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142 Multibody Systems Approach to Vehicle Dynamics
A final, important element of the study of handling load transfer is one that
allows a single, unified treatment but rarely receives it. The notions of antidive/squat
and ‘roll centre’ are rarely discussed together and yet their influence
on vehicle handling loads is via the same mechanism; they lend
themselves to the consistent rational treatment given here. The so-called
‘roll centre’ concept has to be one of the most non-intuitive and misunderstood
modifiers of vehicle behaviour since empirically observed in racing
circles in the 1920s. The understanding developed then is most applicable
to beam axles and adds more difficulty than clarity for independent suspensions.
The authors prefer the concept of ‘sprung’ and ‘unsprung’ loadpaths
to the vehicle body.
Figure 4.8 shows a single wheel of an independently suspended vehicle
viewed from the side. A braking load is applied at the contact patch, reflecting
the fitment of outboard brakes as is common practice. Were the brakes
inboard or were this a tractive case, the load would be applied at the wheel
hub height and the diagrams would be redrawn appropriately. In this case
the suspension is of a leading arm type. Since all independent suspensions
may be represented as some form of equivalent length virtual swinging arm
(although the length and pivot location vary with some types more than
others) this is a useful notion.
Three different orientations for the swinging arm are shown. The first has
the swinging arm pivot on a direct line between the contact patch and the
(a)
(b)
Notional pitched
geometry
(c)
Nominal
ground
Fig. 4.8 (a) No-dive suspension – pitch moment transfer solely via an
unsprung loadpath. (b) Pitch moment transfer solely via a sprung loadpath.
(c) Typical arrangement; pitch moment carried by a combination of sprung
and unsprung loadpaths