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Re: [Tigers] Fulcrum pin survey

To: <tigers@autox.team.net>
Subject: Re: [Tigers] Fulcrum pin survey
From: "Rense, Mark (GE Indust, ConsInd)" <mark.rense@ge.com>
Date: Wed, 11 Feb 2009 12:01:38 -0500
Both my cars have had the cross members rebuilt and up-graded by Doug
Jennings, replacement of the fulcrum pins was include. The lower pins on
B382001465 were clean and looked normal when they came out and showed no
sign of cracking under both radiograph and die penetrant tests, I kept
them as spares. This car only had 48K miles at the time.

Both of the lower pins that came out B382000991 were bent slightly
inward on the rear minor diameters. This car was autocrossed in a
previous life, so I expected that. Although these pins were bent,
neither had started to crack, indicating the material choice was a good

I've been a Mechanical Engineer for 30 years now, and I have had no
small experience in the reliability and fatigue sciences. I do not know
what the exact material Lord Rootes chose for his pins, but I'll wager
it was a medium carbon cold rolled steel. In industry, 1045 CRS is the
most common drive shaft material, used in gearboxes, motor shafts and
the like. That type of steel material is fairly cheap, machines well,
can be surface hardened and has enough strength to withstand cyclic
stresses. It is also is ductile enough to allow some yield before it
begins to crack.

The forces that create failure modes come in several varieties. There
are elastic lateral displacements due to bending moment and transverse
shear, and there are angular elastic displacements from transmitted
torque. Also, the way the energy is transmitted to the part will cause
different types of failures. Impact loading, such as when your Tiger
hits a curb while backing up at full lock, can cause a pin to fail
instantly, as the impulse energy will overcome the yield strength of the
material at the weakest point and cascade into a rapid failure. Slow
cycling, such as normal suspension movement combined with braking and
steering, can cause work hardening until the fatigue limits are passed
and the material yields and fails through crack propagation. It might
take years to reach the critical point, but the failure will take a few
milliseconds to complete.

I believe all the pin failures are due to fatigue failures caused by
cyclic forces and/or by abnormal high static loading. Big, fat, sticky
tires combined with four-piston brakes will put an order of magnitude
more forces into the suspension over what the original designer
intended, and those forces are resolved through the fulcrum pins. The
fact that many pins are found to be bent indicates that the original
material choice was sound. Sudden catastrophic failures are due to the
material failing at one spot and rapidly propagating. That initial
failure spot could have been a tool chatter mark, a gouge, a squared-off
shoulder, or even a chunk of carbon in the steel. Bushing material and
fit also contribute, a hard bushing transmits higher and more
concentrated energy spikes to the pin.

So, what does it all mean? Well, fundamentally, the original design was
flawed from the onset, and most of us have exceeded those design limits,
repeatedly. Every pin will fail eventually given enough time and miles
driven. Pins made correctly out of higher strength material will help by
increasing the yield strength of the material, but if you're doing track
days and running sticky R8s and a stroker motor, I would still suggest
you check your pins often.

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