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Multibody systems simulation software 79
SPH
REV
04
03
02
M
Z G
07
04
03
SPH
02
X G
REV
01
GRF
Y G
05
SPH
06
05
UNI
01
GROUND
Fig. 3.2
Double wishbone suspension system schematic
items such as the parts, joints, imparted motions and applied forces. The
model may well include other elements that include, for example, springs,
dampers and beams. The drawing of a schematic is an important first step
as it will help the user not only to plan the data that will need to be collated
but more importantly to estimate the degrees of freedom in the system and
develop an understanding of how the mechanism will work. The use of a
modern GUI should not discourage this process. Figure 3.2 provides an
example of a system schematic for a double wishbone suspension system.
Users may develop their own style when drawing a schematic but the symbols
shown in Figure 3.3 are provided as a suggested starting point for the
sketching of the various elements of a model.
3.2.2 Reference frames
The three-dimensional description of a multibody system requires the use
of reference frames, not only to set up the configuration and physical properties
of the model, but also to describe the calculated outputs such as the
displacements, velocities and accelerations. There are three types of righthanded
Cartesian co-ordinate systems that will be used in this text:
(i) The Ground Reference Frame (GRF). This is by definition the single
inertial reference frame that is considered to be fixed or at ‘absolute’ rest.
Any point defined to belong to this reference frame has zero velocity and
acceleration. The ground reference frame is taken to be fixed on a body or
part known as the ground part, the physical significance of which may vary
from model to model. For a single suspension model the ground part may be
taken to encompass the points on the vehicle body or subframe to which the
suspension linkages attach. For a full vehicle model the ground part would
relate to the surface of the road used to formulate the contact forces and
moments in the tyre model. In addition to providing the single inertial reference
frame the ground reference frame can be considered to be the origin of