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Simulation output and interpretation 437 on. The combination of design variables is selected to maximize signal-tonoise ratio to maximize robustness of the system. The following steps in the process are the adjustments to the sensitivity, , being made through those variables showing themselves as affecting but of less influence on . Such an approach to the ideal function for the steering and handling behaviour covers a great many different situations although for motorsport applications, in particular rallying, it may lack one important aspect. Rally drivers use the non-linearity of the car to their advantage. Recall the force–moment diagram in Figure 7.29. This is derived on the basis of quasi-static calculations. However, professional rally drivers use the dynamic amplification of the yaw–sideslip resonance along with features such as ruts and road surface edges (Figure 7.35) to achieve yaw accelerations significantly greater than that suggested by the Milliken moment method, as shown by the comparison in Figure 7.36. The lightest data in the centre represents the highest speeds achieved during the stage and fits well within the estimated boundaries of the F–M diagram. However, the two other shades show extremely large levels of yaw acceleration at high lateral acceleration in strong contrast to the general form of the Milliken diagram. This is a symptom of the use of the dynamic amplification in the yaw–sideslip mode as used by the rally drivers. The use of an ideal function that relates steer angle to yaw rate will promote an increase in yaw damping, which is good for road cars but precludes the ‘flick’ style of driving preferred by rally drivers. Figure 7.37, by contrast, shows the behaviour of the rally car on tarmac and suggests that the vehicle stays well within the quasi-static boundary predicted by the Milliken moment method. Fig. 7.35 Tommi Makinen hooks the front left wheel of his 2003 Subaru WRC off the tarmac and ensures a large yaw moment will be available shortly (courtesy of Prodrive)
438 Multibody Systems Approach to Vehicle Dynamics Yaw acceleration (deg/s/s) 1000 800 600 400 200 0 200 400 600 Predicted CN – A y boundary Measured data 0–90 kph Measured data 90–130 kph Measured data 130–250 kph 800 1000 2.0 1.5 1.0 0.5 0.0 0.5 1.0 1.5 2.0 Lateral acceleration (g) Fig. 7.36 Yaw acceleration versus lateral acceleration for the Subaru WRC 2002, Petter Solberg, Argentina Yaw acceleration (deg/s/s) 1000 800 600 400 200 0 200 400 Predicted CN–A y boundary Measured data 0–90 kph Measured data 90–130 kph Measured data 130–250 kph 600 800 1000 2.5 2.0 1.5 1.0 0.5 0.0 0.5 1.0 1.5 2.0 2.5 Lateral acceleration (g) Fig. 7.37 Yaw acceleration versus lateral acceleration for Subaru WRC 2002, Petter Solberg, Germany Examination of the recorded data shows there is little ‘steady state’ about the rally stage on loose surfaces and the character of the handwheel angle versus yaw rate trace for the entire stage has a distinctly ‘circular’ quality about it (Figure 7.38). This indicates that output (yaw) has a phase shift of around 90 degrees – an indication that the system is indeed at resonance. In
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Simulation output and interpretation 437<br />
on. The combination of design variables is selected to maximize signal-tonoise<br />
ratio to maximize robustness of the system. The following steps in the<br />
process are the adjustments to the sensitivity, , being made through those<br />
variables showing themselves as affecting but of less influence on .<br />
Such an approach to the ideal function for the steering and handling behaviour<br />
covers a great many different situations although for motorsport applications,<br />
in particular rallying, it may lack one important aspect.<br />
Rally drivers use the non-linearity of the car to their advantage. Recall the<br />
force–moment diagram in Figure 7.29. This is derived on the basis of<br />
quasi-static calculations. However, professional rally drivers use the dynamic<br />
amplification of the yaw–sideslip resonance along with features such as<br />
ruts and road surface edges (Figure 7.35) to achieve yaw accelerations<br />
significantly greater than that suggested by the Milliken moment method,<br />
as shown by the comparison in Figure 7.36.<br />
The lightest data in the centre represents the highest speeds achieved during<br />
the stage and fits well within the estimated boundaries of the F–M diagram.<br />
However, the two other shades show extremely large levels of yaw<br />
acceleration at high lateral acceleration in strong contrast to the general<br />
form of the Milliken diagram. This is a symptom of the use of the dynamic<br />
amplification in the yaw–sideslip mode as used by the rally drivers. The<br />
use of an ideal function that relates steer angle to yaw rate will promote an<br />
increase in yaw damping, which is good for road cars but precludes the<br />
‘flick’ style of driving preferred by rally drivers.<br />
Figure 7.37, by contrast, shows the behaviour of the rally car on tarmac and<br />
suggests that the vehicle stays well within the quasi-static boundary predicted<br />
by the Milliken moment method.<br />
Fig. 7.35 Tommi Makinen hooks the front left wheel of his 2003 Subaru WRC<br />
off the tarmac and ensures a large yaw moment will be available shortly<br />
(courtesy of Prodrive)
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