How All-Wheel Drive Works

An eight-step lesson on a 3D model. See how power gets from the engine to all four wheels, why a differential is needed, how a clutch engages the second axle and what fails most often in an all-wheel-drive system.

8 steps
about 7 minutes
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Step 1

All-wheel drive pulls with all four wheels

In an ordinary car the engine drives only two wheels: the front axle or the rear one. With all-wheel drive, power goes to all four, so the car spins its wheels less and pulls harder on wet roads, snow and uphill. Inside the body are the gearbox, the transfer case, the driveshafts and two final drives (reduction gears) at the front and rear axles. The lesson uses a BMW 3 Series with xDrive: its main axle is the rear one, and the transfer case engages the front.

Step 2

More wheels pulling means more grip

Traction is limited by the grip between tyre and road. On dry asphalt there is plenty of it, but on a wet road, snow and ice each wheel can pass very little force. When all four wheels pull, the total traction is twice as high, so an all-wheel-drive car pulls away more easily on a slippery hill.

All-wheel drive helps you accelerate and keep moving, but it does not help you brake: the tyres brake, not the drivetrain.

Step 3

From the engine to the wheels: five units in a row

GearboxReceives rotation from the engine and selects the gear; the output is a single shaft.
Transfer caseSplits the rotation into two flows: back and to the front axle. Inside there are gears, a centre differential or a multi-plate clutch, as on BMW xDrive.
DriveshaftsSteel tubes with joints at the ends: they carry the rotation while the body and the axle move up and down.
Final drives (reduction gears)Turn the rotation through 90° and reduce the speed; each contains a differential.
Half-shafts with CV jointsShort shafts with constant-velocity joints deliver the rotation to the wheels and let them steer.

Step 4

Wheels turn differently, so a differential is needed

Without a differentialboth wheels on one shaft: one slips, the tyres squeal, the driveline winds up

With a differentialplanet gears let the wheels turn at different speeds while splitting the torque equally

In a turn the outer wheel travels a longer arc than the inner one, so it has to turn faster. A differential is a carrier with planet gears between two half-shafts. The weak spot of an ordinary differential: it splits the torque equally, so if one wheel is on ice, that wheel spins up and the other gets almost no traction. That is why all-wheel-drive cars get locks and clutches.

Step 5

The final drive gears turn the rotation through 90°

The driveshaft turns along the car, while the wheels turn across it. That is why the final drive contains a pair of gears: a small drive gear and a large ring gear, which turns the differential carrier. The teeth of the pair are slanted and hypoid: they mesh smoothly and quietly and withstand a large load. The ratio is usually 3–4: the drive gear makes three to four turns while the ring gear makes one.

Step 6

A multi-plate clutch engages the second axle

The heart of on-demand all-wheel drive is a multi-plate clutch: a stack of thin discs, some linked to the input and some to the output. While the discs are apart, the second axle is disconnected. When the main axle starts to slip, the discs are squeezed together and friction transmits the rotation to the second axle.

BMW xDrivethe main axle is the rear one; the clutch in the transfer case engages the front, an electric motor squeezes the discs through a cam; normal split about 40 : 60

Most crossoversthe main axle is the front one; the clutch sits in front of the rear final drive (Haldex, BorgWarner), a piston pressed by pump oil squeezes the discs

A fully engaged clutch locks the axles together — roughly 50 : 50. The oil and the friction plates wear: change the clutch oil every 30,000–60,000 km (20,000–40,000 miles), more often with hard use.

Step 7

Three ways to build all-wheel drive

Permanent (centre differential)Works all the time, nothing to engage; the differential itself splits the torque between the axles. Lada Niva (with a lockable differential), Audi quattro with longitudinal engines and Subaru with a manual gearbox are built this way.
Clutch-engaged (on-demand)One axle always pulls, a clutch engages the other. A light, economical layout, the most common in crossovers; BMW xDrive works on the same principle.
Part-time (locked)A transfer case with a lever or button locks the axles together. Great off-road, not allowed on dry asphalt: the driveline winds up. UAZ, Suzuki Jimny, Jeep Wrangler.

Step 8

What fails most often and how to look after the drive

Oil in the clutch, transfer case and final drivesChange it every 30,000–60,000 km, more often with hard use: it heats up and the friction plates wear.
Driveshaft joints and CV jointsA crunch in turns or vibration at speed is a sign of wear; replace a torn boot at once.
TyresAll four should be identical with equal wear: even a small difference in diameter makes the clutch work constantly and overheat.
WheelspinLong wheelspin overheats the clutch: if you are stuck, do not keep the throttle down for minutes.

What to read next

Repair guides for your carsections on the transfer case, driveshafts and final drives How an engine works3D breakdown How suspension works3D breakdown How brakes work3D breakdown

3D models (Sketchfab, CC BY): BMW M340i xDrive by 007; Transmission 3D Scan by CIMtech Inc.; Jaguar differential by 3D scan to CAD by peel-3d.com; Driveshaft by PolyPhantom; Hypoid Gear by C. Yamahata.