4569846498

Simulation output and interpretation 407
(deg)
Understeer
Ackermann
K Understeer gradient
angle
180 L
R
Oversteer
Lateral acceleration (g)
Fig. 7.12 Determination of understeer gradient (Gillespie, 1992)
Vehicles with stiff suspension, high steer axis angles and wide tyres are
more prone to this effect.
Returning to the more general steady state cornering behaviour, the constant
radius turn test procedure (ISO 4138), the procedure may be summarized
as:
● Start at slow speed, find Ackermann angle.
● Increment speed in steps to produce increments in lateral acceleration of
typically 0.1g.
● Corner in steady state at each speed and measure steering inputs.
● Produce a graph similar to that shown in Figure 7.12.
Considering the diagram, two regions are apparent. In the ‘understeer’ region,
more steer angle is necessary compared to the Ackermann angle to hold the
chosen radius. This may not seem intuitive unless the view is taken that the
vehicle steers less than is expected (‘under’ the Ackermann response) and
more steer angle is needed to compensate for it. Similarly, the ‘oversteer’
region needs less steer angle compared to the Ackermann angle. If the oversteer
is large, the steer might need to become negative to trim the vehicle in
the steady state. For many, oversteer is marked by the use of steer in the
opposite direction to the corner – so-called ‘opposite lock’. However, the
strict definition only requires that less steer than the Ackermann angle is
applied – the transition to opposite lock merely marks a further degree of
oversteer but there is nothing especially significant about the sign change.
If the steer angle does not vary with lateral acceleration the vehicle is said
to be ‘neutral steering’.
At low lateral acceleration the road wheel angle can be expressed using
(7.12):
⎛180⎞
L
KA (7.12)
⎝ ⎠
y
R