tigers
[Top] [All Lists]

Re: Fulcrum Pins

To: "Tom Hall" <modtiger@home.com>,
Subject: Re: Fulcrum Pins
From: "DrMayf" <drmayf@teknett.com>
Date: Sun, 14 Oct 2001 17:24:21 -0700
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

<Prev in Thread] Current Thread [Next in Thread>