I would like to give everyone a status update on the spacer project, as we
had hoped to go into production this week. I have been following the various
inputs and working with FOT members and engineers off-line as well. Frankly,
I am having some concerns about product liability from failures resulting
from or appearing to result from the change. There are two failure modes
that are of concern, breakage of the stock spindle due to the newly induced
tension forces, and breakage of the spindle due to inadequate bearing
clearance.
Regarding the first, the issue is that the design stiffens the spindle by
clamping a larger diameter spacer between the bearings. This transfers the
bending forces in to tension forces. While this approach has been used
without issue in other British cars, and most seem to agree that the spindle
itself can take the load, I worry about fatigue at the threaded portion at
the end. It is the smallest cross section of area and is further reduced by
the flat for the D washer. It also has threads with sharp root radii (the v
between the threads) which is a stress concentration area. All of this
combined with long service stock axles or even newer ones from who knows
where makes me nervous. We are working on an uprated spindle as a follow on
with the original intent of further stiffening the assembly. while I am
still trying to determine the stiffening effect, one thing I am sure about
is the new design will reduce the possibility of tensile spindle failure. We
plan to make the spindle from heat treated 4340 which will increase the
yield point in tension. I also plan to take the thread up to .625 dia, the
diameter of the smaller end of the spindle. As in horsepower, there is no
substitute for cubic inches (square inches of cross section actually in this
case). I plan to eliminate the flat, and the threads will be J type meaning
the have a rounder root radius to reduce stress concentration (common
aerospace application). I am investigating rolling the threads in as well
instead of cutting as this is the strongest approach (think forging). The
1.000 inch shoulder at the big end will also e extended as we have found the
Southwick hub bearings extend past the current shoulder, leaving no place to
self-locate for the spacer. This should strengthen the part in bending as
well, and we will strive for smooth transitions everywhere to reduce stress
risers. Finally the harder material will reduce the deformation under the
inner races of the two bearings on the spindle, which appears to be common.
I am giving serious thought to delaying the spacer production until we have
the spindle available, or even requiring then to be purchased together. It
is unfortunate, but we live in litiguous times and my small company could
not even fight a lawsuit much less withstand a judgement. Product liability
insurance would blow the pricing out of the water, and requiring a waiver is
not an airtight protection. Some of you might think I have a profit
motivation in bundling the parts, but the reality is I am concerned about
your safety and my liability so I hope you see that.
Regarding the second failure mode, Kas points out that too tight bearing
clearance can heat and seize the bearings and break the spindle. It would be
tough to design a spindle impervious to this treatment. If we proceed this
will be up to the user to do it correctly.
In addition to these issues, strengthening the weak link moves the forces
elsewhere, potentially to parts that can handle it or potentially to ones
that can't. This is inherent in racing and modifying but you should realize
it. Hubs, wheels, studs , A arms and other bits should be regularly checked
for cracks and deformation by a qualified source, even if you don't make any
changes. Conversely, finding you have no brakes at the critical moment is
not a great thing either.
The bottom line is that we are delayed while sorting through the spindle
design and pricing and pondering the legal implications. If we choose not to
proceed, I wil gladly supply the data to anyone who wants to fab their own,
with Uncle Jacks permission. If any of you who have ordered are having
second thoughts based on the concerns raised here or in the thread, please
feel free to back out at any time, no questions asked. There has been a
tremendous response to the project and many of you are anxious to install
before your next race but I ask for your patience as we sort through it.
Mark
72 TR6
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