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424 Multibody Systems Approach to Vehicle Dynamics
7.5 Steering feel as a subjective modifier
A further difficulty for theoretical dynamicists is the question of steering ‘feel’.
Subjectively, impressions of vehicle behaviour are gathered to a significant
extent through the handwheel, whether consciously or subconsciously. A
great deal of effort is concentrated in modern road cars on the manner in
which torque is transmitted back to the driver up the steering column. Those
skilled in the art have no difficulty distinguishing between steering issues
and vehicle issues. However, a lack of clarity can lead to confusion if steer
effects are not separated from vehicle effects. For example, two otherwise
identical vehicles with different steering ratios will be judged quite differently
by most drivers. Presuming the underlying behaviour of the vehicle is
satisfactory, most drivers will rate a numerical reduction in steering ratio
(‘quicker’ steering) as giving ‘better’ handling due to the increased yaw rate
gain of the vehicle as seen by the driver from the handwheel. Yet the vehicles
are identical and it would be possible to reproduce the behaviour of one
vehicle by steering at a different rate in the other, to different final positions.
Steering feel is correctly given a great deal of importance in road car design
since it is the primary means by which the customer comprehends the
dynamics of the vehicle. Accurate modelling of steering feel is difficult and
requires a great deal of data about friction in individual joints, plus also a
good characterization of the hydraulic or electrical power assistance used
in the steering system. Nevertheless, work to understand the relative importance
of individual contributions is possible with comparatively inaccurate
models so long as good judgement is used and conclusions are correlated
with work on real systems.
Changes in steering torque are a primary input for skilled drivers to detect
vehicle behaviour. When driving normally, the tyres generate forces by distortions
in the contact patch (see Chapter 5) that result in a moment attempting
to return the tyre to a zero slip angle condition. This is referred to as
‘aligning torque’ and, if the steering system is well designed, is delivered
with very little corruption from vehicle weight and frictional effects directly
to the hands of the driver. As the front tyre gets close to its frictional limit, the
deformed shape of the contact patch changes such that the aligning torque
falls substantially and may even reverse slightly. Attentive drivers note this
and are thus pre-emptively aware they are approaching the friction limit. In
addition, if a vehicle starts to spin then the steering system informs the
driver within around 0.1 second using the ‘castoring’ torque generated by
operating the entire vehicle at a large slip angle. This mechanism ensures
minimum handwheel torque when the wheels are placed so as recover the
skid; in this way the steering system fairly directly signals the current body
slip angle to the driver. Skilled drivers are extremely sensitive to these messages,
which arrive ahead of the brain’s processing of the results of its data
from the inner ear and significantly ahead of messages decoded purely
from the visual environment.
7.6 Roll as an objective and subjective modifier
So far, no mention has been made of body roll. There are two important effects
of roll, one objective and one subjective. In Chapter 4, some discussion of