Since my name has been thrown out a couple of times in relation to the
fulcrum pin situation, I thought I should respond at least to level the
field and separate fact from fiction or hearsay. To begin with, I am a
Mechanical Engineer by training and work experience although I will be the
first to admit that I am not PhD level. I am registered with the state of
California as a Professional Engineer in both the Mechanical and Nuclear
Fields. Secondly, I am familiar with several of the failures of fulcrum
pins on Tigers. The evidence is clear in all cases that the failures are
cyclic fatigue failures. The primary cause of the failures is also clear
but there is no justification in pointing fingers at any individuals or
groups at this point in time.
Essentially all designs are by nature compromises of one sort or
another. This is certainly true of the creation of the Tiger. The
geometry of the Alpine steering was not compatible with the installation of
the Ford engine. The steering compromise was first applied at Shelby's
shop in the creation of that prototype. They looked at the situation and
they did what was possible within the physical and financial constraints
they had. Rootes engineers also looked at the situation and essentially
came to the same conclusion. The cost to re-engineer and re-tool the
steering and crossmember to eliminate the Ackerman situation would have
prevented the car from going into production. So looking back after 35
years, I don't think their compromise appears that unrealistic.
So here we are today with fatigue failures starting to increase in
frequency, and we wonder what we should do. I need to address the few
Pins, manufactured by QH in the early eighty's as replacements for
Alpines. They are very different in that the manufacturer turned both the
bushing area and the location notches in a lathe. I am confident that this
was a simple cost reduction decision by production personnel and that no
knowledgeable engineer ever reviewed or approved this change. Suffice it
to say that this method of manufacture is the worst possible design
scenario and any such pin in use is doomed to a very early failure. If you
have them, take them off.
The normal OEM fulcrum pins typically fail at the reduction in diameter
from 7/8" to 5/8" at the rear bushing. They fail at this location because
of the combination of the cantilever design, poor Ackerman geometry and the
stress risers that the change in pin diameter cause. The stress in the
pins in this area are highest when the car is backed up in a turning
situation. If you observe a Tiger in this situation, you will find the one
of the tires is being dragged almost sideways. This reverse turning
activity will be the highest normal loading outside of hitting an object
like a curb or a rock. That is why the pins fail so often when someone is
backing into a parking space.
Unfortunately you can't count on the fact that the pin will fail only at
this time. Every time the car is backed up, autocrossed, etc., a peak
stress cycle occurs. Gradually cracks begin to form and the direction of
cracking has been observed and is always almost vertical. They cracks
begin form on the inside of the pin and on the outside, gradually working
their way toward the center. Each cycle that loads the pin beyond the
yield point increases the propagation of the cracks. Eventually the pin is
weakened to the point that the next load applied results in a braking
failure, and the rear of the A-arm is no longer constrained and is free to
move. Obviously, powerful engines and wide sticky tires only accelerate
the impending failure.
Another factor related to this situation is the bushing material. As Dave
Johnson has pointed out, stiff plastics will also tend to accelerate pin
failures. The stiffer the material, the more concentrated the load will be
in the pin. Dave also pointed out that his first failure was caused by the
formation of rust on the pin due to lack of lubrication. You can imagine
how much faster the pin would fail if you add the torsional (twisting) load
every time the wheel goes up and down to the longitudinal loads from the
Ackerman situation. This is why I advocate drilling the A-arms for grease
fittings when replacing all but rubber bushings. While Dave is correct in
pointing out that stiff plastic, Delron in his case, will accelerate
failures, several failures have occurred on cars with OEM rubber bushings.
We now understand that there is a new risk associated with driving our
cars. The problem is that detection of the symptoms, the initial cracks
that start to form on the pins, not only requires a total tear down of the
lower A-arms and fulcrum pins, but expensive non-destructive testing to
made the potential cracks visible to the eye.
There are only two current manufacturers of replacement pins for
Tigers. One is Doug Jennings, and the other is Dale Akuzsewki. Doug's
pins accept the currently available Urethane bushings and Dale's are
totally custom, with integrated pins and stiff plastic bushings. Either of
them seem to be functional replacements. Because of my personal
interests, I have developed a new design for the fulcrum pins that are
compatible with Rick's new Urethane bushings. My concept was to increase
the diameter of the pin to match the increased ID if the new urethane
bushings. This change essentially doubles the sectional area in the
location of the loading. I have had prototypes on the road for 3 months,
manufactured from a carburized steel alloy (recommended by a PhD
metallurgist familiar with the historic failures) which makes the outside
of the pins very hard with a more ductile core. They are NOT for sale. I
would be happy to give my design to anyone that would care to accept the
liability cause I'm not about to. Otherwise they are for family use only.
I hope that this sheds some light on a touchy subject.
Tom Hall
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