So did you figure anything similar out for the 4 cylinder cars?
-----Original Message-----
From: A.J.Wood [mailto:wood@dandrade.freeserve.co.uk]
Sent: Thursday, September 25, 2003 4:24 PM
To: friends of triumph
Cc: kaskas@cox.net
Subject: 6-Cylinder crank damper
Amici-
Thank you for the enquiries - briefly, we suffered a major failure on a
new steel crank after only 4 hrs track time, several years ago, and
decided that something needed done. After some research, we found that
there is a problematic 3rd harmonic torsional vibration on pretty much any
inline 6-cylinder motor. The 6-cylinder Triumph motor is especially
susceptible since the crank has hardly any overlap on the journals, hence
is "floppy", for want of a better term. At 7200 rpm, the crank is twisting
itself into a frenzy; the original elastomer damper cannot cope with the
forces, so something has to give way. The first victim is the flywheel,
normally as a result of an early downshift, leading to failure of the
flywheel fasteners. So we go 8 bolts, dowels & so on. The flywheel then
stays on. Then the rotor arm fails. Then the cam drive falls to bits in
various ways - cam sprocket drops off with the snout of the cam, or, in
extremis, the chain breaks - seen it , it's true!. The tensioner smashes,
and even valve springs start to fail. I've seen one motor where the engine
backplate decided to part company with the engine.
OK - it's ad break time - my kit allows either original or billet cranks
to rev through the critical rev point without risk of destruction. It's
easy to fit, but the motor needs to go backward in the frame by around
10mm, and there's some adjustment of pulley offsets to do. If you're
unlucky, you got a long drive shaft, which'll need shortening by a bit.
Most are OK. Kit costs #525 - I will be able to reduce this to FOT if we
get a few orders. Steel crank = $3000 minimum - I think it's worth it, due
to lack of collateral damage...
Final puff- the TR-6 I drove at Mid-Ohio, which set fastest lap of the
week (with a fat rusty old tub behind the wheel), has a standard,
unlightened crank in it, wearing said kit. The telltale said 7*00..
Please e-mail or whatever - I already posted the flywheel bolt specs &
so on a while back.
Roll on VIR!!
Jon Wood
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