all wheel drive

Many people go through their entire motoring lives without bothering about what goes on under the hoods of their cars. And that includes not knowing which wheels provide the drive that propels the vehicle. That said, how your car gets its power down to the road surface should be a fairly important consideration for any buyer.

Traditionally, cars tend to fall into two broad categories, front wheel drive and rear wheel drive. Of late however, all-wheel drive has become an increasingly popular layout in road cars, thanks chiefly to the cachet generated by its successful application in the muddy world of rallying. But what are the actual advantages, Air Compressors need clean air too! if any?

Front-Wheel Drive
In the case of front-wheel-drive-cars, the front wheels handle both the steering and the task of getting all the engine's power effectively to the road surface. One advantage is improved packaging; there's no need for a bulky drive shaft to transfer power from the engine to the rear wheels so that saves on interiorspace.

One disadvantage of this layout however, is the fact that there is a limit to how much power you can apply through wheels that also have to do the job of steering. Anybody who's had to multi-task at work will understand how difficult it is to perform two jobs as well as if you had to do just one. Aside from steering, the front tyres' also lose traction (or 'grip') under hard acceleration as the inertial weight of the car transfers rearwards and momentarily keeps the front driving wheels from pressing onto the road surface.

When this happens, the driver can feel the car steering left or right under acceleration. Among the techno-minded in the car industry, this phenomenon is dubbed 'torque steer. ' This is why the engines in front wheel drive cars are rarely very powerful.

Rear-Wheel-Drive
Manufacturers of more sports-biased cars usually adopt a rear-wheel-drive layout, BMW being one notable example. By keeping the front pair of wheels free to focus solely on steering, and leaving the pair at the rear to focus solely on delivering torque to the road surface, a real-wheel-drive car can deliver an arguably purer driving experience than a front wheel drive car. You can also afford to have much more powerful engines as the inertial weight transfer that causes traction problems in front-wheel drive cars works to the rear-wheel drive car's advantage. Some would also argue that it takes more skill to drive a powerful rear-wheel drive car at the limit; although today's electronic traction control devices do make them much more manageable.

All-Wheel-Drive
All-wheel drive, or four-wheel drive cars, as the names imply, distribute engine power to all four wheels. The distribution is often not equal, depending on the manufacturer and also on intended use. The first four-wheel drive systems were solely the preserve of high-level motor racing. Even Formula One teams dabbled with four-wheel drive setups at one point but development costs and regulations prevented their being more widely accepted.

Subaru built the world's first mass-produced AWD car in 1972 in the shape of the Leone Station Wagon. Until this time, four-wheel-drive vehicles had been strictly limited to 'off-road' utility vehicles from companies like Jeep and Land Rover. It wasn't until the 1980s when Audi introduced its famous quattro rally car that four-wheel drive really started to come into vogue. Since then the World Rally Championships (WRC) have been dominated by all wheel drive rally cars like the Subaru WRX. The success of cars like the WRX hasn't been exclusively due to their having more grip over cars with two wheel drive, which is a common misconception, but rather their heightened ability to appropriately channel the engine's torque to the road surface.

The application of all-wheel-drive becomes easier to understand in the context of a very powerful engine. Imagine a 500bhp car with 600Nm of torque channelled through only the front or rear wheels, without sophisticated traction control. It would be very difficult for the car to effectively use all that power. There is a limit to how much engine torque a wheel can transmit to the road surface before it overcomes the wheel's ability to grip the road surface. If you split torque amongst four wheels instead of two, there's less chance of torque overwhelming grip, and hence better traction.

Of course, the application of all-wheel drive isn't limited to cars with powerful engines, an average family saloon can also take advantage of the traction benefits too.

Types of All-Wheel-Drive Systems
For road cars, there are several different ways of apportioning power to all four wheels. Some systems utilise a device called a Haldex clutch coupled to the gearbox. Suitable for cars with transversely mounted engines that sit perpendicularly to the car's body length.

The Haldex clutch enables torque to be sent rearwards. The system however, is also not 'full-time'. This means it operates only under conditions when sensors detect a significant loss of traction at the front. Then as much as 50 percent of the engine's torque is sent to the rear wheels - at all other times torque is sent to the front wheels alone.

Other cars have longitudinally, or lengthwise mounted engines, necessitating a different setup to that in Haldex-based systems. The longitudinal mounting of the engine and likewise the gearbox means that torque only has to split at the front and rear axles, and this can be accomplished with a system called a Torsen, or TORque SENsing differential.

This system, unlike the Haldex clutch, is purely mechanical. Conventional differentials are not able to allow for a large degree of torque difference between the drive shafts. The Torsen differential takes extra torque from the shaft that is travelling more quickly and transfers it to the slower moving shaft. This helps mitigate the effects of different front and rear wheel speeds.

Subaru's Symmetrical All-Wheel Drive System
Subaru has cultivated a strong racing heritage through the World Rally Championship and, like Audi, it has brought its race-bred all-wheel-drive (AWD) technology to its road cars. One of the key features of Subaru's all-wheel drive system is the laterally symmetrical layout of the drivetrain.

Due to layout and packaging constraints, most other four-wheel-drive cars have all their mechanical components 'asymmetrically' laid out. For example, the engine and gearbox may not be perfectly oriented in line with the car's longitudinal axis and this would naturally present some problems with left-right weight distribution.

Subaru's symmetrical AWD system ensures that all major components are oriented to their cars' longitudinal axes. So if you were to 'chop' the cars in half lengthwise, each section would be a mirror image of the other.

Subaru uses a viscous coupling instead of a mechanical differential to split the drive shaft. This consists of two sets of plates, one connected to the front drive shaft and one connected to the rear. A viscous silicone fluid is used to bridge the gap between the plates. If one driveshaft starts to spin faster than the other, the fluid heats up and becomes even more sticky or 'viscous' and this helps transfer torque to the slower moving half of the driveshaft.

The main advantage of Subaru's Symmetrical AWD layout however, is equal weight distribution on the left and right sides of the car. This has obvious benefits for handling and traction. Traction, as we mentioned earlier, is the ability of a car to transfer torque to the road surface. If you start off with even loading on the left and right sides of the car you minimise any traction imbalance right from the start. Subaru is also the only manufacturer to offer an AWD system for all its vehicles, not just for its top spec models.

If its packaging and interior space, then a front wheel drive layout may suit you best. If you're an old-school purist then nothing less than rear wheel drive will suffice, but if you want the added peace of mind that comes from sure-footed traction then all-wheel-drive is for you.
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