Every car with driven wheels needs a differential. Its purpose is deceptively simple: to allow the left and right wheels on an axle to rotate at different speeds while still receiving power from the engine. Without a differential, a car could not turn a corner without the inner wheel skipping and the drivetrain binding catastrophically.
But while the fundamental purpose is simple, the implementation varies enormously—and the type of differential fitted to a car has a profound influence on its handling characteristics, traction capability, and driving feel. Here's how each type works.
The Open Differential
The open differential is the simplest and most common type. It uses a set of bevel gears (typically four spider gears and two side gears) arranged in a cage that allows free relative rotation between the left and right output shafts. When a car drives in a straight line, the cage rotates as a unit. When the car turns, the spider gears rotate on their own axes, allowing the outer wheel to speed up and the inner wheel to slow down.
The disadvantage of an open differential is that it always distributes equal torque to both wheels. This means that if one wheel loses traction—on ice, gravel, or during hard cornering—the maximum torque the differential can send to the wheel with grip is limited to whatever the low-traction wheel can handle. In extreme cases, one wheel spins freely while the other receives almost no power.
The Limited-Slip Differential (LSD)
Limited-slip differentials address the open differential's traction limitation by introducing a mechanism that resists relative rotation between the two output shafts. There are several sub-types:
Clutch-type (Salisbury): The most common performance LSD uses a stack of friction clutch plates, alternately splined to the differential cage and the output shaft. Spring pressure and/or ramp angles create a baseline locking torque, and the clutch stack tightens further as input torque increases. This type is used by BMW's M division, Nissan's GT-R, and most rear-drive performance cars. It provides excellent traction out of corners but adds a subtle—and for enthusiasts, desirable—resistance to the steering that communicates the car's mechanical state to the driver.
Torsen (Torque-Sensing): The Torsen differential uses helical or worm gears instead of bevel gears, creating an inherent resistance to speed differentiation through the friction of the gear meshes. Torsen units are popular in all-wheel-drive systems (notably Audi's quattro) because they respond instantly—with no clutch slip—and require no maintenance. Their limitation is that they cannot proactively send torque to a single wheel; they can only react to existing speed differences.
Viscous coupling: A viscous LSD uses a sealed unit filled with silicone fluid and interleaved plates. When one wheel spins faster than the other, the shearing action of the fluid generates resistance that partially locks the differential. Viscous units are smooth and require no maintenance but are less precise than clutch-type or Torsen alternatives. They're common in all-wheel-drive systems as a centre differential.
Electronic Differentials
Modern electronic differentials come in two forms. The first is a conventional limited-slip differential with electronically controlled clutch engagement—used by Ferrari's E-Diff and Porsche's PTV+. These systems use wheel speed sensors, steering angle data, and accelerometer inputs to optimise clutch pressure in real-time, hundreds of times per second. The result is a differential that can be fully open for cruising comfort and progressively locked for maximum traction, adapting to conditions faster than any mechanical system.
The second form—increasingly common in electric vehicles—eliminates the differential entirely by using independent motors for each wheel. A Tesla Model S, for example, uses software-controlled torque vectoring to achieve the same effect as a differential, but with far greater precision and speed. This approach represents the future of the technology, as it has no mechanical parts to wear and can be updated with software changes.
The choice of differential defines a car's character as much as its engine or suspension. It is the final link in the chain between combustion chamber and contact patch, and understanding how it works enriches every drive.
