Suspension
design starts off as a kinematic problem that the designer must solve. There
are very easy methods to evaluate the basic sanity of the solution that one
might have thought of. An independent suspension with its steering locked has
one degree of freedom. This degree of freedom (DOF) is the travel of the
suspension. When a design solution for the suspension is thought of, it is important
to do the degree of freedom analysis.
For example,
this double ‘a-arm’ suspension in the figure below has an upper and lower
control arm with a toe rod which has ball joints on either end.
*The A-arms
being joined to the chassis by 2 ball joins is actually an over constrain. The
ideal solution would be to have a ball joint at one of the pick-ups and a ball
joint in a slider on the other pick up. This is however not practical and
usually the kinematic over-constrained is persisted with, to better distribute
loads. Think is this in the same way as a door having multiple hinges when
kinematically a single hinge would do. This is also the ‘a-arm’ must be
manufactured with precision on a jig. Any misalignment will cause compliance in
the structure.
** The 2
ball joints are usually rod-ends in a toe rod. The rod-ends are not ideal ball
joints. The limited articulation does not allow the tie rod to spin about its
axis and this constrains another degree of freedom associated with the spinning
of the toe-rod within its place.
It has one
degree of freedom which means that baring interference the suspension travel is
easily achieved without any of the members flexing.
This degree
of freedom analysis is important to determine if a suspension configuration
would work or not. For, example these pictures below illustrate another
solution to toe control in the rear where the tie rod is welded on to the
control arm.
People who
have read this blog post would be quick to point out that the toe base here is
too small. The load path is not great because forces apply bending moments on
the control arm. However, this suspension solution also does not fare well with
the DOF analysis.
Such suspension
solution might function like a 1 DOF system if the over-constrains are
redundant. However, slight misalignment or manufacturing tolerance error would
cause components to go through high stress cycles and eventually break. For the
sake of completion the correct way to get the DOF correct in the above solution
is to have a control rod, with ball joints at either end as the lower control
arm instead of an ‘a-arm’.
DOF is only
one of the criterions that must be kept in mind. The DOF solution in the
suspension system in the picture below is good. However, there can still be an
issue with the toe compliance here. Even though the toe base is reasonably large
the toe rod pick-up is a cantilever on the upright. It will see a lot of
bending moments and cause compliance due that bit flexing.
This blog post is originally written for the Formula Student India website and has been cross posted from here.
This blog post is originally written for the Formula Student India website and has been cross posted from here.
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