Urethane bushings

["Bain, Bill" <Bill.Bain@xxxxxxxxxxxx>]

I have the "AlfaBill specials" in both ends of the trailing arms, as
well as the trunnion bushing and the front caster bushings.  I think
that there is a bit of a misunderstanding about modern polyurethane.
Like just about any plastic, it can be made in just about any hardness
and elasticity.  Andy Kress (Perfomatek) states that the urethane
bushings he sells are about 15% harder than the OEM rubber ones; the
Superflex ones that I installed were not all that much "harder" than the
OEM ones so that sounds about right.  Delrin, etc. are different breeds
altogether and even older poly bushings were much harder than modern
ones -- I helped a friend replace some 8 yo poly bushings in his 'Vette
-- they were much more like Delrin than the poly I put in my Spider.

Poly bushings are sold as a "performance" upgrade, but their real value
is their longevity -- poly is *much* more resistant to environmental
degradation than rubber.   For the front of the trailing arms, for
example, that would be a factor since they, particularly the driver's
side, take up the torque on take off quite directly and excessive
deformation will cause the car to track left on acceleration since the
wheelbase is now incrementally shorter on the driver's side.  Simply
put, poly will give more consistent performance over a longer period
than rubber, but how "hard" it is a design factor, not an inherent
property of the compound.  I would suspect that most of the reason that
production cars don't routinely use more poly is one of cost.  In the
case of an Alfa, most cars we see were designed and built before modern
poly compounds were widely available so rubber was used since there were
no alternatives that would meet the service life requirement and cost
mandate.  However, even Alfa used poly as replacements -- the OEM
replacement rear swaybar to body mounts I obtained to replace the
original rubber ones were poly.

From that aspect, it's important to keep in mind the distinction between
a race car and a street car, even one that's a track toy.  Not all
racing practice translates to the street, nor should it.  Colin Chapman
allegedly built race cars to literally fall apart as the crossed the
finish line -- a race car is not intended to last for 100K miles with
minimal service.  By the very nature of their use (you're supposed to be
either off or at 10/10ths), race cars are maintained in a very different
fashion than a street car so bushing checking and replacement would be a
regular item (perhaps every winter?).  Moreover, if the suspension
builder has tuned the suspension for rubber, then that's what you use
since the object is to maximize the ability to drive at 10/10ths. In
some cases that's because the OEM parts are perceived to be more
consistent in their manufacture than aftermarket parts so the tuning
would not be upset by a straight replacement of a part.  No offense to
the racers out there but even a track toy/street car is not at 10/10ths
for most of its life, and I'd prefer not to have to take the suspension
apart on a yearly basis, even if I were driver enough to wring that sort
of performance out of the car (which I'm not).

Poly, judiciously used, is an improvement over rubber.  I think that the
front of the rear trailing arms and possibly the trunnion are good
examples, and would likely not impact the handling in a negative fashion
-- their function is much more to hold things in place in vertical
travel which providing some cushioning, unlike, say, swaybar end links
which are subject to twisting to lever the car body.

YMMV, My $0.02, all the usual disclaimers apply. 

Bill Bain 
AROC Atlanta 
'83 Spider (garaged -- it's raining) 
'87 Milano (daily driver) 
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