Miata Mailing List: September 1993, Message #127
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   From: "Michael Butler" 
Subject: Why is a Torsen diff so great?
   Date: Thu, 16 Sep 1993 18:42:34 -0400

Here's my attempt at explaining the benefits of a Torsen diff.

First, let me explain an open diff.  An open diff distributes torque
equally to the two rear wheels (in a Miata) while allowing the wheels
to rotate at different speeds.  Contrast this to a spooler (locked diff),
which distributes torque unequally while requiring the wheels to rotate
at the same speed.  Requiring the wheels to rotate at the same speed
causes problems in tight turns (when the inside wheel turns more slowly
than the outside).

The problem with an open diff occurs when one wheel has traction problems.
For the sake of illustration, let's oil one of the tires.  Since the diff
distributes torque equally, the tire with good traction gets just as much
as the tire with no traction.  Net result: oiled tire spins wildly, other
tire sits there.

The Torsen behaves like an open diff until one tire is spinning twice the
speed of the other (this is due to the clever kinematics that Andy alluded
to).  At that point, the torque distribution becomes unequal  -- i.e., the
non-spinning tire gets more torque.

Why is this so wonderful compared to a viscous LSD or clutch type LSD?
The clutch type suffers from non-linear operation.  At some pre-determined
slip rate, the clutch engages.  This can result in a sudden transition to
understeer or oversteer (if the car is cornering to begin with).  The
viscous LSD (as in my car) is linear, but limited in the amount of torque
that can be transferred to the non-spinning wheel.

The Torsen should also be maintanence-free and more efficient than the VLSD.

Michael Butler (mcb@vnet.ibm.com)