4569846498

Simulation output and interpretation 423
a
a
b
b
c
c
Fig. 7.27
Dynamic Index influences the centre of rotation of the vehicle in yaw
vehicle is cornering at the critical slip angle, it can be seen that a DI less
than unity will increase the rear slip angle and therefore reduce the cornering
force available at the rear axle – promoting the tendency to spin. Low
yaw inertia does indeed promote agility and speed of response, but excessively
low inertia – as quantified by DI – makes the car difficult even for
skilled drivers to manage in yaw. There is thus a trade-off between
response and ease of control; for a broad spectrum of vehicles from the Mk
1 Lotus Elan to the BMW M5, this trade-off is a DI of just over 0.9.
Motorsport vehicles differ significantly from road cars and the exact figures
are jealously guarded. It can be seen with some reflection that a lowmass
mid-engined car errs towards a DI that is ‘too low’ since the wheels
have to be a certain distance from each other to accommodate engine and
passengers. The interested reader is invited to compare the mass and
wheelbase of a Fiat X/1-9 and an MG-F.
In general, an important goal for vehicle dynamicists is to have the vehicle
‘look after itself’ from a body slip angle point of view. Most drivers have
no conscious knowledge of body slip angle and beta-dot until they become
large. The authors estimate that a body slip rate of less than around
5 degrees/second is probably the threshold between subconscious and
conscious awareness of body slip rate, but this is necessarily extremely
sensitive to context. This means that a body slip angle of 5 degrees or more
can develop more or less unnoticed by a typical driver. In general, body slip
angles of greater than 10 degrees are difficult for the majority of the driving
population to recover control from, and so 5 degrees represents something
‘halfway to irrecoverable’. When the body slip rate and/or angle exceeds
some threshold, drivers suddenly become aware that something is amiss
and so they report that vehicles ‘abruptly’ skid because the event was well
developed before they recognized it. For this reason, an unexpected skid
during otherwise normal driving can be surprisingly traumatic and leave
people with a large amount of anxiety since they felt ‘ambushed’ by the
vehicle. Drivers become sensitized and acclimatized to variations in body
slip rate and angle through familiarity and for this reason, skid pan training
for normal drivers is an excellent idea.