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Four-Wheel Drive

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Originally the preserve of off-roaders, four-wheel drive has been achieved in many different ways. Here’s how it’s done – and what to beware of.

Clever chap, that Ferdinand Porsche - among many pioneering concepts credited to him is the Lohner electric car, with a motor in each wheel – a layout now being considered by so-called motoring innovators – in 1899. The first four-wheel-drive car with a single engine was the 1903 Spyker 60hp, now in the Louwmann Collection in the Netherlands.

In 1907, Daimler combined four-wheel drive with four-wheel steering, and by WW1 the system was commonly used for army trucks. By WW2, it had been combined with up to three differential locks and all-independent suspension. Bugatti had tried four-wheel drive for racing in 1932.

Gaz beat Willys during the war by introducing a production passenger four-wheel-drive vehicle, but the 1948 Land Rover was the first non military-derived production car to feature it. Citroën headed up another route in 1960 with the 2CV Sahara, fitting a second engine and drivetrain in the back that could be used together or separately, giving four-wheel drive and all-independent suspension. Jeep’s Wagoneer brought independent front suspension and auto transmission to single-engine four-wheel-drive cars in 1963, pioneering luxury SUVs, followed by Range Rover in 1970, but by then an entirely new route had been trodden by Jensen: four-wheel drive for a high-performance production sports car.

Stirling Moss won a Grand Prix in 1961 with the four-wheel-drive Ferguson P99, and the 1966 Jensen FF used Harry Ferguson’s innovative Ferguson Formula complete with anti-lock brakes: 320 were built but it would be decades before other performance car makers followed suit. Other F1 manufacturers tried four-wheel drive but the extra weight and drag outweighed its benefits.

There were big steps on two fronts in 1980: Audi launched the quattro for rallying, changing the sport for ever and giving four-wheel drive a sporting cachet that would make it far more desired in Europe, while in the USA the new AMC Eagle saloon, coupe and estate range had four-wheel drive controlled by a viscous coupling that gave it high off-road ability without compromising on-road drivability.

Mid-engined four-wheel drive soon came with rapid rally development, in cars such as the 1984 Ford RS200.

Today, four-wheel drive is a feature that we take for granted on millions of vehicles; but how many of us understand how it works, how to use it and how to avoid it becoming an expensive nightmare?

Land Rover followed the Jeep layout with a front-mounted engine and gearbox in-line, and a transfer box behind for low ratio plus propeller shafts to front and rear differentials. However, it had big differences. When a four-wheel vehicle turns a corner, all the tires follow different arcs and therefore cover varying distances. If all are geared directly together, as in the Jeep when the front axle is engaged, considerable drag and tire scrub results – which is fine when on grass or mud, but not acceptable on tarmac.

Land Rover solved the problem with a freewheel on the forward propshaft, which allowed the front wheels to run faster but still be driven if the rears started slipping. Later, the freewheel was deleted and two-wheel drive made normal in high range, with four-wheel drive in low range and selectable in high. However, freewheeling front hubs proved a popular option.

On Range Rovers a centre differential was added, making permanent four-wheel drive the only logical route. The centre diff was a limited-slip unit with locking facility, though neither of the axles had locks or limited slip.

The Audi quattro of 1980 brought to the fore a system introduced for the German military by VW in 1978. Both the centre and rear differentials could be locked via knobs on the dashboard. It offered phenomenal grip and handling, while its efficiency and compactness meant it had few downsides. It came to be offered right across the Audi range.

As four-wheel drive became more popular, makers cast around for cheaper and smoother methods of achieving it: AMC’s Eagles had shown the way.

Viscous couplings had been popular since the 1960s to control engine fans: they contain a series of interleaved plates, like a multi-plate clutch but with holes through which the viscous fluid flows. At low revs and temperatures, the fluid flows freely, allowing a difference in speed between either sides of the coupling. Increase temperature, however, by boosting the speed differential, such as when a tire spins on mud or ice, and the fluid’s viscosity increases, until it forces the plates together and locks the coupling.

For manufacturers they are a cheap and effective way of cushioning the transition between unlocked and locked differentials, and over the last 20 years they have been increasingly used to provide four-wheel drive without a central differential at all. They are found in such diverse vehicles as the Land Rover Freelander and Lamborghini supercars.

Mechanical differential locks are rarely used now, as drivers can’t be trusted to disengage them when on grippy tarmac, leading to rapid wear. More expensive and durable solutions than viscous couplings include the Torsen (torque-sensitive) differential,
a form of limited-slip used by Audi for some quattro centre diffs, Lancia on the Integrale, and Hummer.

Others (eg Mercedes 4matic) rely on traction control, which electronically senses a spinning wheel and applies the brake on that wheel only, to provide a limited-slip effect. Another option is the Haldex hydraulic multi-plate clutch, computer-controlled to engage when the computer detects slip and used on most VW/Audi 4wds including the Bugatti Veyron; reliable and effective, early examples gave a jolt on engaging.

The trouble with viscous couplings is that they don’t last for ever – if forced to run out of sync for long, the viscous fluid is overheated and ruined: it gels and locks the coupling permanently. It is rare for them to last more than 75,000 miles, and some fail a lot quicker than that. Even running the car down the motorway with one tire 10psi down on its correct pressure is enough to ruin a coupling – and as the vehicles get older, the couplings become more expensive or even unobtainable.

So if you’re thinking of buying a 4x4 and you find it drags its tires when cornering tightly on tarmac, know this: its viscous coupling has failed, it will be over-stressing and wrecking the rest of the drivetrain, and replacement may cost more than the car is worth.

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