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Fulcrum Pins

To: "Tiger News Group" <tigers@autox.team.net>
Subject: Fulcrum Pins
From: Tom Hall <modtiger@home.com>
Date: Sun, 14 Oct 2001 16:15:26 -0700
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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