All-wheel or four-wheel drive | what's the difference?

10 June 2015
 by 
, CarsGuide

It sometimes seems the world of motoring goes out of its way to be obtuse and overly technical. 

Take the terms "four-wheel drive" and "all-wheel drive" for example, how can the two be any different? Unless we're talking about three-wheeled Reliants, which surely only exist on TopGear, they must mean the same thing, right? 

Sadly not, and the amount you'll hear both these phrases in the modern market is a reflection of the fact that, since cars like Toyota's RAV4 went on sale here, two decades ago, the desire to have a car that looks like it could go off-roading, even if it never does, has been rampant.

As a result, the marketing types have appropriated "four-wheel drive" to apply to everything from hard-edged, boulder-bashing machines with snorkels to soft city SUVs that effectively run on two-wheel drive 99 per cent of the time. 

"All-wheel drive" on the other hand, is very popular with brands like Subaru, who sell it as a potentially life-saving way of improving your grip levels, not just on dirt but on the good old tarmac most of us spend our lives driving on.

So what are the differences, the promises, and the pitfalls of each?

Part-time four-wheel drive

In most traditional, off-road focused 4WD vehicles, the drive from the engine is sent to the rear wheels by default through a box known as the transfer case.

This case houses two cogs, which can be connected by a chain. You disengage the chain for two-wheel drive - just the rears - and it's operational in four-wheel drive. This locks the speed of the front axle to the speed of the rear one.

Four-wheel drives run in two-wheel drive on the road because roads are a high-grip surface and you don't need all four for grip the way you do on dirt, or snow.

Using four-wheel drive on the road for extended periods will chew fuel and tyres unnecessarily and cause serious damage to the entire system due to even more technical terms like transmission wind-up or driveline binding.

In part-time 4WD systems, engaging the transfer case gives you maximum drive in tricky off-road situations. The wheels will still slip and scrabble due to the loose surface, though, which ensures that any overwinding on a wheel will resolve itself by spinning to release the tension.

On the road, however, wheels need to spin independently to drive around corners. If each wheel's rotation is constricted by the four-wheel-drive system, cornering will cause tyres to slip or spin to try and maintain a constant rotation rate. 

If the tyres can't slip to relieve the pent-up energy, it stresses the wheel hubs and driveline to breaking point, and possibly a four-figure repair bill.

Full-time four-wheel drive

Full-time 4WD does what it says on the box: powering all four wheels, all the time. To get around the problem of transmission wind-up, it employs a centre differential, which allows a "different" speed for each axle.

Even though the transfer case is constantly engaged to power the front and rear wheels, the diff allows for different rotation speeds. 

This means that, on the road, the 4WD system won't try to hold each wheel at a fixed speed, avoiding the potential for transmission wind-up.

Vitally, in full-time systems the differential can be "locked", forcing the wheels to rotate at the same rate, and thus providing the same gravel-grappling off-road ability as its part-time counterparts.

Low-range four-wheel drive

Part-time and full-time four-wheel drives will also generally have a low range transfer case. Low-range is for hardcore off-roading only, so basically if you don't have a beard, a pocket knife on your belt and a collection of King Gee clothing, it's probably not for you.

The four-wheel-drive system works the same way as in high-range, but the actual drive is transformed by changing the final drive ratio.

This ratio is modified in the transfer case by a small gearbox, which allows the engine to deliver peak torque at low speeds, but severely limits your top speed.

For technical off-road driving - basically crawling very slowly up or down rock-strewn hills - low-range is sometimes necessary to ensure plenty of torque at safe speeds and to limit the speed of steep descents without needing to ride the brakes, which can also cause you to skid out of control into oblivion.

All-wheel drive

The term "all-wheel drive" is more closely associated with traversing dirt roads at stupidly high speeds, in rally cars.

AWD doesn't use a transfer case, opting instead for a system that shuffles torque to where it's needed most, while still allowing for individual axle speeds.

"In a lot of all-wheel-drive systems, the engine drives a front-mounted gearbox, which drives the front axle through the front differential first," Subaru Australia's tech guru Ben Grover explains.

Not all kinds of all-wheel drive are the same

"The rotation of the front axle in turn runs a central shaft, which turns the rear axle.

"This means that the majority of the torque goes to the front axle, with the rear driveshaft receiving a maximum of 40 per cent.

"On the other hand, Subaru's system powers the centre differential first, which means that the system can send up to 70 per cent of the torque to the rear axle."

You guessed it, not even all kinds of all-wheel drive are the same.

On-demand AWD

This is commonly used on passenger cars and soft roaders SUVs, which are more popular with female buyers because of their raised seating position than because of which wheels they're driving.

Instead of constantly powering all wheels, the vehicle runs in two-wheel drive (generally the front wheels) by default. When the fronts begin to spin, sensors detect a loss of traction and redirect engine torque to the other axle to ensure maximum grip.

It's a clever system, because it doesn't give you what you don't need, until you actually do.

The reduced friction of only driving two wheels most of the time also tends to use less fuel than full-time all-wheel drive systems, which can represent big savings over the life of a vehicle.

So which is best?

Despite what the seemingly endless Jeep commercials might tell you, two-wheel drive is all you really need in most Australian road conditions - hence the fact that the Commodore and Falcon have never bothered with all-wheel drive.

The vast majority of drivers never attempt to leave sealed roads and the few that do usually stick to nicely graded dirt tracks and the odd muddy paddock.

It's surprising just how far you can get with an all-wheel drive with decent ground clearance

If all-wheel drive is necessary – or if you want the safety of all-wheel traction, a constant all-wheel drive system is best, followed by its on-demand equivalents.

The reaction speed of on-demand AWD is nothing short of astonishing; when combined with modern traction and stability control, the practicable difference is minute. A proactive, full-time system is still better, of course.

True four-wheel-drive systems only come into their own in the kind of terrain you probably don't need to take a car into at all.

It's surprising just how far you can get with an all-wheel drive with decent ground clearance, especially if you stick to marked trails or fit off-road tyres.

So really the choice between all-wheel and four-wheel drive is as clear as mud - if there's a huge amount of it, you might need the latter.

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