Tom, very good explanation of the failure mode! It would seem that a "fix"
might be to support the end of the pin to change it's end fixity from
cantilever to simply supported. This might be done with a plate that fastens
under the pin with a support for the pin at each end. This is probably not
clear, but I could sketch one up and pass it along.
mayf
----- Original Message -----
Subject: Fulcrum Pins
> 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
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