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Open / Standard Carrier Differential
The standard differential, or what is referred to as an open carrier, is what comes with most OEM vehicles. The open carrier holds the ring gear in place and within the open carrier is generally a set of gears called spider gears. These spider gears are responsible for allowing a vehicle to negotiate a turn and allow the outside wheel to travel farther and turn faster than the inside wheel. This type of open design works great for most of vehicles on the road today. However when a vehicle with an open differential meets a lack of traction, it directs power to the wheel with the least amount of resistance. The result is the wheel on the traction-less surface spins free, while the opposite wheel of that axle on the better traction surface provides little or no power.
Limited Slip Differentials, Posi-Traction (Posi, Posis)
Limited Slip and positraction (posi) differentials are designed to "limit" the tendency of open differential to send power to a wheel that lacks traction and redirect the power to a degree to the other wheel of the axle. The Limited Slip and Positraction differential will send power to both wheels equally when traveling straight, however when one wheel spins due to a lack of traction, the differential will automatically provide torque to the other wheel with traction. Limited Slip and Positraction (posi) differentials limit the loss of torque to a slipping wheel through various mechanisms such as clutches, gears cones, and other methods dependant on the unit. The limited slip and positraction will not provide 100% lock up of the differential in extreme situations such as when a wheel completely looses traction. Limited Slip and Positraction (posi) differentials are recommended for daily driven vehicles and are used in many applications where traction is sometimes needed as in emergency vehicles. They are also ideal for front axles of 4x4 vehicles that are not equipped with front hubs that can be disengaged. The term "positraction" ("posi" for short) was used by General Motors years ago for their limited slip differential and has been used to refer to limited slips since.
Automatic Torque Biasing (Quaife)
The second mechanical method of improving traction is to use a Quaife Automatic Torque biasing differential. Rather than using clutch plates as a method of operation, the Quaife ATB uses sets of floating helical cut gear pinions that run in pockets and mesh during normal driving.
Should one of the driven wheels start to spin however, the helical gears start to generate a torque bias thanks to the axial and radial thrust of the helical gear pinions in their pockets. The result is a progressive transfer of torque away from the spinning side of the axle to the driven wheel, which is now capable of transmitting a greater proportion of torque.
