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Simulation output and interpretation 397
Z
Yaw rate
Roll angle
Lateral
acceleration (A y )
Y
PLANE Y–O
X
PLANE X–O
Forward speed (V x )
PLANE Z–O
Fig. 7.1 Typical lateral responses measured in one of several possible vehicle
co-ordinate frames
Modern satellite-based instrumentation has improved on this somewhat.
For each handling manoeuvre, or simulation, it is necessary for vehicle
engineers to decide which responses are to be measured during the testing
process.
All test and analytical activities are directed at understanding and improving
the overall dynamic behaviour of the vehicle. As discussed in Chapter
1, the difficulty with the vehicle dynamics field is not the complexity of the
effects in play but the level of interaction between them. A further difficulty
is the level of change in behaviour required for a vehicle to be usefully
‘improved’. Typically, strong impressions of change are made with
only modest variations of physical measures. When the difficulty of repeatable
testing and the variation of impression with individual testers are both
thrown in, the vehicle dynamics process lacks ‘capability’ – in the sense of
quality control – compared to the task it is set. To explain this further, consider
the following example.
7.2 Case study 8 – Variation in measured data
Several attempts have been made to define single number measures for
vehicle dynamic performance. A popular measure in the USA is limiting
lateral acceleration – frequently referred to as ‘grip’. Table 7.1 shows data
that might be recorded during a steady state test, using a stopwatch to time
a lap of a marked 200 m diameter circle.
If an honest error estimation is made in the recorded data then the results
with the stopwatch are probably accurate to 0.1 second. Thus the raw
data would appear as shown in Figure 7.2, with error bars on the figure as
shown. This is a typical set of data for such a test; prolonging the test further
might well degrade the tyres on the vehicle and lead to a ‘skewed’
result favouring the first test configuration with fresher tyres. It could be
argued that tests should be repeated on fresh tyres for each different configuration
to be examined but while academically sound, this argument
neglects the commercial and temporal pressures placed on vehicle testing,