Eaton's Posi is easily identifiable...
Eaton's Posi is easily identifiable by the springs that preload the clutch discs. This is the same unit that was standard on GM products in the 1960s and 1970s.
Traction-Control Differentials
Basically there are two types of traction-enhancing differentials-limited slip and locking. Limited slip responds to the amount of torque being applied, and locking rearends respond to the difference in wheel speed.
Over the years there have been a number of companies producing variations of both types. Eaton limited-slip rearends were first introduced in 1961 on half-ton GM pickups, and cars shortly thereafter, and just about every rodder has heard of the Detroit Locker. Recently the Eaton Corporation acquired the parent company that makes the Detroit Locker and Detroit Truetrac. Along with the Eaton Posi limited-slip differential and the Eaton Elocker electronic locking differential, Eaton has just about covered all the bases.
The clutch discs can be seen...
The clutch discs can be seen in this exploded view of an Eaton Posi. These units require special lubricants to prevent the clutches from sticking.
Limited-Slip Differentials
Limited-slip differentials come in various designs, but they all react to the amount of torque applied to them.
Inside the Eaton Posi case are clutch packs and springs. When torque is applied to the differential and both wheels have traction surface, the clutch packs clamp the side gears to the differential case. This clamping action is directly related to the torque being applied-more torque means higher clamping force (the Eaton Posi essentially prevents wheel spin before it happens). However, when one tire does begin to spin, the clamping force is reduced; consequently, the torque to that wheel is reduced. When it comes time to turn a corner, the clutch discs allow the slippage necessary to allow the axles/wheels to turn at different speeds.
A variation on the limited-slip theme is the Truetrac. These are unique in that they are gear-type limited-slip design with no clutch or friction plates, so there is nothing to wear out.
In operation, the Truetrac's side gears and the differential's worm gears are forced away from each other. Similar to the clutch-style unit, this action clamps the axle to the differential case. So, if one of the tires loses traction, the friction between the worm gears and the case is reduced, and power to that wheel is reduced.
Here we have a Detroit Gearless...
Here we have a Detroit Gearless differential. The cross pin connects the unit to the differential case.
Limited-slip rearends are relatively seamless in operation; they're quiet and smooth, and you'll never know they're there until you need them. However, be aware that most clutch-style limited-slip differentials require special gear oil or an additive. Always check with the manufacturer for their recommendations on lubrication.
Locking Differentials
When it comes to the ultimate in traction, lockers are hard to beat. This style of differential is speed sensitive; that is, both wheels are locked together until one rotates faster than the other.
The venerable Detroit Locker uses what's called a dog clutch to deliver power to each wheel. When one wheel goes faster than the other, as in turning a corner, the unit disengages the outside wheel (or the one without traction) and sends all the power to the inside wheel (or the one with traction).