List,
A couple of days ago I posted the following question:
>What is the difference between polyurethane bushings and stock (black
rubber) bushings? Is one better than the other? If so, why?
>
>On the one set of polyurethane bushings that I bought, the cost was higher
than stock bushings. Is this common for all polyurethane versus rubber?
I purposely did not tell "The Rest of the Story" at that time because I
wanted to see if my conceptions were unique to me or not.
This is the rest of the story (and what I learned from several VERY helpful
individuals who replied). About a month ago I asked the list for suggestions
to repair my steering rack that would slip every once in a while on my 74
TR6. Almost everyone suggested new bushings. So after considering the
different options I decided on polyurethane. I was surprised to find that
these new polyurethane bushings were softer than the black rubber bushings
that I took out. Everyone always described the polyurethane as being harder.
Being a professional test engineer I put each bushing on the test machine
and measured the load and displacement. What I found was that the modulus
(hardness or spring rate if you want to think of it in those terms) of the
old rubber bushing was 1/3 higher than the polyurethane
One list member pointed me to the SuperFlex web site to learn more about
polyurethane bushings. What I learned is that there are several benefits to
polyurethane other than just stiffness. They advertise that they make
bushings in several different hardnesses depending on the application.
I believe that polyurethane can be fabricated to a wide range of hardnesses
(probably over the same range that black rubber can be fabricated). Everyone
has seen black rubber as soft as an inner tube and as hard as an antique
child's toy. So the bottom line is, if you buy polyurethane because you are
looking for a stiffer suspension, that is good as long as the particular
manufacturer that you are buying from makes its polyurethane bushings harder.
They could be softer like my steering rack bushings.
By the way, it turns out that the problem that I had with my steering rack
was the CO! Everyone knows what a DPO is. Well, I've had my TR6 so long now
and have worked on so many different parts of it that I am now my own DPO or
CO (Current Owner). Several years ago I had the rack loose when I had the
engine out rebuilding it. When I put the rack clamps back on, I let the
bottom plate (bend up ledge) get under the ears of the clampdown plate. This
kept the clamp plate from tightening down the way it should have. In fact
when I later torqued the nuts tighter to try to tighten the rack, I bent the
ears of the top clampdown fixture. When I took the parts off to replace the
bushings one of the ears fell off.
One other thing I learned about bushings is that nylon and polyurethane are
not the same. That's obvious to even a second grader when I state both words
in the same sentence like I just did. But for some reason I had three types
of bushings in my head; rubber (stock), metal (brass etc), and harder than
rubber materials (I didn't think of nylon and polyurethane as being
different).
That is my lesson for the day. I hope that sharing it will help others.
Creig Houghtaling
74 TR6
St. Louis, MO
PS Would anyone have a steering rack clampdown bracket that they would like
to sell. Mine with one ear bent and one broken off works but if I could find
one that was right, I'd replace it.
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