How 4x4 works: differentials and the main types of system

• Four-wheel drive sends power to both axles. Between the axles it is split either by a centre differential or by a controlled multi-plate clutch. Hybrids and electric cars drive each axle with a separate motor.
• A differential lets the wheels turn at different speeds in a corner. An open differential, however, leaves the car stranded when one wheel loses grip. Limited-slip differentials, diff locks or braking the spinning wheel help.
• Automatically engaged four-wheel drive with a multi-plate clutch, as in the Škoda Octavia Combi III 4x4, can bring in the rear axle as soon as the car pulls away, before the front wheels start to slip.
• Part-time four-wheel drive with low range, as in the Suzuki Jimny IV, locks the axles together rigidly. In 4x4 mode it therefore belongs on snow, mud or gravel, not on dry tarmac.
• Trade names such as quattro, 4Motion or xDrive identify the brand, not the design. The same name can refer to a different mechanism on different models and generations.

The 4x4 label covers several quite different designs. Some drive both axles all the time. Others continuously vary how much power the second axle receives, and on others the driver engages it. In hybrids and electric cars the axles may not be linked by any hardware at all, only by electronics. To understand how four-wheel drive works, you have to start with a corner: each wheel covers a different distance in it, and the drivetrain has to cope with that so that the tyres do not scrub and the gears are not put under needless strain. Which used all-wheel-drive car to choose is covered in our guide to the best 4x4 cars.

In a mechanical four-wheel-drive car, power from the engine travels through the gearbox to both axles. A longitudinal propshaft carries it to the more distant one. Škoda adds a flexible coupling to the propshaft on its 4x4 models to damp shocks and vibration. At the axles, a differential and the driveshafts then turn the wheels.

In a corner, the outer wheel follows a longer arc than the inner one, so it has to turn faster. An axle differential compensates for the different speeds of the left and right wheels, even though both are driven by the same engine.

The same problem arises between the axles, because the front and rear wheels follow different paths through a corner. Mechanical all-wheel drive deals with it using a centre differential, or a clutch that allows the speed difference and links the axles only as firmly as necessary.

Why a car with an open differential can get stuck

An ordinary open differential splits torque roughly equally between the two wheels. On tarmac this does no harm. But when one wheel is on ice, it can transmit only a small amount of torque, and the other wheel receives just as little, even though it is on a firm surface.

The result is one wheel spinning rapidly and a car that goes nowhere. People often say that "all the power goes to the wheel with no grip". More precisely, an open differential cannot send more torque to the wheel with better grip on its own. That is why 4x4s use limited-slip differentials, diff locks or brake intervention on individual wheels.

The type of drive is defined by what connects the front and rear axles. Electrified systems dispense with the mechanical link altogether.

How the individual types differ in fuel consumption, on snow and in maintenance is covered in our article on permanent and on-demand four-wheel drive. Here the focus is on how they work.

Permanent four-wheel drive: what the centre differential does

Permanent four-wheel drive with a centre differential drives both axles even on dry tarmac. It compensates mechanically for the difference in their speeds, so the drivetrain does not fight itself in corners.

The basic torque split need not be 50:50. The design may favour the front or rear axle and vary the split further according to grip.

The classic solution is the Torsen, a mechanical limited-slip differential with worm gear sets. Friction in the gears prevents one output from spinning freely. The differential therefore sends more torque to where the wheels can use it. It is not a rigid locker, though.

Subaru's Symmetrical AWD is more of a layout concept, as the engine, gearbox and axle drive sit on the car's centreline. The actual mechanism varies by version. The Subaru Forester III (2008–2012) with a manual gearbox had a centre differential with a viscous limited-slip unit. With an automatic it got Active Torque Split AWD with an electronically controlled clutch. One model name thus hides two different designs.

Automatically engaged four-wheel drive: how the multi-plate clutch works

A multi-plate clutch contains a pack of alternating plates. One set is connected to the shaft from the gearbox, the other to the rear axle drive. The harder the mechanism clamps them together, the more torque the clutch sends to the rear.

The control unit monitors wheel speeds, accelerator position and steering angle, among other things, and adjusts the clutch pressure accordingly. A modern system does not have to wait for wheelspin: it engages the rear axle as soon as the car pulls away or the driver accelerates hard. The electronically controlled clutch in the Škoda Octavia Combi III 4x4 and the Škoda Kodiaq I 4x4 in their 2018 specification worked predictively in this way.

This type of clutch is widely known as a Haldex, after its original manufacturer. Not every automatically engaged 4x4 has one, however; other designs use a similar principle. Nor does the clutch simply switch between front-wheel drive and 4x4. It varies the degree of engagement continuously.

The clutch can also be used the other way round, to disconnect an axle completely when the car does not need it. For its quattro ultra system, which disconnects the rear axle, Audi quotes average fuel consumption in everyday driving 0.3 l/100 km lower than with conventional all-wheel drive.

BMW xDrive also uses a controlled multi-plate clutch in the transfer case. In its basic setting, however, it sends power mainly to the rear rather than the front.

Part-time four-wheel drive: what 2H, 4H and 4L mean

A classic off-road design drives only the rear axle in its default mode. The driver engages the front axle through the transfer case with a lever or selector. A system without a centre differential then locks the axles together rigidly. This is how the Suzuki Jimny IV (from 2018) works, with part-time four-wheel drive and a low-range gearbox.

Low range lets you drive more slowly at the same engine speed and deliver considerably more torque to the wheels. It comes into its own when climbing over obstacles, crawling up steep slopes off-road or making a controlled descent of a steep hill.

A rigid link between the axles needs a surface on which the wheels can slip. On snow, gravel or mud, the difference in distance travelled evens itself out. On dry tarmac, transmission wind-up builds up in the drivetrain, the tyres scrub and the steering feels heavy in corners. Some off-roaders also have a centre differential and a mode for road use. The 4H label will not tell you that, though.

Hybrid 4x4: an electric motor drives the rear axle

In many hybrids, the combustion engine and the electric part of the system drive the front wheels, while the rear axle has its own electric motor. No propshaft or centre coupling is needed. Electronics decide when and how strongly the rear motor engages, according to the demand for traction. The fifth-generation Toyota RAV4 Hybrid AWD-i (2019–2025), for example, drives its rear axle in this way.

The same layout appears in full hybrids and plug-in hybrids. It applies only to some of them, though, as certain hybrids retain a mechanical power transfer between the axles. How an electrified 4x4 behaves in winter and with a depleted battery, and how it affects range, is covered in our article on electric and hybrid 4x4s.

Electric 4x4: two motors instead of a mechanical link

An electric car with all-wheel drive usually has one motor for the front axle and another for the rear, and the electronics control the torque of each separately. There is no centre differential, but a conventional axle differential usually remains between the left and right wheels of each axle. Two motors therefore do not mean that each wheel has its own drive.

The two motors need not both work all the time. The first-generation Hyundai Ioniq 5 AWD (from 2021) can mechanically disconnect the secondary driven axle. When the car does not need its traction, this reduces drivetrain losses.

A diff lock restricts or completely blocks the speed difference between the two outputs of a differential. On an axle it links the left and right wheels; in the centre of the drivetrain it links the front and rear axles. A fully locked differential will not let one wheel spin while the other stands still.

A limited-slip differential works more gently. It restricts the speed difference but does not lock the outputs together rigidly. This category includes the Torsen, the viscous coupling and plate-type differentials.

The electronic substitute for a diff lock uses the brakes. The system brakes the spinning wheel, increasing the resistance on its side of the open differential. The other wheel can then transmit more torque. This is how Trail Mode works on the fifth-generation Toyota RAV4 Hybrid, for example.

Braking individual wheels is no substitute for a mechanical diff lock. During prolonged intervention the brakes heat up and the electronics reduce their effect to protect them. Even a "Lock" button on a 4x4 with a multi-plate clutch may not actually lock anything. Often it simply sets the clutch to a permanently firmer clamp, which is released again at higher speeds.

A trade name mainly tells you which brand the system belongs to. You often cannot tell the design from it.

The abbreviation AWD tends to appear on road cars, 4WD on off-road designs. That is no hard and fast rule, though. Nor does a 4x4 badge mean that all the wheels always receive the same torque.

• Four driven wheels are not four diff locks. Most road-going 4x4s have open axle differentials and maintain traction by braking individual wheels.
• Permanent drive does not mean a fixed torque split. In most systems the ratio between the axles changes according to grip.
• Four-wheel drive does not shorten braking distances. It helps when pulling away and accelerating, but all cars brake with all four wheels. On snow, safety therefore depends mainly on winter tyres, which four-wheel drive cannot replace.
• The drivetrain is sensitive to mismatched tyres. A more worn tyre has a smaller circumference and turns faster, so the drivetrain sees a constant speed difference. BMW states that an extremely worn tyre can keep the drive permanently engaged on xDrive systems with the ATX13-X transfer case. What this means for a used car is covered in our article on buying a used 4x4.

The four-wheel-drive system of the same model has changed between generations and engine versions. Whether it works as it should, only the car itself will tell. Before sale, Carvago has every vehicle checked by a technician as part of the CarAudit™ inspection with up to 270 checkpoints. The inspection includes an examination of the technical components, diagnostics and a test drive, which also shows how the drivetrain behaves when pulling away and in corners.

The customer knows the result of the inspection before deciding to buy the car, and Carvago guarantees the condition stated in the CarAudit™ certificate. If Carvago does not recommend a car after the inspection, the CarAudit™ payment is transferred to the inspection of another car.

It pays to look for a specific drive system in a large selection. Carvago has the widest selection of used cars: roughly a million cars from dealers in several European countries. Even a less common combination, such as a 4x4 with an automatic gearbox or an off-roader with low range, is available in many times the numbers found on a local market.

Is every car with four-wheel drive suitable for off-road use?

No. Off-road ability is also limited by ground clearance, approach and departure angles, suspension travel and underbody protection. A road-going 4x4 has enough traction on mud, but it will get stuck on its underside at the first bigger obstacle.

Does every 4x4 have low range?

No. Low range is found mainly on off-roaders and pickups with part-time four-wheel drive, such as the Suzuki Jimny IV. Road-going SUVs and estates with automatically engaged drive, such as the Škoda Kodiaq I 4x4, do not have it.

How does a viscous coupling work?

A viscous coupling is a sealed housing containing plates and a thick silicone fluid. When one side starts to turn significantly faster than the other, the fluid heats up, thickens and links the two sides more firmly. It reacts on its own, without electronics, but only once a speed difference has arisen. That is why manufacturers gradually replaced it with electronically controlled clutches that engage predictively.

What is torque vectoring?

It is the targeted distribution of torque between the left and right wheels to help turn the car into a corner. The outer wheel receives more power and the car turns in more willingly. Torque vectoring uses an active differential with clutches, separate electric motors or, more simply, braking of the inner wheel.

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