4569846498

Simulation output and interpretation 413
(a) (b) (c)
Fig. 7.18
Possibilities for departure from linearity
close to the subjective linearity limit and then the vehicle speed is increased
in order to increase the lateral acceleration. The increase in vehicle speed is
gradual and so details such as driveline layout are not relevant because the
drive torque is low. This situation might occur, for example, on a long, constant
radius, downhill motorway interchange ‘cloverleaf’.
The three scenarios illustrated in Figure 7.18 can be summarized as:
(a) reduced AyG and further reduced YRG
(b) reduced AyG and YRG in proportion to each other
(c) reduced YRG and further reduced AyG
In scenario (a), yaw rate gain is reduced further than lateral acceleration
gain. In order to accommodate the changes in both lateral acceleration and
yaw rate, the radius of the path must increase and so the vehicle has a
period of adjustment to a new, wider line in the curve. Most drivers notice
this and instinctively reduce vehicle speed to restore the desired path over
the ground. It is described subjectively as an ‘understeer departure’ or
‘pushing’ or perhaps in the USA as ‘plowing’ (ploughing). If uncompensated,
it leads to a vehicle departing the course (road, track, etc.) in an
attitude that is basically forwards. This is by far the most common behaviour
for road vehicles. It is desirable since, if the vehicle does leave the road,
it is least likely to roll over and will correctly present the engineered crash
structure between the occupants and any obstacles encountered. For sporty
drivers the sensation of the vehicle ‘turning out’ of the corner as it departs
from linearity can become tiresome.
In scenario (b), lateral acceleration and yaw rate gain change in some connected
manner and the vehicle will maintain course although it might need
some modification to steering input. Subjectively this vehicle will be
described as ‘neutral’ although objectively it might well be understeering.
Excess speed for a curve will lead to the vehicle running wide but with no
sense of ‘turning out of the curve’. Such a vehicle generally feels benign
although the progressive departure can mean it is unnoticed by inattentive
drivers. Enthusiastic drivers will not be so frustrated by this behaviour.
In scenario (c), lateral acceleration gain reduces more than yaw rate gain. This
leads to an ‘over-rotation’ of the vehicle when viewed in plan. Depending
on the severity of the mismatch, the change may lead to a spin out of the
curve. From inside the vehicle there is a pronounced sense of the rear end