All-Wheel Drive Explained: History, Technology, and How AWD Systems Work

This article started as a straightforward technical guide — something like “Everything you ever wanted to know about all-wheel drive, but didn’t know who to ask.” We planned to cover how open differentials differ from visco-coupler or Haldex-type units, what self-locking differentials actually do, and why any of it matters. But the deeper we dug into the history, the more surprised we became. It turns out the first passenger car with permanent all-wheel drive was built in the Netherlands over a hundred years ago. And in 1935, a four-wheel-drive American racing car came astonishingly close to changing the course of world history.

Why does a passenger car need all-wheel drive? In the 21st century, the answer seems obvious: better traction, less wheelspin on slippery surfaces, and improved handling under power. Four driven wheels are simply better than two. But humanity took a surprisingly long time to act on this basic truth. Ask any automotive historian and they’ll tell you the all-wheel drive era for mass-market passenger cars began in 1980 with the Audi Quattro. They might also mention rare predecessors — the 1966 British supercar Jensen FF and the 1972 Subaru Leone 4WD. A real expert, however, will quickly note that early four-wheel-drive Subarus weren’t permanent AWD systems at all — they were part-time. And as we’ll explain, that’s a crucial distinction.

Part-Time 4WD: A Stop-Gap Solution

Part-time drive on one axle is a compromise solution, and not a particularly elegant one for road cars. The term “Part-Time 4WD” originated in the world of SUVs and off-road trucks. In this configuration, one axle drives permanently while the other is rigidly connected on demand — but this rigid connection can only be used off-road. On paved surfaces, the part-time system must be disengaged entirely. Here’s why:

• When a car turns, the front wheels travel a longer arc than the rear wheels and must therefore rotate faster.
• With a rigidly connected all-wheel drive system, traction at the front wheels is reduced while torque at the rear increases.
• In some cases, the front wheels can actually generate braking force rather than driving force — increasing resistance and making the car harder to steer.
• On loose surfaces like mud or snow, this effect is manageable, but on tarmac it causes severe drivetrain binding and handling problems.

When a car navigates a turn, every wheel follows its own arc and must rotate at a different speed. This is why a permanent all-wheel drive system requires three differentials: two inter-wheel differentials (one per axle) and one inter-axle differential to allow both driven axles to rotate independently of each other.

Despite these drawbacks, rigidly connected all-wheel drive did appear on some road-going vehicles — though they were closer to off-road trucks in character. In the USSR, for example, the GAZ-61 “Emka” — a four-wheel-drive saloon with a six-cylinder engine and a part-time front axle — entered small-batch production as early as 1938. After the war, similar drivetrains appeared in the GAZ-M72 “Pobeda” off-road variant and the Moskvitch-410. The 1972 Subaru Leone 4WD followed the same logic: it was built for off-road use, with a higher ride height than standard front-wheel-drive Subarus, and a manually engaged rear axle.

The Subaru Leone 4WD Station Wagon (1972–1979) was a four-wheel-drive adaptation of a front-wheel-drive platform, with a manually connected rear axle. Key specifications included:

• Engine options: 1.4-litre (72 hp) or 1.6-litre (80 hp)
• Body styles: station wagon, saloon, and pickup truck
• Rear-wheel drive engagement: manual on manual-transmission cars; automatic via a multi-disc friction clutch on automatics
• This part-time arrangement continued on all four-wheel-drive Subarus until 1989

The core problem with part-time all-wheel drive is that it’s useless on the paved roads where most cars spend most of their time — yet the car must carry the extra weight of a transfer case, a second driveshaft, and a secondary axle assembly at all times. Converting a part-time system to full-time, however, requires just one additional component: an inter-axle differential in the transfer case.

Full-Time All-Wheel Drive: How It Works and Why It Matters

The inter-axle differential is the key to permanent all-wheel drive. Two inter-wheel differentials — one at each axle — allow the left and right wheels on each axle to rotate at different speeds through corners. The inter-axle differential does the same job between the front and rear axles. A car equipped with all three differentials can run permanent all-wheel drive on any road surface without drivetrain binding or handling penalties.

Simple in theory — yet until the early 1980s, the automotive mainstream considered full-time AWD unnecessary for road cars. The conventional wisdom was that constantly spinning a second pair of wheels and all associated drivetrain components on dry tarmac added noise and wasted fuel. The Audi Quattro changed that thinking permanently. By distributing engine torque across all four wheels at all times, a full-time AWD system:

• Leaves a larger grip margin available for handling lateral forces in corners
• Significantly improves stability when accelerating or braking mid-corner
• Reduces the risk of sudden oversteer or understeer triggered by throttle inputs

The Audi 80 Quattro of the late 1980s illustrates how refined this layout became. The Quattro architecture is more compact than the rival Ferguson Formula transmission. From 1984, Audi adopted the Torsen self-locking differential — a purely mechanical device that responds to changes in torque on each output shaft rather than to wheel speed differences. Unlike a visco-coupler-based differential lock, the Torsen only locks under traction, not during braking, which means it is fully compatible with ABS systems and improves stability during deceleration.

It’s worth noting that the Range Rover (1970) and the Russian Lada Niva (1976) are generally considered the first mass-produced vehicles with inter-axle differentials — but both are firmly in the off-road category. The Audi Quattro claims the pioneer title specifically among passenger cars.

Early Four-Wheel Drive Racing Cars: From Spyker to Bugatti

Did racing car designers explore full-time all-wheel drive before the Quattro era? The answer is a definite yes — and the story goes back much further than most people expect.

Ferdinand Porsche’s first post-war project was a four-wheel-drive racing car: the Cisitalia 360, with a mid-engine layout and a 1.5-litre twelve-cylinder engine. However, its front-wheel drive was part-time — the driver engaged it only on straight sections of track, switching back to rear-wheel drive before corners.

But Porsche had actually built a four-wheel-drive vehicle far earlier: an electric car with four individual wheel motors, dating back to 1900. The real shock for automotive historians, however, is the 1902 racing car built by the Dutch manufacturer Spyker. At a time when even brakes were fitted only to the rear wheels, this car had genuine full-time all-wheel drive — complete with an inter-axle differential.

The Spyker company had been founded in 1880 by the Spijker brothers as a horse-drawn carriage maker. Their first car appeared in 1900, and two years later, working with Belgian designer Joseph Valentin Laviolette, they produced the four-wheel-drive Spyker 60 HP racing car (1902–1907). Its specification was extraordinarily advanced for the era:

• Three differentials — inter-axle and both inter-wheel
• Three separate braking mechanisms — two on the rear wheels, one on the front driveshaft
• A four-wheel drive system that would not be matched in concept for decades

The full-time four-wheel drive concept is, therefore, well over a century old. Not many four-wheel-drive Spykers were built — they were enormously expensive and never achieved significant racing success. Two more ambitious AWD racing projects followed in the early 1930s: the Bugatti Tipo 53 and the Miller FWD.

The Bugatti Tipo 53 project originated with Fiat engineer Antonio Pichetto, who proposed the idea to Ettore Bugatti in 1930. Three cars were completed in 1932, each featuring:

• A 300 hp supercharged straight-eight engine
• Full-time all-wheel drive with three differentials
• A transfer case and inter-axle differential integrated with the separately mounted gearbox
• Drive shafts for both axles positioned on the left side of the car
• An independent front suspension on a transverse spring — unusual for Bugatti

Despite outpacing contemporary rear-wheel-drive cars through gravel corners, the Tipo 53 suffered from excessive steering effort due to the use of standard Cardan joints rather than constant-velocity joints in the front driveshafts. The three cars competed until 1935.

The Miller FWD came about in part because American designer Harry Miller had studied a front-wheel-drive Bugatti purchased specifically for disassembly. Inspired by Bugatti’s approach, Miller developed his own four-wheel-drive chassis with sponsorship from the FWD truck company. One of the four-wheel-drive Millers led the 1934 Indianapolis 500 before retiring with mechanical problems in ninth place.

These cars are also connected to one of the strangest “what if” moments in automotive history. During a race at the AVUS track in Berlin in 1935, a four-wheel-drive Miller was running third when its straight-eight engine failed catastrophically, sending debris flying toward the grandstands. Adolf Hitler was present in the stands that day. If even a small fragment had reached him, the course of the Second World War — and world history — might have been entirely different.

The Ferguson Formula: The AWD System That Changed Everything

To understand the next critical chapter in all-wheel drive history, it helps to revisit a fundamental limitation of open inter-axle differentials. An open differential allows one axle to spin freely while the other receives no torque. If the rear wheels lose grip completely, the front wheels can remain stationary while the rears spin — the differential does nothing to prevent this.

The solution developed for SUVs was positive locking: the driver manually engages a mechanism that rigidly locks the differential gears, converting the differential drive into a solid connection. This approach was used in early Range Rovers, the Lada Niva, and many other off-road vehicles — including the first-generation Audi Quattro, which required the driver to manually lock the centre differential until 1984. But manual locking is another compromise: it must be disengaged on hard surfaces, and it offers no protection if wheelspin begins unexpectedly on a slippery road.

The first automatic self-locking inter-axle differential was the creation of British racing driver and engineer Tony Rolt. Along with his friend and fellow racer Fred Dixon, Rolt had run the Rolt/Dixon Developments workshop before the war. Afterwards, the two became fascinated by the potential of permanent all-wheel drive. After building an experimental four-wheel-drive testbed called the “Crab,” they joined forces in 1950 with Harry Ferguson — the successful tractor manufacturer — to form Harry Ferguson Research.

Ferguson’s vision was not a racing car but a genuinely safe road car: one whose wheels would neither spin under acceleration nor lock under braking. Rolt and Dixon resolved to design such a car entirely from scratch — body, transmission, and powertrain included. With experienced designer Claude Hill (formerly of Aston Martin) brought on as chief engineer, the experimental Ferguson R4 saloon was completed after six years of development. Its specification was remarkable for 1956:

• Permanent all-wheel drive with a self-locking inter-axle differential
• A flat-four engine
• Disc brakes on all four wheels
• The Dunlop MaxaRet electromechanical anti-lock braking system, adapted from aviation

The heart of the Ferguson Formula transmission was an ingenious self-locking mechanism inside the transfer case. In addition to the differential, the unit contained an extra gear set, two ball overrunning clutches, and two packs of friction discs. Under normal conditions, these elements idled quietly. But when one axle’s wheels began to slip — causing a difference in output shaft speeds — one of the clutches would engage, squeezing its friction pack against the differential gears and instantly converting the differential drive into a solid connection.

A second prototype, the 1962 Ferguson R5 estate, was even more capable. Autocar magazine testers noted that it reached the limits of adhesion at speeds that seemed almost impossible. Despite this, no manufacturer agreed to put the Ferguson into production — the complexity and cost were too high. However, in 1962 Tony Rolt convinced the management of Jensen Cars to adapt the Ferguson Formula transmission for their upcoming CV8 coupe, which used a 300 hp Chrysler V8 engine. Three years later, an experimental four-wheel-drive Jensen CV8 FF was completed.

In 1966, the Jensen Interceptor replaced the CV8 — and alongside the standard rear-wheel-drive coupe, Jensen offered an all-wheel-drive variant badged with a discreet “FF” nameplate. The Jensen FF became the world’s first production car to combine a self-locking inter-axle differential with ABS. The “FF” designation stood for “Formula Ferguson.” Key specifications included:

• 6.3-litre Chrysler V8 big-block engine producing 325 hp
• Three-speed TorqueFlite automatic or four-speed manual gearbox
• Asymmetric torque split: 63% to the rear axle, 37% to the front — to preserve rear-wheel-drive handling character
• Single-channel Dunlop MaxaRet ABS
• Rack-and-pinion power steering and disc brakes all round
• Top speed of 212 km/h; 0–100 km/h in 7.7 seconds; kerb weight approximately 1,800 kg
• UK price in 1968: approximately £6,000 — similar to the cheapest Rolls-Royce
• Total production: 318 cars (1966–1971)

Every automotive journalist of the era praised the Jensen FF’s exceptional stability and what they described as “an almost unlimited traction margin on wet asphalt.” Tragically, Harry Ferguson himself never saw the Jensen FF — he died in 1960.

Why spend so much time on the Ferguson Formula? Because Harry Ferguson Research was the first organisation anywhere in the world to treat all-wheel drive primarily as a tool for active safety — not simply as a solution to off-road traction problems. The asymmetric torque split was a deliberate choice to avoid the unpredictability that plagues symmetrical AWD systems. On a rear-wheel-drive car, applying too much throttle in a slippery corner causes predictable oversteer. On a front-wheel-drive car, it causes predictable understeer. On a symmetrical AWD car, the response depends on which axle has the worst grip — which can be ambiguous and dangerous. By biasing torque to the rear, the Ferguson Formula gave the Jensen FF near-predictable rear-drive handling under most conditions.

The Invention of the Visco-Coupler

The Ferguson Formula’s self-locking mechanism had one significant limitation: its overrunning clutches operated in a binary, on-off fashion. The transition from open differential to full lock was instantaneous, which could create its own handling ambiguity at the moment of engagement. What was needed was a mechanism that could vary the degree of differential lock smoothly and progressively.

In the late 1960s, Tony Rolt and Derek Gardner — later the chief designer of Tyrrell’s Formula 1 cars — began experimenting with the silicone fluid used in viscous fan drive couplings. The result was the visco-coupler: a cylindrical housing filled with silicone fluid, containing alternating packs of friction discs connected to each output shaft.

Here is how it works:

• Under normal conditions, with all wheels rotating at similar speeds, the disc packs barely move relative to each other and the coupler has no effect on the differential.
• When one axle begins to slip, its output shaft spins faster, causing the disc packs to rotate relative to each other and shear the silicone fluid.
• The shearing increases temperature and pressure inside the coupler, dramatically raising the fluid’s viscosity.
• This viscosity increase causes the discs to drag against each other, progressively braking the faster-spinning shaft and partially or fully locking the differential.

After patenting the visco-coupler, Tony Rolt established FF Developments (FFD) in 1971 to supply all-wheel drive transmissions commercially. Early projects included four-wheel-drive Bedford vans for British forestry services, a batch of Ford Zephyr FF police cars, and Opel Senator 4×4 saloons for the British military mission in Berlin.

FFD’s most significant production achievement was the transmission for the AMC Eagle (1979–1988) — a raised, all-wheel-drive version of the AMC Concord saloon, fitted with larger tyres and a 75 mm body lift. The AMC Eagle was the first production car in the world to use an inter-axle differential locked by a visco-coupler. Though conceived as a mild off-roader rather than a performance car, its transmission architecture became the direct ancestor of some of the most celebrated performance AWD cars ever built — including early generations of the Subaru Impreza WRX and the Mitsubishi Lancer Evolution.

Self-Locking Differentials: From Torsen to Electronic Control

When the Audi Quattro entered production in 1981 — two years after the AMC Eagle’s debut — it used a conventional open inter-axle differential with a manually operated positive lock. The elegance of Audi’s solution lay in the packaging: the longitudinally mounted engine pointed directly toward the rear axle, and an inter-axle differential was integrated directly into the gearbox. The secondary shaft of the gearbox was made hollow, and the front driveshaft was routed through it. Ferdinand Piëch’s team chose a symmetrical 50:50 torque split between front and rear.

In 1984, the manual differential lock levers finally disappeared from Audi cabins, replaced by the Torsen (TORque SENsing) self-locking differential. The Torsen offers several key advantages:

• It is entirely mechanical — no electronics, fluid, or driver input required
• It responds to changes in torque on the output shafts rather than to speed differences, meaning it can react before wheelspin actually begins
• Unlike a visco-coupler, it locks only under traction, not braking, making it fully ABS-compatible
• Locking and unlocking is smooth and continuous, with no binary transitions

The Torsen’s proven ability to deliver handling and stability improvements on performance cars later attracted SUV engineers seeking car-like dynamics. Today it is used in the transmissions of vehicles including the Range Rover, the Volkswagen Touareg, the Porsche Cayenne, and the Toyota Land Cruiser Prado.

Back in the 1980s, the Audi Quattro’s rally dominance triggered an all-wheel drive arms race among Group B competitors. Within a few seasons, the following four-wheel-drive rally cars had all appeared — each using FFD visco-coupler technology in their self-locking differentials:

• Peugeot 205 T16
• Austin Metro 6R4
• Lancia Delta S4
• Ford RS200

Stuart Rolt, Tony’s son, managed FFD’s relationships with the rally teams during this period.

In the early 1990s, the AZLK factory in Russia also turned to FFD to develop an all-wheel drive rally version of the Moskvitch 2141. Using the same three-differential layout as the Ford RS200, the experimental four-wheel-drive Moskvitch achieved remarkably predictable handling in extreme conditions. Testing revealed an important principle: by adjusting the locking stiffness of each visco-coupler individually, engineers could tune the car’s handling balance across a wide range:

• Stiffer rear inter-wheel differential → more oversteer tendency
• Stiffer front or inter-axle differential → more understeer and stability

This tunability is why modern WRC rally cars use electronically controlled multi-disc clutch packs rather than passive visco-couplers in all three differentials. Hydraulic actuators and an onboard computer can vary the lock-up of each differential in real time — releasing the clutches when entering a corner to allow the car to rotate freely, then progressively clamping them as the driver accelerates onto the straight to maximise traction while avoiding understeer.

Two manufacturers pioneered electronically controlled differentials in road cars:

• Mercedes-Benz 4Matic (1986, W124 E-Class): Three electronically controlled clutches sequentially connected the front axle, then locked the inter-axle differential, then locked the rear differential as conditions required. The system was effective but overly complex, and the electronics could cause the front wheels to connect and disconnect noticeably on loose surfaces.
• Porsche 959 (1986): Two electronically controlled clutches operating across four selectable driver modes. The 959’s system was more sophisticated and better suited to high-performance use.

Replacing the Differential: Haldex and Simplified AWD Systems

While rally engineers were pushing self-locking differentials to their limits, designers of mainstream passenger cars moved in the opposite direction — eliminating the inter-axle differential entirely and replacing it with a visco-coupler alone. The 1985 Volkswagen Golf II Syncro was the first European passenger car to use this approach. The transmission was developed by engineers from GKN, which had acquired FFD in 1969.

The simplified visco-coupler layout offered clear advantages for mass-market production:

• The all-wheel-drive model shared most components with the standard front-wheel-drive version, reducing manufacturing cost and complexity
• Under normal conditions, the car drove identically to a front-wheel-drive car
• When the front wheels slipped, the visco-coupler could transfer up to 70% of torque to the rear within approximately 0.2 seconds

However, this delayed engagement created a handling liability: a car that initially behaved like a front-wheel-drive vehicle (pushing wide at the front) could suddenly shift to rear-biased behaviour when the visco-coupler engaged, catching drivers off guard. Japanese manufacturers explored various solutions, including fitting multiple visco-couplers — some models, such as the 1988 Nissan Sunny/Pulsar, used three: one to engage the rear drive and one to lock each inter-wheel differential. The Mazda Concerto 4WD went even further, using visco-couplers in place of both the inter-axle and rear inter-wheel differentials.

The next evolutionary step replaced the visco-coupler with an electronically controlled hydraulic multi-disc clutch — a much faster and more precisely controllable device. The Haldex coupling, which replaced the visco-coupler on the Volkswagen Golf IV and its platform siblings, is the best-known example of this technology. Here is how it operates:

• Face cams detect any rotational speed difference between front and rear shafts
• Rollers riding over the cam surfaces push pistons in ring cylinders, pumping hydraulic fluid
• Fluid pressure compresses the multi-disc clutch pack, transferring torque to the rear axle
• A solenoid valve, controlled by the vehicle’s electronics, can release pressure at any point — allowing infinitely variable torque distribution

Today, most AWD passenger cars and crossovers use some variant of this electronically controlled clutch architecture — whether it is Haldex on Volkswagen Group vehicles, Honda’s VTM-4, or BMW’s xDrive. The speed of modern clutch systems has reduced engagement delay to the point where it is imperceptible in normal driving. The tuning of the control software now matters more than the hardware itself: the Golf 4Motion and the Audi A3 Quattro use mechanically identical transmissions, but different software gives the Volkswagen a symmetrical torque split while Audi’s calibration sends 60% of torque to the front for a more familiar front-wheel-drive character.

AWD Technology Today: Which System Is Best?

Part-time all-wheel drive systems with a manually engaged second axle have mercifully disappeared from passenger cars. The remaining architectures each have their merits:

• Full-time AWD with a self-locking inter-axle differential (visco-coupler as in Subaru, Torsen mechanical as in Audi A4/A6/A8 Quattro and Volkswagen Phaeton, or electronically controlled clutches as in Mitsubishi Lancer Evo): the most sophisticated and rewarding systems, capable of genuinely improving handling on both road and track when properly calibrated.
• Full-time AWD with an open inter-axle differential (as in Mercedes-Benz 4Matic): relies on anti-slip electronics to compensate for the lack of self-locking. Effective on the road, but mechanically less proactive.
• Part-time rear drive via a controlled clutch (Haldex, as on Volvo, Saab, and various Volkswagen Group crossovers): the most common layout in modern crossovers — cost-effective, light, and increasingly capable thanks to faster electronics.

The dominant trend in advanced AWD is torque vectoring — not just distributing torque between front and rear axles, but actively varying it between the left and right wheels on an axle. The Mitsubishi Lancer Evolution X represents the state of the art: its S-AWC system combines an electronically controlled centre differential (ACD) with an Active Yaw Control (AYC) rear differential capable of transferring torque between the rear wheels independently. Additional gear sets can shift the torque balance proactively, before grip is lost, rather than reactively once wheelspin has already begun.

In practical terms, the real-world handling differences between modern AWD systems continue to narrow as control electronics become more sophisticated. A well-calibrated Haldex-based system in a crossover can deliver stability that would have seemed remarkable from a mechanical Torsen differential a generation ago. That, ultimately, is the direction the technology is heading — and the endpoint may be the electric car with four individual wheel motors, each delivering precisely controlled torque with no mechanical drivetrain at all.

This is a translation. You can read the original here: https://www.drive.ru/technic/4efb336400f11713001e4f54.html