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426 Multibody Systems Approach to Vehicle Dynamics
this is currently true is a consequence of pragmatism rather than any underlying
principle – it is very quick to have someone experienced and skilled
in the art develop damper tuning for these subjective qualities, compared to
instrumenting a vehicle and going through a research programme to define
numerical goals at which to aim. In general, a fluid development of initial
roll rate and a progressive deceleration to the final roll angle are recognized
as necessary to reduce the perception of roll angle. Directional changes
towards this end are certainly amenable to predictive analysis with multibody
system models, and simulation work using the final, released damper
calibrations would go a long way towards improving the quality of work on
the next vehicle programme. However, institutionally there is little time in
modern engineering organizations for such work since it does not immediately
contribute to the task at hand. Historically it has been difficult to get
good data to define vehicle dampers (primary modifiers for roll transients)
although modern system identification techniques mean this is more possible
than it once was.
There is some work that suggests that roll-pitch interaction is important for
subjective evaluation of roll (Kawagoe et al., 1997) and this certainly seems
plausible. Again, such behaviour is amenable to analysis with multibody
system modelling and allows directional selection of design alternatives if
not final tuning on the real vehicle. Essentially, a pitch nose-upward that
accompanies roll is often subjectively described as ‘the rear of the vehicle
rolling more than the front’ – a statement that is quite mystifying to objective
vehicle dynamicists but common currency among skilled development
drivers. It is recognized as undesirable and a small but not excessive amount
of nose-down pitch is preferred for road vehicles. The exact amount varies
with market segment and changes over time as market tastes change. This
is something of a challenge for vehicle dynamicists since often there is a
desire to have the vehicle roll onto front bump stops before rear in order to
guarantee limit understeer, and hence stability – unfortunately this promotes
subjectively undesirable pitch nose-up with large roll transients. Development
work with multibody system models and real vehicles allows combinations
of damper tuning and anti-roll geometry to overcome this difficulty.
Once again, a fluid development of initial pitch rate and progressive deceleration
to the final pitch angle are recognized as necessary to reduce the
perception of pitch angle.
7.7 Frequency response
As mentioned in section 7.4, a transient demand for yaw rate change (a
non-zero steering rate) is also a transient demand for a change in body slip
angle. That is to say there is a strong link between expectations of beta-dot
and rate of handwheel motion. Delays between the two are also acutely
remarked.
The seminal IME papers in 1957 showed the existence of a yaw/sideslip
mode of vibration for the vehicle, which can be illustrated with only a
2-degree-of-freedom model. The mode of vibration may be thought of
as analogous to a pendulum but in the ground plane. It is this mode of
vibration that rally drivers use when they ‘flick’ the car from one side to
the other before a turn. Like any other vibrating system, the gains of the