Subject: Polyurethane bad??

[RonHorowitz <RonHorowitz@domain.elided>]

Jon,

This is a case of a little knowledge being a bad thing. For the specific
examples described in the article, what's said is correct, but not
necessarily relevant.

If your suspension is suited to poly bushings, they'll work. If it's not,
they won't. It's not a "one size fits all" situation. Understand that in
many cars, the geometry of the suspension comonents is such that if all the
bushings were solid bushings, the suspension would bind up and not move at
all. Probably the best example of this is the Ford Mustang "quadra-bind"
rear suspension.*  This is easy to visualize, but difficult to explain! It
can happen in control arms if the centerlines of the pivots don't lie along
the same line, or because of non-parallel radius rods and so forth. If
you're using soft rubber bushings, the rubber bushing can deflect around
and accomodate all of this.  In any suspension design where the geometry is
"correct" for solid bushings, binding is a non-issue. In some instances,
like with non-parallel radius rods, Heim joints may work even if solid
bushings won't. It just depends.

Re the issue of lubrication, poly needs to lubricated, yes, so what's new?
(Anybody remember kingpins?) The better poly lubes work just fine, so it's
not a problem. IMHO, squeaking is a result of improper design, assembly, or
lubrication.

Part three: if you're lowering your car, a bonded rubber control arm
bushing will be deflected from its "normal" rest position at ride height. A
non-bonded poly bush just spins around and doesn't care whatthe rest
position is.

Part four: Harshness and vibration _tend_ to increase as the bushings get
more rigid, true. If you want more "road feel" this may be welcome, if not,
there may be workarounds such as making relatively thin, compliant elements
to fit at the leading edges of the control arm bushes, or by making
co-axial hard / soft bushes, or whatever. Just depends.

* (As an aside, it's been said that the Mustang's rear suspension was
designed like that because the early transmissions couldn't handle the
torque. Ford designed a rear suspension that would break traction instead
of transmissions! Since the "typical" Mustang owner tends to be somewhat
unsophisticated, poor suspension design and loss of traction under
acceleration was simply mistaken for higher power output and proof of
manhood. Don't know if it's really so, but that's what they say....)
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