The discrete element can be either a spring or a damper
that connects two nodes. The connection between constituent
nodes is defined with the *ELEMENT_DISCRETE keyword, while
nodal locations are defined with the *NODE keyword. The
*SECTION_DISCRETE keyword is used to choose the movement
option. The discrete elements in this truck model are either
translational (dro=0) or rotational (dro=1).
The discrete elements are assumed to have no mass. However,
to solve the equations of motion at unconstrained discrete
element nodes or nodes joining multiple discrete elements,
nodal masses must be specified at these nodes. For that
purpose the ELEMENT_MASS keyword was used to specify a
miniscule mass of 1g where required.
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