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Simulation output and interpretation 429
Increasing agility,
reducing grip
1
0.8
0.6
0.4
0.2
Circuit car
Gravel rally car
Road car
C N
0
2 1.5 1 0.5 0
0.2
0.5 1 1.5 2
0.4
0.6
Increasing stability,
reducing grip
0.8
1
A Y
Fig. 7.29 F–M diagrams for circuit racing, rally and road car performance –
based on static weight distribution
circuit-racer’s knowledge. Rally driving is all about control and adaptation.
The driver’s inputs have a very small open loop content and very large and
active closed loop (trim) content. During a typical drift event lasting 2 seconds,
there are five changes of steering angle direction (i.e. from left to right)
and handwheel rates as defined in Bartlett (2000) exceed 1000 degrees/second
regularly despite the use of ultra-low (‘fast’) steering ratios. The yaw moment
versus lateral force diagram is typically skewed much more towards control,
which often produces stability concerns at the very highest speeds
(Figure 7.29). The right-hand vertex is above the A Y -axis, indicating a
departure that is ‘loose’ – that is to say the vehicle will spin unless actively
trimmed by the driver.
7.8.3 Accident avoidance
Accident avoidance in road cars is quite different to both the previous scenarios.
In terms of the percentage of miles driven it is statistically insignificant;
if it weren’t for the fact that collisions have such serious consequences we
would ignore it altogether. This is important because it means the driver is
(in general) unpractised and unprepared. The driving task may be thought of
as possessing two components – the command component, concerned with
the speed and path of the vehicle, and the control component, concerned with
maintaining the heading of the vehicle in line with its path. That this split
mimics the way the authors model the driver behaviour (Chapter 6) is no
coincidence. Under normal circumstances the control part of the driving
task is minimal. However, during aggressive manoeuvres it emerges to
become an important component of the overall task because of the transient
and frequency domain issues outlined earlier. Typically, the importance of
command decisions is also high at these times, leading to either a loss of
control or poor command decisions under circumstances where the vehicle
could have been physically able to evade the emergent hazards, had the