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414 Multibody Systems Approach to Vehicle Dynamics
of the vehicle departing first but objectively the vehicle may not actually
oversteer in the classical sense – it may simply move ‘towards neutrality’.
This is the nature of rear-wheel-drive vehicles when driven to departure
using the throttle. Subjectively, there is a pronounced sense of ‘oversteer’ –
sometimes described as ‘loose’ in the USA. Vehicles that preserve yaw rate
gain as they lose linearity are widely regarded as fun to drive and sporty.
A further difficulty between theoretical and practical dynamicists is that the
former group often consider the vehicle on the basis of ‘fixed control’ and
‘free control’ where the latter almost always use ‘driver input to complete
a set task’. With fixed and free control, the inputs are consistent and the
response of the vehicle is used to evaluate it. With driver input, the vehicle
response is substantially constant and the vehicle is evaluated on the basis
of the required changes in driver input to complete a task. More and more,
so-called ‘black lakes’ – large flat areas of high grip surface – are being
added to vehicle testing facilities to allow the evaluation of fixed and free
control manoeuvres for experimental correlation purposes. Also gaining in
popularity are theoretical ‘driver models’. These range from simple pathfollowers
to sophisticated multi-loop, multi-pass adaptive controllers. At
present there are many such models and none has gained precedence, suggesting
perhaps that none is ideal for the task at hand – understanding and
improving vehicle behaviour. Methods for modelling driver behaviour are
discussed in Chapter 6. A very real problem with such models is that there
is a fine line between evaluating the quality of the driver model and evaluating
the change on the vehicle. Did the vehicle performance improve because
the modification suited the driver model? Does that behaviour reflect a typical
driver in an emergency situation? These criticisms are not unique to
modelling and can also be levelled at highly skilled development drivers.
Indeed, within motorsport circles this particular difficulty is widely recognized.
There are drivers who can drive a given setup in the fastest way that
it can be driven but who cannot articulate how the vehicle could be faster.
Such drivers are an asset on race days but less so during development testing.
For this reason, ‘test’ or ‘development’ drivers are frequently employed
who have a different set of skills to event drivers.
In summary then, the subjective evaluation of a vehicle depends largely on
the nature of its departure from linearity while objective evaluation is an
absolute positioning against a neutral datum. From inside the vehicle it is
generally difficult to distinguish between a vehicle that is operating at a
large body slip angle and one that is truly oversteering. In any case, to control
a large body slip angle it is frequently necessary to reduce or reverse
steering input (‘opposite lock’), changing the measured yaw rate gains substantially
towards oversteer.
7.3.5 Mechanisms for generating under- and oversteer
To understand the vehicle parameters that have an influence on oversteer,
understeer and departure behaviour, we can use the roll stiffness model
described earlier to consider a series of free bodies and the force moment
balance on each during steady state cornering. Figure 7.19 shows a version
of the roll stiffness model while travelling in a curved path, with a lateral
acceleration A y .