<VV> Front end shimmy

[Jim Simpson]

There are a couple of things people think of as front end shimmy.

One is more accurately called cowl shake.  Usually it's visible as vertical
motion of the front hood.  That's a body flexure issue and Corvair
convertibles -- particularly late models -- are susceptible to that.
Without the structural support of the top, a convertible can bend in the
middle.  Think of what happens as a car goes over something like a railroad
track.  First the front is pushed up and then drops down again followed by
the rear doing the same.  The car's body is somewhat flexible and tends to
bend a bit.  But it's also "springy" so if undamped, it will tend to keep
on flexing up and down somewhat like a ruler if you hold one end down on
desk and "twang" it.  The "cocktail shakers" installed in the front and
rear of the late Corvair are intended to counter that.  They're basically
just weights suspended in oil that act as vertical vibration dampers.  When
the car body flexes vertically, they absorb the energy and damp that
vibration.

Front end shimmy is normally a front wheel/suspension/steering issue where
things tend to vibrate horizontally.  It mostly shows up in the steering
wheel although if severe enough can shake the entire body.  There are many
causes as the earlier posts have noted.  Usually it's a combination of
problems/issues any one of which might be the trigger but usually indicates
that there are several things you probably ought to take care of .

In diagnosing and fixing front end shimmy, keep in mind that the entire
front suspension and steering system -- including the body structure -- is
a combination of springs.  All of those steel and rubber components are
flexible to some degree and will act as a spring.  Shimmy results when you
hit a resonance point in that spring system that amplifies and maintains a
severe vibration.

Normally the shimmy is triggered by the tires and wheels.  It may be a road
irregularity or perhaps an inherent issue with the tire or wheel such as
balance, broken belt, out-of-round condition, etc. But once the initial
impulse is given then it's other components that sustain the vibration.

Alignment:  When a car is moving, there's a certain amount of drag from the
wheels.  If there is ANY flexure or play in the front suspension (and there
is!), then the wheels will tend to splay outward from that drag.  Toe-in
compensates for that drag.  In a perfect situation, the wheels would be
perfectly straight and parallel, but given the suspension and steering
flexure, it's best to have the wheels pointing slightly inward (toe-in) so
that under normal conditions the front wheels are hunting back and forth
over dead straight ahead.  That hunting can cause shimmy.

Wheel bearings:  If the wheels start "hunting" back and forth over straight
ahead, significant looseness in the bearings will allow them to move
farther back and forth, accentuating the movement.

Steering components:  Tie rod ends, pitman arm bushing and loose steering
box (including the mounting bolts and rusted frame) all contribute to
allowing the front wheels to wobble in something other than a straight
path.  Keep in mind that the steering components are all designed to allow
motion (normally some kind of twisting), but because of that, they have to
have some play.  But as they wear, the allow more and more play.  So if
something triggers that initial wobble (shimmy), any loose or worn
component in the steering will accentuate the problem.

Suspension:  These are normally the most rigid components in the front end,
but if there are seriously worn ball joints or bushings, they can
contribute as well.

My recommended approach is this:

First rotate the tires.  Poor quality, old worn, or damaged tires are
frequently the trigger to shimmy.

Second check the alignment, particularly the toe-in.  Sometimes just
increasing the toe-in will eliminate the problem.  But that usually is just
a band-aid to a more fundamental problem -- something loose or worn in the
steering.

Then check all the steering components, paying particular attention to the
tie rods and pitman arm bushing.  Play in these will allow a sudden jolt on
one of the tires be amplified into a continued vibration (shimmy).

Finally check the basic suspension -- ball joints and such.

Sometimes it can be hard to pinpoint a single item.  As I pointed out
above, the entire front end is a spring system with many components all
working together.

Jim Simpson
Group Corvair