On 12/6/13, 7:54 AM, John Hasty wrote:
> For what it is worth, we got 10 race weekends on our last rebuild running a
> damper from BFE. The crank broke between # 1 rod and front main on last
> race, last lap, last turn while running 1st. in EP. I regularly turned 6500
> in 2nd, 3rd, 4th and 6200 in OD. The motor always ran smooth with no
> vibration I could feel even with the front engine plate bolted directly to
> the frame.
>
There is a nasty harmonic a little over 6000 RPM that affects the crank
towards the middle. It is probably the one that took out your crank.
The commonly sold harmonic dampener was picked to tame the harmonic at
around 5200 RPM that tends to break the crank at the flywheel mounting
flange. Which is why the stock engine is red lined at 5000 RPM and a
number of people have added harmonic dampeners to rev higher.
The story I once heard was that some folks had access to British
Leland's crank test sheets that included harmonic testing. They
compared other cranks to the TR3/4 crank to find one with the closest
harmonic match. They choose the harmonic dampener for that crank to use
on TR crank to tame the 5200 RPM harmonic. The harmonic dampener that
is commonly sold for the TR3/4 crank was specifically chosen because it
had the correct characteristics to tame that harmonic.
That leaves me wondering if the other harmonic dampeners have been
harmonic tested with the stock TR crank to see what it does to the
harmonics? I'm assuming that different weight and diameter dampeners
affect harmonics differently. Just wondering
TeriAnn
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