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102 Mutibody Systems Approach to Vehicle Dynamics
z
02
x
y
I
Line-of-sight
force element
x
y
z
J
03
Fig. 3.23
Line-of-sight force element
The cylindrical joint is used to prevent an unwanted degree of freedom that
would result in the central link spinning about its own axis.
3.2.9 Force elements
There are two fundamental types of force element that may be defined in a
multibody systems model. The first of these are force elements that can be
considered internal to the system model and involve the effects of compliance
between bodies. Examples of these include springs, dampers, rubber
bushes and roll bars. As such these forces involve a connection between
two bodies and due to the principle of Newton’s third law are often referred
to as action–reaction forces. For the translational class of force elements
used to define springs and dampers the force will act along the line between
two markers that define the ends of the element and as such this form of
definition is referred to as the line-of-sight method.
The second type of force is one that is external and applied to the model.
Examples of these include gravitational forces, aerodynamic forces and any
other external force applied to the model where the reaction on another
body is not required. As such they may be referred to as action-only forces.
The forces generated by a tyre model and input through the wheel centres into
a full vehicle model can also be considered to be this type of force. These
forces may be translational or rotational and as they require the definition
of a magnitude, line of action and sense the method of definition is referred
to as the component method.
The definition of line-of-sight forces is illustrated in Figure 3.23 which
shows a force acting along the line of sight between two points, an I and a J
marker, on two separate parts. The forces acting on the I and the J marker
are equal and opposite. As the line of the force is defined entirely by the
location of the I and the J marker the orientation of these is not relevant
when defining the force.
The component method applies to translational action-only forces where
the direction and sense of the force must be defined and to rotational forces
where the axis about which the torque acts is required. In MSC.ADAMS it
is the z-axis of the J marker that is used to define the direction and sense of
a translational action-only force. The force acts on the I marker as shown in
Figure 3.24 and there is no reaction on the J marker.