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Simulation output and interpretation 425
the so-called ‘roll centre’ was put forward as well as the notion of suspension
movement leading to adjustments of the wheel camber and toe angle as the
body rolls.
Both these effects lead to a modification of the way the tyre is presented to
the road. These in turn lead to variations in the yaw moment on the car and
therefore some real (objective) influence on the behaviour of the vehicle.
These are mostly quite straightforward. For example, toe out on bump on
the front suspension will lead to a reduction in the toe angle of the wheel as
the vehicle rolls out of the turn, reducing the yaw moment on the vehicle
and hence reducing the steady state yaw rate if no adjustment is made by
the driver. A slightly more subtle effect is associated with the typical inclination
of the so-called roll axis. If the vehicle is imagined to rotate purely
about its longitudinal axis (to roll in a pure sense) then the lower front antiroll
geometry will lead to a greater lateral velocity of the front wheels in
comparison with the rear. This modification of the lateral velocity will modify
the tyre slip angles, with a correspondingly greater increase in front slip
angle than rear. As a result, the tyres will produce a yaw moment out of the
turn – against the yaw rate but phased with roll rate. Although the motion
of the vehicle is not purely roll when entering a turn or reacting to a disturbance,
this mechanism may be seen to be one that couples roll and yaw.
Modern road vehicles are comparatively taut in roll, with compliances of
6 degrees/lateral g and less being commonplace on quite ordinary vehicles.
This is in comparison with 20 years ago, when roll compliances of
12 degrees/lateral g were quite normal. Circuit competition cars are typically
at something under 2 degrees/lateral g. Low levels of roll compliance mean
that body roll is no longer the modifier to vehicle dynamics that it once
was – its effect is now quite small in the overall scheme of things.
Subjectively, roll has an important effect. Upon entering a turn, the vehicle
may be thought of as ‘relaxing out’ to a final body roll angle. This means
the lateral response of the driver’s head, high in the vehicle, is reduced during
the transient roll-out section of turn-in. Consideration of the expression
for beta-dot, above, shows that subjectively this leads to a momentary overestimation
of body slip rate by the driver and hence roll transients often
degrade driver confidence by introducing a delay in perceived lateral acceleration.
Note that for vehicles typically instrumented, delay is not captured
by an accelerometer mounted on the floor of the passenger compartment
and some processing of roll rate is necessary to compute the lateral response
of the vehicle at driver’s head height. This is also true of multibody system
models. Although the driver subconsciously compensates for motion of
their head on the flexibility of their neck, they do not generally compensate
for motion of the platform of the vehicle in the same manner.
The nature of the roll-out event is also important, with many in the vehicle
dynamics community believing that roll acceleration profiles and roll jerk
(the time derivative of roll acceleration) are important modifiers in the perception
of roll as a modifier to platform dynamics. Makers of motion simulators
understand and use this effect to ‘simulate’ angular events, applying
an angular jerk and then providing a visual environment to suggest the roll
rate is persisting. Although quite reproducible, these small events are difficult
to capture with instrumentation on vehicles and so there exists a belief
that these phenomena are not amenable to objective quantification. That