.38.
6.12.2 Steering ratio
In order to implement the ratios used in the couplers shown in Figure 6.38
linking the rotation of the steering column with the steer change at the road
wheels it is necessary to know the steering ratio. At the start of a vehicle
dynamics study the steering ratio can be a model design parameter. In the
examples here a ratio of 20 degrees of handwheel rotation to 1 degree of
road wheel steer is used. On some vehicles this may be lower and on trucks
or commercial vehicles it may be higher. To treat steering ratio as linear is
a simplification of the situation on a modern vehicle. For example, the
steering ratio may vary between a higher value on centre to a lower value
towards the limits of rack travel or vice versa. This would promote a feeling
of stability for smaller handwheel movements at higher motorway
speeds and assist lower speed car park manoeuvres.
Using the multibody systems approach the steering ratio can be investigated
through a separate study carried out using the front suspension
system connected to the ground part instead of the vehicle body. The