In a corner the outside wheels travel further than the inside ones. The differential lets the driven wheels turn at different speeds while still sharing the engine's torque.
Top view, left turnWheel speeds in a corner
Outer wheel covers 14.3 % more ground than the inner one
On 225/45R17 tires: 312 rpm inside, 357 rpm outside. An open differential allows this while sending equal torque to both wheels.
The radius is measured to the middle of the axle. Each wheel follows its own circle, half a track width inside or outside that line, and the speed of each wheel is in proportion to its radius.
With a solid axle both wheels would have to turn at the same speed, so one of them would scrub through every corner, wearing the tire and pushing the car wide. Tighten the radius or widen the track and the gap grows.
Non-driven wheels don't need a differential; each simply turns on its own bearing.
Schematic, not to scale. The pinion turns the ring gear, which is bolted to the carrier. The spider gears ride round with the carrier on their cross-pin and push on both side gears, which are splined to the axle shafts.
What happens inside
Pinion and ring gear turn the drive through 90° and reduce the speed. Their ratio, such as 3.9 : 1, is the final drive ratio.
In a straight line the spider gears don't turn on their pin. They act as a solid link and both side gears turn at the carrier's speed.
In a corner the spider gears start to rotate on the pin, letting one side gear run faster and the other slower by the same amount.
The rule that follows: the carrier always turns at the average of the two wheel speeds. If one wheel is held still, the other spins at twice the carrier speed.
The catch: equal torque
An open differential always sends the same torque to both sides. If one wheel is on ice and can only take a little torque before it spins, the wheel with grip gets that same small amount, and the car stays put.
Ways around it
Clutch-type limited slip: friction plates resist a speed difference and pass extra torque to the slower wheel.
Torsen and other helical-gear designs: worm or helical gears bias torque towards the wheel with grip, up to a fixed ratio.
Viscous coupling: thick fluid between plates stiffens as one side outruns the other.
Locker: ties both sides together for off-road use.
Brake-based systems: traction control brakes the spinning wheel, which gives the open differential something to push against.
Diagrams of the limited-slip and Torsen designs are next on the list.
Sources
Bosch Automotive Handbook — final drives and differentials
Gillespie, T. D., Fundamentals of Vehicle Dynamics, SAE International — cornering kinematics and differential torque distribution
Values checked 22 September 2026. Found an error? Tell us.