What Is a Differential?
A differential is a gear mechanism that splits engine torque between two output shafts (the drive wheels) while allowing them to rotate at different speeds. This is essential because during turns, the outer wheel must travel a longer path — and therefore rotate faster — than the inner wheel. Without a differential, the inner wheel would skid, causing tire wear, handling problems, and drivetrain stress.
How the Open Differential Works
The standard open differential consists of:
- Ring gear: A large bevel or hypoid gear driven by the driveshaft pinion. This is the input
- Differential case: Attached to the ring gear, it rotates as a unit with the ring gear
- Spider gears (planet gears): Typically two or four small bevel gears mounted on a cross pin inside the differential case
- Side gears (sun gears): Two bevel gears, each splined to one of the axle shafts
When the vehicle goes straight, the spider gears do not rotate on their pins — they simply carry the side gears around with the case, and both wheels turn at the same speed. During a turn, the spider gears rotate on their pins, allowing the outer wheel to speed up and the inner wheel to slow down by the same amount. The total speed always equals twice the case speed.
Torque Split
A key characteristic of the open differential is that torque is always split equally — 50/50 — between the two wheels. This is both its defining feature and its weakness. If one wheel loses traction (on ice, mud, or gravel), the differential sends almost no torque to the wheel with grip because the low-traction wheel spins freely.
Limited-Slip Differentials (LSD)
To overcome the traction limitation, various limited-slip designs have been developed:
- Clutch-type: Friction clutch packs resist the speed difference between axles. When one wheel starts spinning, the clutches transfer torque to the slower wheel. Common in sports cars and trucks
- Torsen (torque-sensing): Uses worm gears that create friction proportional to torque difference. No clutches to wear out. Used in Audi Quattro, some Toyota and GM vehicles
- Viscous coupling: Silicone fluid shears between plates connected to each axle. Speed difference heats the fluid, increasing its resistance. Simple but less responsive
- Electronic: Uses the ABS brake system to brake a spinning wheel, effectively transferring torque to the opposite wheel. No mechanical LSD needed — software-controlled
Locking Differentials
For extreme off-road use, locking differentials can lock both axles together, forcing both wheels to rotate at the same speed regardless of traction. Types include manually selectable lockers, automatic lockers (Detroit Locker), and air-operated lockers (ARB). Locked differentials should not be used on paved roads — the inability of wheels to turn at different speeds causes tire scrub and handling problems.
Differential in AWD Systems
All-wheel-drive vehicles use additional differentials to split power between front and rear axles. A center differential (or transfer case) distributes torque front-to-rear, while separate front and rear differentials handle left-right distribution. Modern AWD systems often use electronically controlled clutch packs or Torsen differentials to vary the front-rear torque split continuously.