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Home How a Differential Works

A car’s rear axle, opened up

How a differential works

Round a corner, the outer wheel has farther to go than the inner one. The differential lets one engine drive both wheels while each turns at its own speed.

Take it apart
2.4 m
farther for the outer wheel in a 90° bend
3.73:1
ring and pinion, 41 teeth to 11
50:50
torque split of an open differential
1827
patented for a steam carriage in France
The rear axle of a car cut open on a test track: a blue pinion gear drives a copper ring gear, with green spider gears and cyan and magenta side gears inside.
Cut open: the gears inside the rear axle

Video

How a Differential Works - Why Your Wheels Turn at Different Speeds

Watch on YouTube

3D model · drive · steer · turn the wheels

Drive it, steer it, turn the wheels

It’s already cut open. Press Drive, steer into a corner and watch the green spider gears start to spin. Drag to look around; scroll or pinch to zoom.

Try itPress Drive, then steer. The cyan side belongs to the left wheel, the magenta side to the right.
Loading the 3D model…
Where
Differential

Scroll to take it apart

A gearbox inside a cast pumpkin

Keep scrolling: the housing is sawn in half, then the gears inside the carrier slide apart.

Loading frames…
  1. Step 1Closed upThe cast housing, the “pumpkin”, with its rear cover. The driveshaft bolts on at the front.
  2. Step 2Saw it in halfInside: the blue pinion, the copper ring gear and the carrier it is bolted to.
  3. Step 3Slide the gears apartThe cross shaft lifts out, the green spiders slide off it, and the side gears slide out along their axle shafts.
Meet the parts

Ten parts, three jobs

Tap any part of the opened axle or pick a name below. The final drive turns the power through the corner, the differential shares it out, and the axle carries it to the wheels.

The rear axle cut open with the gears slid apart: the cross shaft lifted out, the spider gears above and below, the side gears out to each side, the carrier case see-through.

Hover or tap the picture. The dots mark every part.

Parts diagram

Follow the power

Numbers 1 to 8 follow the power from the driveshaft to the wheels. Numbers 9 to 15 hold everything in place and keep the oil in.

The rear axle cut open, with fifteen numbered parts from the pinion yoke to the axle shafts.
    How it works, step by step

    Fifteen steps round the corner

    One idea at a time, from why the wheels must turn at different speeds to cars with three differentials. Every step has a button that sets up the 3D model to show it.

      Top view of the yellow car leaving a bend, with the tracks of its two rear wheels drawn behind it: the outer track is longer.

      Two wheels, two distances

      Demo · turn a corner

      Two wheels, two circles

      In a turn each rear wheel runs round its own circle. The outer circle is bigger, so the outer wheel must turn faster, by exactly as much as the inner one turns slower.

      Steer, then swap the differential for a solid axle. With both wheels forced to turn at the same speed, the tyres have to scrub across the road.

      0left wheel, rpm
      0carrier, rpm
      0right wheel, rpm
      0spider gears, rpm

      Demo · one wheel on ice

      Open, limited slip or locked

      Three cars wait with the right rear wheel on ice. Each needs a push of about 200 Nm at its rear wheels to get going. Open the throttle and watch what each kind of differential does.

      The spider gears push equally on both side gears, so an open differential can only give the gripping wheel as much as the icy one can take: about 60 Nm.

      The yellow car with its right rear wheel spinning on a sheet of ice, throwing up chips of ice, while the left wheel stands still.
      Open differential: the icy wheel spins, the other one stands still.

      Demo · all-wheel drive

      Four wheels, three differentials

      In a turn the front wheels run round bigger circles than the rear ones, so the front axle as a whole has to turn faster too. A centre differential lets it.

      Lock the centre differential on a dry road and steer: the front and rear driveshafts are forced to the same speed, the driveline winds up like a spring, and a tyre has to give.

      Top view of the see-through car turning, with its front, centre and rear differentials lit blue.
      Front, centre and rear differentials, lit blue.
      0front left, % of car speed
      0front right, %
      0rear left, %
      0rear right, %

      More to know

      Kinds, history and care

      Six kinds of differential

      OpenSpider gears and side gears, nothing else. Smooth, cheap and quiet, so most cars have one. Its weak spot is a wheel without grip.
      Clutch limited slipClutch packs behind the side gears drag when the wheels differ and send torque to the wheel with grip. Popular in sports cars and tow trucks.
      TorsenWorm gears that are hard to drive backwards. Their friction lets the gripping wheel take several times the torque of the slipping one, with no clutches to wear. A wheel in the air still beats it.
      Viscous couplingThin plates in a sealed drum of thick silicone fluid. When one side spins faster the fluid shears, heats and drags the two sides together. Often found in the middle of all-wheel-drive cars.
      LockerA dog collar or pins lock a side gear to the carrier, so both wheels turn together. Some are switched by air, a cable or an electromagnet; automatic ones lock by themselves.
      Torque vectoringExtra clutches or a small motor push more torque to the outer wheel, so the car turns in more eagerly. Electric cars can do it with a motor for each wheel.

      From a chariot to a supercar

      1. In China, Ma Jun builds a south-pointing chariot whose little figure keeps pointing the same way as the cart turns. Some reconstructions use differential gears.
      2. Joseph Williamson, a London clockmaker, uses a differential gear in a clock.
      3. In France, Onésiphore Pecqueur patents a differential for steam carriages: the ancestor of the one in every car.
      4. James Starley fits his “balance gear” to a tricycle, so both of its big wheels can be pedalled round corners.
      5. A limited-slip differential designed for Ferdinand Porsche’s Auto Union racing cars tames their wheelspin.
      6. Vernon Gleasman patents the worm-gear differential later sold as the Torsen.
      7. The Audi quattro brings permanent four-wheel drive, with a centre differential, to fast road cars and rallying.
      8. Mitsubishi’s Active Yaw Control uses clutches to push torque to the outer rear wheel: torque vectoring.
      9. Most cars still have an open differential. Electric cars with one motor for each wheel can do its job in software.

      Keeping one happy

      • Change the gear oil. It is thick hypoid oil. Limited-slip differentials need oil with a friction additive, or their clutches chatter in tight turns.
      • A whine that changes when you lift off or accelerate usually means worn or badly set ring and pinion gears or bearings.
      • A clunk as you pull away can be worn spider gears, too much backlash or a worn driveshaft joint.
      • Don’t spin one wheel for long on ice or in mud. The spider gears and their washers run very fast and can overheat. Ease off and pull away gently.
      • Never run the engine in gear with only one driven wheel off the ground. That wheel spins at twice the speed the speedometer shows, and the car can jump off the jack.
      • Keep the tyres matched. Different sizes make the differential work all the time, and in an all-wheel-drive car they can damage the centre differential.
      • Watch for oil round the pinion seal and the cover. A small leak today is a dry, noisy axle later.

      Things worth knowing

      2.4 m, whatever the bendIn a 90° turn the outer wheel goes farther by the track width × π ÷ 2: 1.55 m × 1.57 = 2.4 m. The radius of the bend doesn’t matter.
      A stuck car doing 15Many speedometers read the driveshaft, so they show the average of the two wheels. Stuck with one wheel spinning at 30 km/h, a car shows 15 km/h and doesn’t move.
      A gear that addsCarrier = (left + right) ÷ 2. Mechanical computers on warships used dozens of differentials to add and subtract numbers turned in as shaft angles.
      Go-karts have noneA kart’s rear axle is one solid bar. In a turn the frame flexes and lifts the inside rear wheel a little, so it can slip.
      Hypoid gearsIn most rear axles the pinion sits below the ring gear’s centre. That lowers the driveshaft and the floor, but the teeth slide more, which is why the oil is special.
      Tractors lock theirsMany tractors have a pedal that locks the differential for pulling through mud. The driver lets go of it before turning.
      Quick check

      Four questions