Date: Thu, 3 May 2001 19:20:40 -0400
From: "Gary Derian" <gderian@ameritech.net>
Subject: Re: [uuc] Quaife diff comparo

Here is the low down as I see it.

The differential is the part inside the ring gear that allows the outside
tire to turn farther in a turn.  The problem is when cornering hard, and
applying power, the inside tire is lightly loaded and spins first.  The
torque to the rear wheels is evenly split so once the inside tire spins, the
torque to the outside tire is also limited to that same level, often much
less than the driver would like to apply or the engine is capable of
supplying.

A clutch type limited slip has clutch plates in the differential that slip
when the axles turn a different speed from each other.  The differential
locks the clutches tighter as more torque is applied.  Now, when powering
out of a corner, the clutches prevent the inside tire from turning faster
than the outside one, and more torque can be applied to the outside rear
tire.

When the car is driving at constant speed around the corner, there is little
torque applied and the clutches slip easily.  As the torque is increased to
accelerate, the clutches begin to tighten up.  Since the outside tire is
spinning faster, as the clutches tighten up, torque is transferred to the
inside tire, up to the point where it begins to slip.  When the inside tire
slips at a speed to equal the outside tire, torque transfers back to the
outside tire.

When torque is applied to the inside rear tire, understeer is induced.  When
torque is applied to the outside tire, oversteer is induced.  With a clutch
type limited slip differential, the handling is upset as torque shifts from
the inside to the outside rear tire.

A Quaife operates like a Torsen differential.  There are worm gears that
apply torque to the axles.  A Quaife divides torque evenly to both rear
tires like a standard non limited slip up to the point that one tire begins
to slip.  At that point, torque gets transferred to the slower turning tire,
but only enough to prevent the faster turning tire from overspeeding the
other.

When powering out of a corner, the Quaife or Torsen (torque sensing)
differential transfers torque to the outside tire as the inside tire tries
to spin.  It is not influenced by the speed of the rear tires, only the
torque applied to them.  In this way, torque is smoothly transferred to the
outside rear tire when powering out of a corner.

The gerotor type differential on the E46 M3 combines both characteristics.
It has clutches but they are applied hydraulically by a gerotor pump that
operates when there is a speed difference between the rear tires, not
through the torque applied to them as a clutch type limited slip.  The
result is Quaife or Torsen like operation with some additional control on
lockup initiation (the inside tire can spin slightly before the clutches are
applied) which makes work much better with DSC controls because the system
can sense the slip of the inside rear tire and begin its control strategy.
With a Quaife or Torsen, the DSC system cannot sense an incipient rear end
slide but must wait for it to happen first.

Gary Derian <gderian@ameritech.net>