Re: Carrera 4 & diff designs

[phile@swamp.stpaul.gov (Philip J Ethier)]

Glen Powell writes \

\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.

OK, here is the deal on the original Quadra-Trac from someone who was
alive and paying attention at the time they came out for the
Cherokee/Wagoneer.  I actually considered buying one despite my strong
allergy to automatic transmissions, which was the only way you could get
the system at that time.

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.  Each of the two intermediate differential gears is
connected to a series of plates which interleave with the plates of the
other intermediate differential gear.  The viscous silicone fluid is
between these plates.  

When the car is going in a straight line with good traction, the plates do
not move relative to each other.  The differential acts like a normal
differential.

When the car is going around a corner with good traction, the total
distance traveled by the average of the two front tires and the average
distance traveled by the two rear wheels differs slightly, so the plates
move relative to each other.  The relative movement is slow, and the
viscous silicone fluid offers virtually no resistance.  The differential
acts like a normal differential.

When one of the tires spins on some ice, one output of the center
differential turns much faster than the other.  The plates move quickly
relative to each other.  The viscous silicone fluid doesn't like this at
all, and offers major resistance to it.  The differential acts like a
locked differential.

Essentially, what you have here is a device that acts just the opposite of
a Positraction-type (slip-clutch) limited-slip differential.  The
Positraction is essentially a locked differential that is allowed to slip
and act like an open differential when enough force is put on it.

The Quadra-Trac style of differential is just what one would want, IMHO.

phil.ethier@stpaul.gov