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Author Topic: Prop shaft vibration  (Read 1989 times)
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davidwheeler
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« on: 30 May, 2012, 08:42:39 AM »

I was sure I saw a posting about this but cannot find it.
I have had bad prop shaft vibration on Old Boot for the past 47 years despite renewing or truing everything except the front shaft and finally got fed up with it a couple of weeks ago.  I jacked up the front wheels and fired her up and went into the pit to watch.  The front shaft wobbled merrily away though it was true at the front and rear, therefore it was bent.    I took it off and went down to my local engineer who put it on his lathe.   The dial gauge showed it to be 0.63mm out of true (i.e. 0.315mm away from straight or a bend of 0.8 degrees).   We tried differential heating without success so put it into the hydraulic pipe bender and gave it a cautious bend.  It sprang back bent as before so we put on more and more force until it bent about 15 degrees and sprang back true within 0.1mm.   
I reinstalled it and the prop shaft vibration is gone.      There is still some vibration (is there always on a Lambda?) partly because I am driving it much faster (!) and there is engine vibration despite the best efforts of Vibration Free.  I think some of this must be coming from the irregular sparks.   Now, where was that posting about rebuilding the distributor with a VW(?) shaft?
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David Wheeler.  Lambdas, Aprilia, Fulvia Sport.(formerly Appia and Thema as well).
GG
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Posts: 458


B20 s.2, Appia C10 unrestored


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« Reply #1 on: 30 May, 2012, 01:00:31 PM »

You have found the secret: most balancing efforts are trying to correct alignment issues. If the center of gravity for each piece and element were on the central axis from the engine to the rear, there would be no need for balancing. Typically the pieces (some part, a shaft, etc.) is off the central axis. So people put on balancing weights to "realign" that C of G onto the axis. Its much easier to put the whole assembly back into alignment.

On the Aurelia - the little splines on the end of the shafts (early type) are getting a bit off and thus throw things out of alignment. One shaft got back to .006" overall, and the vibrations went away.

Some background noise is a different matter, and more related to resonances from all the sheet metal. Dampening with mats, rubber, etc. can help that - or let it be.

In one B50 (very original), setup by a man who really knew what he was doing, you couldn't tell there was a driveshaft at all in the car. If you are interested, some of this is written up at: <http://web.me.com/geoffreyg/Aurelia_Tips_/driveshafts.html>

   
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B20 s.2, Appia C10 unrestored
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