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Re: Polyurethane bushings

To: "Lotus Cars Mailing List" <lotus-cars@lists1.best.com>,
Subject: Re: Polyurethane bushings
From: "Tim Engel" <tengel@isd.net>
Date: Tue, 2 Feb 1999 21:00:43 -0600
>From the chapman-era list:
02 Feb 99, Gene Carter, <Gene_Carter@avid.com> wrote:
>
> I have a friend with a racing Super 7 who is looking
> for polyurethane suspension bushings.  In particular,
> his car seems to chew up the A-frame rubber bushings
> that attach at the differential.  He doesn't want to go the
> route of spherical bearings because he's afraid that
> may put too much stress on other suspension parts.
> Does anyone know of a source for polyurethane
> (or some other than rubber alternative).  We've talked
> to Dave Bean Engineering and Lee Chapman racing.
> So far, we've struck out.  --  Gene Carter

Do not put a rigid or semi-rigid bushing in the center/apex pivot of a
7's rear A-frame.  Preferably not in any of the 7's rear suspension
pivots, but certainly not at the apex where it attaches to the axle
housing.   Urethane,  Delrin (DuPont trade name for acetal),  Nylon
(GE?  trade name for polyamide) bronze, steel...  they are all
verboten in apex pivot of a 7's rear A-frame.  It's been tried and it
invariably leads to cracks in the A-frame or in the bracket welded to
the bottom of the axle housing.  It can even lead to cracks in the
axle housing.  Picture experiencing a failure while braking hard for
the right hander at the end of the straight.

The axle and the A-frame are parallel in the static condition.  But
anytime the body rolls or one rear wheel hits a bump, the axle twists
with respect to the A-frame.  The A-frame basically stays parallel to
the body (side to side) while the axle stays parallel to the ground.
The rubber bushing deflects and provides the limited degree of
movement required to absorb that relative motion.  Without that
deflection, huge load spikes would be fed into the center pivot.

For a 7's A-frame center pivot, the only appropriate alternative to
the rubber bushing is a spherical bearing.  Ironically, the spherical
bearing your friend fears will over-stress other suspension parts is
actually the alternative that will save them.  Shock loads?  Well, the
big rubber-donut airbags out at the end of the axle will soften the
blows much more than that tiny little bit of 85 durometer urethane
ever will.

The same is true for all the bushings in the 7's rear suspension...
they all get twisted with body roll and one wheel bumps.  It's just
that the center/ apex pivot is the worst case and the first to fail.
If you elect to replace the rest of the rubber bushings, use urethane
in a softer durometer.  Something that still has some give to it.
Nylon is too hard.   Delrin is harder yet.   Metal bushings are out of
the question.

The front suspension of a Seven is more appropriate for plastic
bushings as long as everything is straight and aligned.  But, ANY
twist that finds it's way into the suspension arms can put huge loads
into the mounting brackets and the arms.  If you use plastic bushings
in the front of a Seven, Elan or Europa, make crack testing part of
your normal maintenance routine.   If you really want to do it right,
convert to spherical rod ends.

Any suspension that induces a twisting motion as it moves up and down
should avoid plastic bushings.  The Europa and Esprit rear suspensions
come to mind.  The trailing arms swing the hub carriers in an arc as
the move up and down.  That arc imposes a twist in the lower lateral
links.  The rubber bushings absorb the twist, but plastic bushings
will feed huge loads into the mounting brackets.  Before considering
using plastic bushings in any suspension, take a long analytical look
at it.  If there's any twisting motions, stay with rubber or go to
spherical bearings.

Nylon (polyamide) was probably the first polymer bushing material.
Primarily because it was invented first.  It's a large family of
resins and some are tougher than others.  If you are considering
buying bar-stock and making your own, talk to a knowledgeable tech or
sales person at a plastics supplier for a recommendation.  Look in the
Yellow Pages under Plastics.  To reduce friction, various additives
(molybdenum, Teflon, graphite, etc.)  are compounded into nylon.
Nylatron is a trade name for one such nylon designed specifically for
bushings.  Ask for Nylatron by name and you can't go too far wrong...
IF it's a Nylon you want.

Delrin came next.  It's harder than Nylon and has a lower coefficient
of friction... that was it's main advantage as a bushing material.
It's also available with various anti-friction additives to further
enhance it's slipperiness.

DuPont has ST (Super Tough) versions of both Nylon (Zytel is DuPont's
trade name) and Delrin.  The ST versions tend to be a little softer
(still feel hard to your hand) and have higher coefficients of
friction.  But they are much tougher than the straight grades.  IMHO,
your ahead to use the ST versions of Nylon and Delrin and then rely on
a modern silicone grease for anti-friction.

Polyurethane came later, but is not the preferred material for
automotive bushings.  It's main claim to fame is much improved
toughness, excellent wear resistance and a wide range of durometers
(hardnesses).  It's coefficient of friction is higher than the other
two resins so it needs to be assembled with a silicone grease.
Without a good silicone grease, PU is too sticky and causes the
suspension to bind.  Don't take an easy out, source a good grease.
PU is very difficult to machine, but it can be done with patience.
Freezing it first helps.  There are a number of companies making cast
PU bushings for automotive applications.  The only one that comes
immediately to mind is Energy Suspensions.  They have a full line
stock and OEM-replacement bushings.

If you can find some UHMWPE (Ultra-High Molecular Weight
PolyEthylene), it's also very good for bushings.  It's tough as nails
and wears like iron and is slippery.  Poor man's teflon.  But it's a
real pain to process into shapes like bar-stock (it doesn't extrude
for beans and basically has to be forged into shape), so not every
plastic supply house carries it.

If you make your own bushings, size the bushing to be a light press
fit into the suspension arm.  A heavy press fit is a waste of effort
since any plastic will deflect over time (creep) to relieve the
stress.   Use a steel sleeve in the center bore that is a light
finger-press fit and flush to a few thousands longer than the bushing.
The best source for the sleeve is from the center of the old rubber
bushing you're replacing.   Grease the sleeve and the center bore of
the bushing.

When you torque the mounting bolts, they clamp down on the sleeve and
hold it stationary.   The bushing, being a light press fit into the
suspension arm, moves with the arm and rotates/ slides on the sleeve.
That's completely different than the way a rubber bushing works.  In a
rubber bushing there is no sliding motion and the rubber twists to
allow motion.

Good luck,
Tim Engel







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