Re: Carrera 4 & diff designs

[Eliot Lim <eliot@u.washington.edu>]

> 
> \I do not believe that the Jeep "Quadra-Track" system used Quattro-like
> \open diffs with 100% mechanical locking for the center and rear diffs,
> \although it may have been one of the first "full time" AWD systems.
> 
> The center differential has a special silicone fluid in it.  This is NOT a
> power coupling between front and rear axles.  The power coupling is a
> regular differential gear-cage, with the trick silicone stuff between the
> differential gears.  

this is exactly what was used in the jensen FF and today's 4wd
mitsubishis.  the bmw 325ix also used it.  i believe the technology was
licensed from a british company called ferguson formula or something like
that. 

the idea of using the viscous coupling as _the_ center differential, thus
providing part time, automatically engaging 4wd was audi's, who then
rejected it and gave it to vw.  lots of cars today use this simple
concept, from minivans to the diablo VT to the latest carrera 4. 

for the record, viscous couplings were rejected by audi even though they
were available during the time of the original quattro.  the reason given
(though i have never been able to find out more) was on the fact that
viscous couplings had locking characteristics that were too sudden and
this would (somehow) lead to premature brake locking. 

also it hindered ABS operation.  recall that mitsubishi was not able to
provide a VC rear diff _and_ ABS for a few years.  porsche, who was in a
blitzkrieg mood at the time came out with computer controlled clutches in
the 959 and original carrera4, also going on record about the evils of the
viscous coupling. 

they were in good company.  mercedes also used CCC's over VC's.  it is
thus supreme irony that the latest C4 uses a simple minivan style VC after
building the most sophisticated 4wd system (in the 959) for reasons that
are completely understandable.. to retain the rwd 911's feel. in
retrospect i think it was a good decision.

i guess the lesson to be learned is that technical perfection is not
necessarily a good thing.  a 911 with all quirks expunged is no longer a
911.  an audi without a 5 cylinder engine is ...(fill in the blanks)..  no
wonder many enthusiasts are shunning the latest cars with honda style
steering and quiet refined engine.  if audi is trying to ape the lexo
barges, what do they have to offer the customer _over_ the lexo-barges? 

one only has to drive one of these so called "perfect" japanese cars to
really get a feel for "perfection"..a driving experience that is utterly
detached, distant and cold.  it is as perfect as talking to a computerised
telephone answering system that doesn't have bad moods or makes any
mistakes.  it makes the high costs of audi ownership worth it for me. 

eliot