What's the difference between 4WD and AWD?

The popularity of four-wheel drive and all-wheel drive vehicles has boomed in the last decade.

Most mainstream manufacturers have released ‘crossover’ vehicles to take advantage of the public’s obsession with SUVs, but it’s not just traditional SUV and 4x4 vehicles that have contributed to the boom.

More and more cars have been offering all-wheel drive drivetrains, from high-performance sports cars to luxury sedans and family-oriented people-movers.

But why do manufacturers use different terms? Isn’t four-wheel drive (4WD) and all-wheel drive (AWD) the same thing? Let’s have a look at the different terms in use and what each of them should mean.

Although each manufacturer tends to use its own name for its chassis systems for branding reasons, we are generally able to separate 4WD/AWD drivetrains into three basic groups: full-time 4WD, part-time 4WD and all-wheel drive.

Each system is different in terms of how and when torque is supplied to each wheel, so it's important to choose the correct system for your typical vehicle use.

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Full-time 4WD operates as the name suggests. At all times torque is supplied to all four wheels, all of the time. The driver usually has several options available to them which affect the operation of the drivetrain depending on the conditions that are encountered.

In normal operation (such as driving on a bitumen surface) the front and rear axles are split by a differential which allows them to operate at different speeds when required -- for example, when cornering.

In most vehicles there will be the option of ‘diff lock’. This locks up the centre differential and restricts any rotational difference between the front and rear axles. It’s a feature that is commonly used when off-roading to gain maximum traction.

There may also be the option of ‘low range’. Usually used in extreme off-road conditions and on steep inclines, this changes the gearing of the drivetrain to provide a torque multiplying effect similar to the lowest gearing on a mountain bike.

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Part-time 4WD is the original 4WD system and the most basic.

As with full-time 4WD there are several options available to the driver. In normal conditions the vehicle is driven in two-wheel drive, usually powering only the rear wheels. 4WD mode can be selected either by a separate mechanical lever or by an electronic switching system.

When 4WD is selected, torque is split evenly between the four wheels.

Part-time 4WD vehicles tend not to have differentials between the front and rear axles. This limits the use of 4WD to off-road use because use of this feature on bitumen surfaces (where tyres have good traction) can cause excess stress and damage to the drivetrain (often called wind-up).

Most vehicles of this type have the option of ‘low range’ for extreme off-road conditions.

All-wheel drive is in some ways similar to the full-time 4WD system in that it also sends torque to all four wheels constantly.

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However, most modern AWD systems don't offer drivers the option to operate in two-wheel drive and, unlike the 4WD systems, the differential between the front and rear axles cannot be locked.

The differentials do, however, have ability to limit slippage between the axles if a low traction situation is encountered.

For example, some high-tech performance AWD systems can direct up to 100 per cent of torque to the front or rear axle depending on conditions (think Porsche 911 Turbo).

Meantime, some AWD systems are essentially only part-time, driving only the front wheels until traction loss is sensed and torque is automatically directed to the rear wheels (think Honda CR-V).

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Usually this ability is provided with a differential known as a viscous coupling or multi-plate clutch, although more recently a similar outcome has been achieved by using electronically-controlled hydraulic or electro-magnetic ‘clutch’ systems.

AWD systems also lack the ‘low range’ feature which is common in most 4WDs, like a LandCruiser, and are  less effective and more fragile than 4WD systems in situations where high torque is required.

Indeed, most AWD systems simply cannot satisfy high-torque demands when the vehicle is off-road, therefore AWD vehicles tend to be more ‘car-like' or 'crossover' wagon style than obvious off-roaders like the Ford Everest.

That said, many car-based 'SUVs' are now also available in cheaper front-drive form and indeed some are produced only in 2WD guise, like the Ford EcoSport.

Given the capability of most modern (and now mandatory) electronic control systems, if you're buying such a vehicle as a higher-riding, more practical substitute for a small hatch or family wagon, then save your money and don't pay more to buy, run and maintain an all-wheel drive.

If you need the extra traction of AWD for slippery or loose unsealed roads where extra ground clearance doesn't suffice, chances are the greater cost will be offset by the benefits of all-wheel drive.

But choose only four-wheel drive – as fitted to the ute-based Everest, Toyota Fortuner, Holden Trailblazer, Mitsubishi Pajero Sport and Isuzu MU-X – if you plan on heading well away from the beaten track.