Rod & Custom

t By Chris Shelton t
Hardly anything can make a car
more miserable to drive than a
persistent shake. And it’s not just
annoying; over time the oscillations
generated by imbalanced or worn
parts can literally tear a car apart.
It’s easy to exorcise demons for
most cars with the obvious remedies
but others shake despite seemingly
all attempts to cure them. But
that doesn’t make them incurable.
Vehicles are nothing but a collection
of parts, each of which can run
true on its own. Often an incurable
shimmy boils down to an easily
overlooked thing, like the way those
parts meet or the condition or tune of
the parts they bolt to.
What follows are some familiar
and uncommon things that can
make the best car develop the worst
shake. Most are universal but a few
apply specifically to our cars, at least
to the combinations of parts that we
use to build them. We won’t address
every potential shake source in a car,
but if your car still shakes despite
these solutions it might need a
minister more than a mechanic.
WHEEL BALANCING
With few exceptions every
shimmy diagnosis should start with
a good wheel balance. There are two
methods: Static and dynamic.
Static balancing equalizes the
wheel’s mass parallel to its axis
(the point around which the wheel
spins, say an axle). A classic example
of an extreme static imbalance is
a bucket swung on a rope. Even if
you stop swinging the bucket its
stored energy will drag your hand
in a circle until it stops. Now think
of your car’s suspension as your
hand and you’ll see why that energy
causes the car to shake.
An equal amount of weight
(a counterweight) applied at the
inverse angle of the imbalance
will statically balance a wheel. In
fact it will achieve static balance
whether the weight mounts to the
1
The Common Causes
Of Shakes, Wobbles,
And Shimmies And
(Hopefully) Their Cures
Imbalance that can be cured by static balancing.
Vibration of the wheel while driving is shown at
the right. At low speeds a wheel rotates around its
principle axis of inertia, its axle (blue line). But at
speed a static imbalance can force the suspension to
deflect so the wheel can follow another axis called
the axis of rotation (red line). Swing a bucket to
experience the force a static imbalance can generate.
wheel’s face or backside. For reasons
explained later, narrow wheels and
tires respond best to static balancing.
Now to dynamic balancing.
Increasing a wheel’s width
creates a balancing conundrum. It
potentially moves the imbalance
and counterbalance points further
away from the wheel’s centerline. A
counterbalance added to the wheel’s
backside to counter an imbalance
at the wheel’s face would statically
balance the wheel; however, that
likely won’t prevent a shake at high
speeds. In fact, doing that might
actually make the wheel wobble as if
it were mounted to a bent shaft.
Imagine a driveshaft to
understand why. If the tailshaft end
were imbalanced at 12 o’clock then
a counterweight added to the yoke
end at 6 o’clock would statically
balance it. However, as momentum
2
Imbalance that can be cured only by dynamic
balancing. Vibration of the spinning wheel shown at
the right. Imbalances away from the wheel centerline
can generate an axis of rotation (red) irrespective
to the axle itself (blue). When the axis of rotation is
close to a wheel’s centerline (left) the wheel won’t
wobble. But moving the imbalances apart (right)
moves the axis of rotation further away from the
centerline and a wobble is likely to follow.
52 rodandcustommagazine.com