Persistent vibration, 87 5KQT

[Peter Golledge]

John,

I've done a bunch of driveshaft work on Audi's and never bothered with alignment other than marking the bracket location
prior to removal.  The "get the driveshaft within 0.X mm of spec" smacks a lot of some documentation writer translating
the engineer without much insight into the "real world".  Consider how much the body flexes during operation and the
whole alignment thing seems ludicrous. :-)  I'd got a 5000 body which is quite bent from 50mph snowbank adventures
(including this year's steamboat near roll), I'm very sure the driveshaft position is in no way close to the original
specified and it is quite vibration free.

I'd bet the yolks were assembled 180 deg from their original location.

Cheers


On 2/12/2011 4:08 PM, john at westcoastgarage.net wrote:
> After a long break, necessitated by family priorities involving my aged
> parents, I'm back on this car.  I thank all who replied last fall when I
> first put this up.  I've looked at the bearings, swapped in a set of
> known good wheels and tires, and looked again at the driveshaft.  Here's
> a further question:  My home made fixture aligned the driveshaft to be
> straight.  The manual doesn't specify an angle, it just says to use the
> factory fixture.  Is the shaft supposed to be straight?  Or, is there a
> specific angle at which to set it?  Another thing.  Prior to the noise,
> I had the center support bearing, the support, and the center U-joint
> replaced at a local driveline shop.  Is it likely I need to take the
> shaft back to them for redo?  Maybe I should drain the rear diff and
> inspect the magnet for debris from a failing bearing.
>
> Recap:  What's the optimum angle for the driveshaft halves, and could
> that be the problem (changing it from just being thrown in to 0 degrees
> made no difference)?  Or, should I send the driveshaft back out? I
> suspect the driveshaft, or its alignment, to be the root of the issue,
> because it all started when I replaced it with the locally reconditioned
> unit (and the 034 sourced center support).
>
> Thanks again for your time!
>
> John
>
>
> On 10/31/2010 10:45 AM, Louis-Alain Richard wrote:
>> To my ear, this is either the propshaft u-joint or the center bearing, or
>> maybe the gearbox output shaft bearing. Might as well be the rear diff input
>> shaft bearing. Maybe you can hear them with a stethoscope while the car is
>> on the lift ?
>>
>> Louis-Alain
>>
>>
>>
> -----Original Message-----
> From: John Larson<john at westcoastgarage.net>
> Sender:quattro-bounces at audifans.com
> Date: Sat, 30 Oct 2010 18:41:13
> To:<quattro at audifans.com>
> Subject: Persistent vibration, 87 5KQT
>
> I'm about at the end of my rope.  I have a customer's well worn, high
> mileage, 5KTQ here that has developed a low pitched humming vibration at
> freeway speeds, although the noise persists as low as 30MPH, maybe even
> lower.  I've checked the axle shafts for bends, replaced a bent RR
> wheel, and aligned the driveshaft with a cool home made fixture.  I
> spoke with a chassis and alignment guy this afternoon (he's gonna drive
> it Monday) and he asked about the wheel bearings.  I've done probably
> 500 double row wheel bearings in my career, and I've never had one sound
> like this.  They mostly howl or make a low frequency rumble, and usually
> (but not always) change tone or volume when side loaded by turning the
> car one direction or the other.  This noise comes on as you build speed
> on the freeway, and is there whether you're going straight or turning.
> The volume, tone, and frequency appear to remain pretty constant.  As
> always, any and all constructive input would be sincerely appreciated!
>
> TIA!  John
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