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330 Multibody Systems Approach to Vehicle Dynamics
finite elements only one half of the vehicle body. This requires the use of
anti-symmetry constraints along the centre line of the finite element model,
for all nodes on the plane of geometric symmetry, to carry out the asymmetric
torsion case.
It is also possible to incorporate a finite element representation of the vehicle
body within the multibody system full vehicle model. An example of this
was shown in Figure 4.3 where the flexibility of a racing cart frame was
included in the model. Despite the capability of modern engineering software
to include this level of detail it will be seen from Case study 7 at the
end of this chapter that a single lumped mass is an efficient and accurate
representation of a relatively stiff modern vehicle body for the simulation
of a vehicle handling manoeuvre.
6.3 Measured outputs
Before continuing in this chapter to describe the subsystems that describe the
full vehicle we need to consider the typical outputs measured on the proving
ground and predicted by simulation. This will be dealt with more extensively
at the start of Chapter 7 but an initial treatment is given here to support the
following discussion and the case study presented at the end of this chapter.
For a full vehicle system simulation the predicted outputs are generally plotted
as time history graphs where the outputs are the computed in a reference
frame fixed in the vehicle body as indicated in Figure 6.5. Typical outputs
can include:
(i)
(ii)
(iii)
Forward velocity
Lateral acceleration
Roll angle
Z
Lateral acceleration (g)
LUMPED MASS MODEL (TYRE A/INTER-
POLATION) - 100 KPH LANE CHANGE
1.0
0.8
0.6
0.4
0.2
0.0
0.2
0.4
0.6
0.8
1.0
0.0
1.0
2.0
3.0
4.0
5.0
Time (s)
Y
Lateral acceleration
PLANE Y-O
Yaw rate
PLANE X-O
X
Roll angle
Fig. 6.5
PLANE Z-O
Typical lateral responses measured in vehicle co-ordinate frame