Posi Replies

[Gary Derian]

All clutch type limited slip differentials work the same way.  A series of
clutches are placed between the axle and the carrier so that when one axle
turns relative to the other, the clutches must slip.  These clutches can be
either multi disc or cup and cone.  The mulit-disc are rebuildable, the cone
type is not but they work the same.  When the spider gears apply torque to
the axles, they tend to spread apart, as do all gears.  The spreading force
from the spider gears applies the clutches.  The higher the torque being
applied, the greater the locking force.  There is also some sort of pre-load
spring to achieve a breakaway torque.  The stiffer the preload, the harsher
the engagement.  For road racing, handling can be tuned with various amounts
of preload as well as number of discs, etc.  Too much anti-slip creates
power understeer, not enough lets the inside tire spin.

Locker types send all the power to the slowest turning axle by using one-way
clutches.  This is OK for drags but too harsh for the street or road racing.

Torsens are really cool.  They use the pressure angle of worm gears to apply
the torque.  They apply torque evenly until one axle tries to overspeed the
other, then torque is shifted to the slower axle.  If one axle brakes, the
diff runs free.

There are also viscous clutches that don't apply the clutches with springs,
instead, a rheopectic fluid, like silly putty which increases viscosity as
it shears, is in between the clutch plates.  When they begin to slip, the
fluid gets stiff and the clutches lock up.

Gary Derian <gderian at oh.verio.com>