Brian
Its not possible to calculate this with any accuracy because of the damping
which is inherent in the system however it is possible to determine exactly
where the torsional natural frequency is by instrumentation and analysis.
There are vibration consultancies who could do this but I'd suggest that an
approach to the engineering faculty of a university may do the job. This is
not difficult but it does involve some instrumentation on the nose of the
crankshaft and is done with the engine running.
If you elect to try this route it is a good idea to have a couple of
alternate dampers (one of which should be an elastomeric type) available to
test so that you have an idea which way they are moving the torsional
natural frequency.
Jim Runciman
Jim.Runciman@donhad.com.au
Phone:08 9279 7611
Fax:08 9279 7173
-----Original Message-----
From: Brian Evans [mailto:brian@uunet.ca]
Sent: Tuesday, 11 July 2000 10:05
To: Mark Palmer
Cc: vintage-race@autox.team.net
Subject: RE: Spridget 1275 crankshaft
Thanks for the info - sounds like you have done the research! That being
so, I have a few questions. BTW, the original poster was the guy breaking
BMC cranks - I'm currently running Ford based engines. I ran A series a
few years ago, and like you I ran essentially stock cranks, well prepped.
The question: I'm running a stroker version of a TwinCam bottom end. I
think one of the problems is that a critical vibration harmonic is
occurring right in the center of my power band, while in motors with
shorter strokes the harmonic happens at a higher RPM than the motor turns
to. From what I've read in books, the FFord has the same harmonic problem,
occurring at about 6800 - 7000 rpm, and they say that FFords that are run
there a lot tend to fail the cranks faster than FFords that don't.
Here's the stroke data: I'm running 82.2 mm stroke, giving 1800 cc (the
engine was designed for the 2 litre sportscar class, in case anybody shouts
"cheater"!). The Twin Cam, apparently safe to well beyond 8,000 rpm, has
about a 72 mm stroke. The FFord, which I've heard has the harmonic around
7,000 rpm, has a 76 mm stroke (I think). Any thoughts on where the
critical vibration is with my stroke of 82.2 mm?
Thanks to anybody who can answer this!
Brian
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