I read Rainer's lengthy post on this with great interest, since I felt last
June that my VC was 'too stiff' therby not allowing a certain amount of
differential action during normal cornering etc.  I believe that the
differential tire pressure suggested by VW for our vans has an effect on
its safe handling characteristics, something about the amount of understeer
in extreme cornering etc.

Re: the tire pressure - it has a definite effect on its loaded driving
radius. In simple geometry consider the radius of a severly underinflated
tire under load at the point directly down from the center of the axle. The
tire as it turns will continually deform on the bottom to create the same
size of footprint (variable determined by the current pressure). The radius
from the center of the axle to the ground contact point will basically stay
the same, for a specific inflation pressure and tire, on a smooth surface,
as the tire turns. In an extreme case (tire nearly flat) this radius could
be several centimeters  less than that of a properly inflated tire. The
simple formula for circumference using [ 2 * r * Pi = C ] gives the
distance travelled by the tire on the ground, which we know in the real
world is different from the unloaded tire circumference. Said another way :
these distances (circumference) will vary directly with inflation pressure
and the resulting variation in height under the wheel centers.

This brings us back to the importance of a properly working VC. If  you are
supposed to have slightly different front to rear tire pressures for
handling characteristics or whatever, there will be a slight differential
in rotation rate between front and rear axles. The properly functioning
Viscous Coupling, I think, doesn't lock until there is some 4 - 6 %
difference in rotation between axles, which is no doubt part of the
engineering design, since I'm sure VC's could be built to lock at a variety
of differntial turn rates (specific to the geometry of each design). So we
realize once again that its important to have equal tires, and use correct
tire pressures.

I agree with Rainer, that I think that a ' stiff' VC, one that locks up at
a rotational differential significantly less than the design, due to
whatever failure (debri  in the viscous fluid making it even thicker, or
...)  is too harsh on the rest of the drive train, and could lead to
premature wear on other parts. I suspect though that a VC gradually
stiffening over time, is hard to detect, and late owners like myself that
acquire their first Syncro with 200,000 km on the clock, may have no VC
'stiffness' experience to compare to. Unfortunately I believe that I have a
'stiff' VC, using Rainer's  description.

The Question I put to fellow Syncronauts  :

Has anyone on the list, besides Rainer,  replaced a VC because it locked up
too frequently ?
Where do we draw the line on a gradually deteriorating item like this ?
Once replaced,  how dramatic was the difference between old and new ?
How would you decide whether to replace it the next time ?

=================
Wolfgang  Carolsfeld
wolfgang@island.net