From: jimo@navaho.uucp (25656-o'keefe)
Subject: Vibration (drivetrain modification)
Organization: Bellcore, Livingston, NJ
Date: Thu, 10 Sep 92 16:07:14 GMT
Keywords: auto, repair, vibration, modification


This is follow-up to a problem that I posted several weeks ago!

The problem I posted was:

I have an 87' Chevy G20 Conversion Van, 3 spd trans, 7.5" rear.  I
experience vibration both on acceleration to 25 MPH (increases with
torque) and above 45MPH (continuous; even when coasting in neutral).
This vibration is felt thru the entire van.

((( Thanks to all those who offered there good suggestions )))

Tests and repairs made to isolate the fault:

- Dynamically balanced all tires (tires/rims inspected for runout)
- Trans test-driven and inspected
- Replaced one of the rear outer bearings (the noiser one)
- While the rear was open, visually inspected diff. and both axles
- Replaced drive shaft (split-shaft) support bearing and all u-joints

** The vibration persisted after all above was done and checked **

Problem isolation and temporary work-around:

((( sorry for the long story )))

Tuesday (9/8), I took the van to a drive shaft specialty shop to have
the drive shaft inspected and dynamically balanced.  The mechanic
checked both shafts for balance and run-out and said that my drive shaft
was good (although he noticed a slight run-out on the small shaft).  He
felt that the vibration was NOT due to the shaft but rather, the two-piece 
drive shaft was AMPLIFYING some excess play in the rear.  He recommended
that I allow him to fabricate a new ONE-PIECE drive shaft because this
would isolate the van chassis from the drivetrain.  I asked him if this 
would cause ANY problems and he said no.   But when he began to install
the new shaft, he discovered that the new shaft (larger circumference)
wouldn't adequately clear the cross-member (where the support bearing is
bolted).  He wanted me to agree to let him cut a half-moon opening in
the cross-member to provide enough clearance.  This, of course, would
have effectively and permanently prevented me from going back to the
original two-piece design.

Rather than rush into this decision, I asked him to let me drive with
the new shaft for a few days.  I've driven it for two days now and have
confirmed that the one-piece HAS eliminated all of the vibration.  I must
decide soon because the shaft clearance is too small to accomodate a
full load (6 people) and road bumps.


The dilemma:

I asked him to fix it; and he did; but the fix may cause me more
problems down the road.  One problem, I already see, is that the rear
trans seal is leaking pretty bad now (it had been leaking somewhat since
I've had the drive shaft rebuilt earlier.)  I understand replacing this
seal is relatively easy but will the one-piece shaft continue to put an
excessive load on the trans due to the spindle travel?  He gave me my
old drive shaft, so that I can go back to original design but he
insists that the vibration will return and his one-piece is my BEST fix.
I'm now confident that the vibration is coming from the old shaft and
may be due to the rear-end play and/or the old shaft.

Question:

Which way would you go?  Keep the one-piece, cut-out the cross-member and
replace the trans seal?  Or, try used driveshafts and/or have the rear
rebuilt?  I realize this could be thought of as an issue of stock vs
custom.

All thoughts welcome, but opinions from any GM experts or people 
with similar conversions would be appreciated.

Thanks
Jim O'Keefe
(908)699-3810