Subject: [TR] TR4A crankshaft extension

From: [email-masked]
Date: Mon, 6 Jan 2014 21:28:26 -0600

Hello List,

Am considering removing the stock fan and going with an electric fan on my 4A.
There is a "Fan Eliminator" kit available that bolts on in front of the
pulley, thus eliminating the fan and the extension.  My question concerns what
I have read on "harmonic damping".  My TR restoration book says that if the
fan is removed the crankshaft extension needs to be retained because it
provides harmonic damping for the crankshaft.  I don't get it.  In my
pedestrian mind, seems like less mass in front of the pulley is better.  Can
anyone shed a little light?  Thanks.

John DeLuca

** [email-masked] **

Archive: http://www.team.net/archive

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From: Randall <[email-masked]>
Date: Tue, 7 Jan 2014 3:53:10 +0000

---- [email-masked] wrote: 
> In my
> pedestrian mind, seems like less mass in front of the pulley is better. 

As I understand it, the issue is that the crankshaft has a resonant frequency (actually several of them), much like a bell does.  At just the right rpm, the pulses from the cylinder firings match the resonant frequency and cause the crankshaft to "ring" and twist/flex farther than it would normally.  Keep that up for long enough, and the metal fatigues and breaks.

The damper works much like holding your hand on a bell, and eliminates the ringing.  If someone had been touching the Liberty bell, it might not have broken <g>

That said, I've covered over 200,000 miles without a fan or damper; and never broken a crankshaft.  I have seen broken crankshafts, but always in racing motors that made considerably more than the stock 100 bhp and turned more than 5000 rpm.  Perhaps it's more of an additive problem.  Or maybe I've just never run for long enough at a "problem" rpm.  I've heard it said that the first problem is around 5200 rpm, and I don't drive up there very much :)

Randall

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From: "Andrew Uprichard" <[email-masked]>
Date: Tue, 7 Jan 2014 09:17:22 -0500

When I put rack and pinion steering on my TR3s it requires removing the
extension and installing an electric fan.  Never had a problem.
Andrew Uprichard

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]] On Behalf Of
[email-masked]
Sent: Monday, January 06, 2014 10:28 PM
To: [email-masked]
Subject: [TR] TR4A crankshaft extension

Hello List,

Am considering removing the stock fan and going with an electric fan on my
4A.
There is a "Fan Eliminator" kit available that bolts on in front of the
pulley, thus eliminating the fan and the extension.  My question concerns
what I have read on "harmonic damping".  My TR restoration book says that if
the fan is removed the crankshaft extension needs to be retained because it
provides harmonic damping for the crankshaft.  I don't get it.  In my
pedestrian mind, seems like less mass in front of the pulley is better.  Can
anyone shed a little light?  Thanks.

John DeLuca

Archive: http://www.team.net/archive
http://autox.team.net/mailman/options/triumphs/[email-masked]