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428 Multibody Systems Approach to Vehicle Dynamics
during emergency manoeuvres, the amplitude and frequency content
of driver’s steer inputs rise tremendously. Published data suggests that
the highest steer rate sustained for 200 ms or more is likely to be around
1100 degrees/second by the population as a whole (Bartlett, 2000). Data from
the World Rally Championship series confirms this is the highest rate used
by those drivers also; the authors’ logged data also achieves – but has never
exceeded – these levels on occasion.
When such large and high frequency inputs are used, the response of the
vehicle is no longer controlled only by the steady state yaw characteristics
but may also be amplified compared to the base level as well having a phase
delay imposed as described previously. The dynamic amplification is a result
of the yaw/sideslip resonance of the vehicle being excited to produce
responses that may be substantially greater as well as delayed by some 150
milliseconds compared to the linear result that the driver might be expecting.
7.8 The problems imposed by …
7.8.1 Circuit racing
Here the authors draw unashamedly on the material (Milliken and Milliken,
1995) presented in Chapter 1 of the Milliken and Milliken book Race Car
Vehicle Dynamics. Circuit racing is about using the acceleration vector of
the vehicle to maximum effect at all times, be it in braking, accelerating,
turning, or some combination thereof. Circuit racing is a highly rehearsed
behaviour with ‘braking points’ and ‘turn-in points’ all being prescribed
for a given type of vehicle. The driver’s task is to apply control in a largely
open loop manner and to add minor trim inputs in a closed loop manner.
The ‘Milliken Moment Method’ uses, as one of several tools, a diagram that
describes the manoeuvring envelope for the vehicle by plotting yaw moment
against lateral force (or some variants thereof). Some examples are given of
the diagrams for various possible configurations. Notionally, the ‘ideal’
circuit car is one which is neutral steering and retains its yaw rate gain
characteristic right to the limit of adhesion since this maximizes the lateral
acceleration possible. In truth, most circuit cars exhibit a small amount of
‘push’ or ‘plough’ right at the limit but to a first approximation, the yaw
moment versus lateral force diagram looks a little like that shown in Figure
7.29. The right-hand vertex aligns with the A Y -axis, indicating a departure
that is neither ‘push’ nor ‘loose’.
7.8.2 Rallying
In contrast, rallying is largely unrehearsed, with line-of-sight driving plus
prompts from co-drivers. These prompts take the form of memory-joggers:
‘left 5, 150 over crest; caution, do not cut’. This is interpreted by the driver
as ‘left [turn, of severity] 5 [where 1 is gentle and 5 is aggressive based on
likely speed before braking commences], 150 [metres] over crest; caution,
do not cut [the inside of the corner because there is some hazard not obvious
on entry]’. A great deal of skill and rapport is required; both driver and
co-driver need to mould each other’s expectations on the style of the co-driver’s
prompts. This information, though, is very incomplete compared to the