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What is a differential?

A differential is a device that sends power from a driveshaft to both sides of an axle.  The rotational torque from a driveshaft is generally laid out horizontally down the length of the vehicle (typical RWD).  But the wheels need to be turned at a 90-degree angle from the position of the driveshaft.  The axle driving the wheels is split into two parts and the inner ends of both sides are connected to the differential.   The differential is made up of a series of gears that can direct the rotational power from the driveshaft to 90-degree angles and turn both sides of the axle.

A differential can also be used between the front and rear axles on a four-wheel-drive or all-wheel-system by splitting power between both axles and ultimately driving all four wheels.

There are three major types of differentials:

Open Differential

Also called a slipping differential, this is the most common differential available on stock cars.  An open differential allows for free slippage between the two sides of the axle.  When a car is turning, the inner and outer wheels must rotate at different speeds.  If they are forced to rotate at the same speed, there will be increased wear on the entire drivetrain and tires as the slippage needs to "rubbed off" somewhere, normally the tires.  An open differential allows for some slippage to occur between the two sides of the axle so that the tires do not rub the ground.   With both tires left off the ground, put the transmission in neutral and with your hand turn one of the wheels.  The other wheel should spin in the opposite direction with an open differential.  If the opposite wheel does not spin, the differential may be damaged.

The downside to an open differential is that it offers the least amount of traction.   In a low-traction setting (dirt, sand, snow, ice, gravel), attempting to accelerate beyond the traction capabilities of the tires causes only a single tire to spin while the opposite tire generally does not spin at all or very little.  A 4WD or AWD vehicle with open differentials on the front and rear axles will spin one tire on the front and one tire on the rear in this situation.

Limited-Slip Differential (LSD)

To solve the problem of the low traction of an open differential, car manufacturers typically offer a limited-slip differential (LSD) on off-road, luxury, and high-performance vehicles.  A LSD is similar to an open differential in that it allows some slippage to occur, but it differs from an open differential by limiting the slippage, usually to a percentage difference.  Under normal conditions, a LSD will allow minor slippage between the wheels.  But when the slippage passes beyond a pre-determined difference, after a slight delay, the axle sides lock together somewhat and send part of the rotational torque to the opposite wheel of the slipping wheel.   There are several types of LSD's available from manufacturers all over the world, each offering a different mechanical solution.  With both wheels lifted up and the car in neutral, turn one wheel with your hand.  The other wheel typically won't move at all until about 1/2 of a full turn, then it will start to spin.

Locking Differential

A locking differential is unlike an open differential or LSD in that it allows no wheel slippage to occur.  The axle sides are ultimately "locked" together with a series of toothed gears, similar to the operation of a manual transmission.  A locking differential causes excessive wear on the drivetrain and tires and therefore is not used on street applications.  It's normally reserved for off-road use where the slippage can be innocently rubbed off by the tires on the low-traction ground, snow, or ice.  A 4WD's transfer case is a type of locking differential as it locks the front and rear axles together for superior traction.