Date:         Thu, 11 May 1995 13:28:59 -0300
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         smitht@jupiter.sun.csd.unb.ca (Tim Smith)
Subject:      Re: Help! Synchro Vibration

>
>The Synchro's had a slight vibration ever since, but it wasn't too bad until
>this spring, at which time I decided to pull it off & have it checked out
>more closely. I found another shop that found joints to fit, so I had new
>ones put in. The shaft vibrated worse, so I found another shop that could bala
>nce the shaft. Now it's a little worse--significant vibration from 30 - 50
>mph.  No vibration with driveshaft out, however, but maybe I shouldn't do
>that.
>

One thing to check, it's a feature of U-joints in any vehicle. If you 
misalign the yokes on _two_ U joints on the same shaft you set up 
vibrations! as simple as that. What is happening is that by its nature one U 
joint will cause a slight steady (sinusoidal) variation in speed of rotation 
of the output shaft. This happens in single joint driveshafts, as seen under 
regular trucks. The variation is small if the shaft angle is small.
         Two U joints in one drive shaft can perfectly cancel this out. The 
first joint makes the long shaft under your Syncro experience the variation 
in rotational speed, but the second joint cancels this out before it gets to 
your front diff. The Syncro shaft is aligned almost straight so I am 
surprised any major speed induced vibrations are occuring anyhow. Check that 
the yokes on each end of the shaft are aligned, ie. if the front Ujoint yoke 
(on the long shaft) is vertical then the rear Ujoint yoke on the same shaft 
should also be vertical.         A mis-indexed rubber coupling may be the 
culprit. They are there to help damp out and soften the effects of the speed 
fluctuations that the long shaft will be undergoing BTW. The shaft is pretty 
thin, low mass, so an imbalance may also be due to a big blob of 
undercoating stuck to the shaft, or a bent shaft. I figure you've checked 
this out. I had ice build up onto the shaft this spring, shook the whole van 
really violently, like a flat tire would! Keep us posted.  tim s.