What is 4WD? All-wheel drive (4x4) systems explained
Get to grips with the intricacies of different four-wheel drive systems with our handy guide
Four-wheel drive isn't quite as straightforward as you think, as you can get cars with various different types of four-wheel drive systems. Certain 4WD configurations are more common than others, and each one goes about the task of giving you as much grip as possible in different ways.
Four-wheel drive transmissions can vary from the purely mechanical to being almost entirely electronically controlled. There are even some trick traction control systems in use, like PSA Peugeot Citroen's GripControl that mimic the effect of 4x4 without actually sending power to all four wheels.
Plus, 4WD isn't just limited to big SUVs; Audi's quattro system is part of almost every car in the brand's range, while performance cars like the Ford Focus RS and the Mercedes-AMG E 63 prove that it isn't just for greenlaning or driving out in the wilderness - all-wheel drive can help you get the maximum from your car's performance too.
Our 4x4 guide will talk you through the various systems available to you and what makes them different, plus the advantages and disadvantages of each one.
For our in-depth explainer into the different four-wheel drive systems, scroll on...
Selectable four-wheel drive
If you’re serious about heading off the beaten track, then it’s likely a selectable four-wheel-drive system will be best.
Essentially the same system that made its debut on the forties Jeep and first Land Rovers, it features a dual-range gearbox that delivers ultra-low gearing for steep inclines and descents. Today, an updated version of this system is favoured by rugged pick-ups at one end of the market and luxury off-roaders like the Range Rover at the other.
With its bulky transfer case and chunky components, this heavyweight system favours ultimate traction and toughness rather than efficiency. Most of the pick-ups have an option to run in rear-wheel drive only in an effort to boost fuel returns on the road, but the gains at the pump are minimal.
Machines such as the Mitsubishi Shogun still rely on a small lever to manually select 4WD and the low and high ratios, but most rivals now get a small rotary controller that allows electronic switching between the drive modes.
In low-range four-wheel-drive mode, these models are hugely capable in the rough stuff. This is partly down to the gearing, but also the use of locking differentials, which fix the torque split between the front and rear axles at 50:50. Some models also have this function for the rear wheels, while the Mercedes G-Class also allows you to lock the front differential, for the ultimate in traction.
Perhaps the most advanced version of this system is found on the Range Rover, which combines the old-school transmission components with a Terrain Response control system. It uses similar hardware to rival models, but employs a clever traction control that helps eliminate wheelspin. It can assess the condition of the ground automatically, or drivers can manually select from Sand, Snow, Mud, Rock and Gravel settings. There’s also All Terrain Progress Control, which is essentially low-speed, off-road cruise control.
On-Demand four-wheel drive
Four-wheel drive really started to gain traction with buyers in the eighties – and as a result manufacturers started to invest more heavily in the technology. The result of this was the introduction of ‘on demand’ systems, which promised the safety and grip benefits of all-wheel drive, but with the efficiency and usability of regular two-wheel drive.
This type of system proved to be particularly well suited to existing front-wheel-drive models that had transverse engines. Cars like the Vauxhall Cavalier Mk3 led the way, featuring transmissions that delivered over 90 per cent of the engine’s torque to the front wheels during normal driving and had the ability to automatically disengage the rear axle during braking for greater stability.
It was the introduction of the electronically controlled Haldex centre differential in 1998 that allowed brands to relatively easily add all-wheel-drive cars to their line-up. This allowed the car to run in two-wheel drive under normal conditions, which helped save fuel. Yet these early systems were reactive and waited for the front wheel to spin before sending torque to the rear axle. In very slippery conditions this led to scrappy handling as individual wheels were flooded with power as the transmission frantically tried to find grip.
But constant development, more powerful electronics and faster acting differentials have meant that this type of system now comes close to matching more traditional all-wheel drive for traction. In fact, even Land Rover has taken to using this type of ‘on demand’ set-up for models like the Discovery Sport and Evoque. In combination with traction control and Terrain Response it allows these cars to perform almost as well off-road as the full-sized Range Rover.
Permanent four-wheel drive
When four-wheel drive left the farmyard and hit the road, it was a fairly simple permanent set-up that was used. Cars such as the sixties Jensen FF and eighties Audi Quattro made these systems famous, and it wasn’t long before mainstream brands such as Ford had an affordable 4x4 saloon in their range.
Advances in transmission tech mean these systems are getting rare, but brands such as Audi and Subaru have kept the faith. As its name suggests, this system is permanently engaged so that all four wheels are always being driven. It’s not as fuel-efficient as ‘on demand’ set-ups, but the upshot is even greater traction in slippery conditions. This is because the transmission doesn’t have to react to surface changes, so there isn’t the odd spike of power being sent to individual wheels as the sensors try and limit wheelspin.
Audi was one of the pioneers of this, and most cars from the A4 above get a redeveloped version of it. Originally using a Torsen centre differential and latterly Audi’s own set-up, it features a permanent torque split of either 50:50 or 40:60. But the centre diff also has the capability of shuffling up to 100 per cent of the power to an individual wheel or axle in extreme circumstances.
Subaru takes matters to the next level with its Symmetrical 4WD set-up. By mounting the longitudinal located engine, propshaft and rear differential in a straight line, engineers can incorporate equal length driveshafts front and rear. Factor in Subaru’s low-line boxer engines and you should get balanced, predictable handling, together with even power distribution for enhanced traction. Like the Audi set-up, a viscous centre coupling or an electronically controlled unit can vary power front to rear depending on the conditions.
Advanced Traction Control
The French firm’s Grip Control set-up features five driving modes which can be accessed using a rotary controller. Fiat’s system is simpler, with drivers simply prodding a single button when grip levels fall away. If all you’re tackling is a snowy drive, these set-ups are effective. But bear in mind 80 per cent of the traction boost comes from the standard mud and snow tyres.
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