RE: Low grade rumble at idle when center diff locked

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

> Thanks for the reply. Still trying to digest the info in an effort to
> understand hoe the diff lock actually works.

A differential is used to allow power to be transmitted to two output shafts
without requiring both output shafts to rotate at exactly the same speed.
When you lock your Audi's diff, it actually ceases being a differential, and
requires that both output shafts turn exactly the same amount.  On our 4kQs
this means that when the rear diff is locked, both rear wheels have to
rotate at exactly the same speed, and that when the center diff is locked
that the shaft that goes to the front diff has to turn at the same rate as
the shaft going to the rear diff.  As long as you travel in a straight line
[and assuming that all 4 tires are exactly the same diameter, etc.] you
really don't need a differential ... but as soon as you begin to turn [or
there is some other inconsistency] different tires are going to want to
rotate more or less as compared to the others.  If you're running with open
diffs all is fine because the differentials allow this, but if any diffs are
locked then something has to give.  Have you ever noticed that when you
drive on relatively high friction surfaces with the diffs locked that the
car loses speed quickly when you try to turn?  What is happening [hopefully]
is that your tires are scrubbing to equalize the rotation, dissipating
energy converting the motion of the car to heat in the tires and the road.
Given that you are in a high friction situation not all of the torque
between the wheels is eliminated though.  You will end up with some amount
of residual torque in the driveline itself.  This can and does cause the
various drivetrain components to shift relative to the body of the car, and
can cause the noise inside the car to change ... even with the car stopped,
out of gear, with the engine idling.  
> 
> BTW did you get that strange chain letter?? Wonder how that got started.

Yes, I got it ... I don't know why they sent it to me but I deleted it.  I
guess I'll be in for some bad luck now, eh?  :-)

Cheers!
Steve