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How does a car differential work?

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Quick answer

A differential splits engine power between two wheels on the same axle while letting them rotate at different speeds. In a turn, small spider gears inside the differential carrier rotate so the outer wheel can spin faster than the inner one. Without it, one tire would scrub and hop around every corner.

A car differential is a gear set on the driven axle that lets the left and right wheels turn at different speeds while both still receive engine power. It is needed because the outer wheel in a corner travels a longer path than the inner wheel in the same time.

Step by step

  1. The driveshaft or gearbox output turns the pinion gear, which spins the larger ring gear and the carrier around the axle.
  2. The carrier carries spider gears on pins, and these mesh with the two side gears that drive the half-shafts.
  3. On a straight road, the carrier, spider gears and side gears rotate together as one block at the same speed.
  4. When the car turns, the inner wheel resists, so the spider gears roll around it and feed the extra speed to the outer wheel.
  5. The average speed of the two wheels always equals the carrier's speed, however the difference between them is shared.

Key things to understand

  1. The driveshaft turns a small pinion gear that drives a large ring gear bolted to the differential's rotating carrier.
  2. Inside the carrier, small spider gears mesh with two side gears, each splined to one wheel's half-shaft.
  3. Driving straight, the spider gears do not spin on their own pins, so both wheels turn at the carrier's speed.
  4. In a corner, the spider gears rotate, letting the outer wheel speed up exactly as much as the inner wheel slows.
  5. Because an open differential splits torque equally, one wheel on ice or mud can spin while the other barely moves.

Labelled diagram

  1. Pinion gear

    Driven by the driveshaft, it turns the ring gear at a right angle.

  2. Ring gear and carrier

    Rotate together around the axle, carrying the small gears inside with them.

  3. Spider gears

    Sit still on their pins when straight, rotate on them during a turn.

  4. Side gears

    Each one is splined to a half-shaft and drives a single wheel.

  5. Left and right wheels

    Can turn at different speeds while both still receive the engine's torque.

Inside an open differential on a driven axle

Real-world examples

  • A rear-wheel-drive car turning into a tight parking space lets its outer rear wheel cover visibly more ground than the inner.
  • Farm tractors in India commonly have a differential-lock pedal that the driver presses when one rear wheel spins in a wet paddy field.
  • Rally cars use limited-slip differentials so power is not wasted spinning the inside wheel as they exit bends.
  • A car stuck with one drive wheel on snow shows the classic open-differential problem as that single wheel spins uselessly.

Common mistake

✗ Myth: A differential sends more power to the wheel with the most grip.

✓ Fact: A standard open differential does the opposite in effect: it splits torque equally, so the slipping wheel limits what both wheels get. Only limited-slip or locking differentials move torque toward the wheel that can use it.

People also ask

Why do cars need a differential?
When a car turns, its outer wheels follow a bigger arc than the inner wheels and must spin faster. If both driven wheels were fixed to one solid axle, one would have to skid, wearing tires and making steering heavy. The differential lets the two wheels turn at different speeds while both are still driven.
What happens if a car has no differential?
With both driven wheels locked to one axle, the inner tire would scrub and the outer one would drag every time the car turned. Steering would feel heavy, tires would wear quickly and the axle would be strained. Go-karts work this way, but they rely on light weight and lifting an inner wheel.
What are the signs of a bad differential?
Common signs include a whining or howling noise that changes with speed, clunking when accelerating or turning, vibration through the car and oil leaking near the axle centre. Worn gears or bearings and low differential oil are frequent causes, so a mechanic should inspect it early.
Why does one wheel spin when a car is stuck?
An open differential sends equal torque to both wheels, so the total is limited by the wheel with the least grip. If one wheel sits on ice, very little torque reaches either side, and the slippery wheel simply spins faster.
What is a limited-slip differential?
It is a differential with clutch packs, special gears or a viscous coupling that resists large speed differences between the wheels. When one wheel starts to spin, it shifts more torque to the wheel with grip, helping cars accelerate out of corners and off slippery ground.
Do front-wheel-drive cars have a differential?
Yes. In a front-wheel-drive car the differential is built into the transaxle, the combined gearbox and final drive at the front. Four-wheel-drive vehicles often have three: one per axle and a centre unit between front and rear.
What does a differential lock do?
A differential lock joins both half-shafts so the wheels must turn at the same speed. That sends drive to the wheel with grip in mud or sand, but it should be released on firm roads because it makes turning difficult and strains the tires.

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