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144 Multibody Systems Approach to Vehicle Dynamics
(a)
(b)
Roll
centre
Nominal
ground
Notional rolled
geometry
(c)
Fig. 4.9 (a) No-roll suspension – roll moment transfer solely via an unsprung
loadpath. (b) Roll moment transfer solely via a sprung loadpath. (c) Typical
arrangement; roll moment carried by a combination of sprung and unsprung
loadpaths
As with the anti-pitch behaviour, the absolute height is of less importance
than the distribution of loads between sprung and unsprung loadpaths. It is
not any kind of centre of motion.
Again as with the anti-pitch behaviour, some fraction that is a function of
the two angles and may be calculated and expressed as an ‘anti-roll’
fraction or percentage, or alternatively the anti-roll angle, , may be quoted
separately. Alternatively, an anti-roll fraction or percentage could be
quoted based on the fraction of the ‘roll centre height’ compared to the
mass centre height. The authors prefer to use a ratio of the two angles and
to express the anti-roll fraction similarly to before:
Anti-roll % 100()/() (4.31)
For lateral handling loads, the same ideas of relative speed between unsprung
and sprung loadpaths apply. This has a particular importance when considered
in the light of the phasing of front and rear axle forces in order to
manipulate the yaw moments on the body. For a vehicle in yaw, the rate of
load transfer may thus be set differently at different ends of the vehicle in
order to modify the transient behaviour as compared to the steady state
behaviour. For example, it is typical for vehicles to run around 20% rear
anti-roll and only around 6% front anti-roll. This means that as a manoeuvre
develops, load transfer from the outside rear tyre may briefly outpace load
transfer from the front tyre. The resulting yaw moment acts to stabilize the