Cars are complicated creatures. We’re happy to hop in to the driver’s seat, turn the key and go merrily on our way, barely giving a second thought to how our pride and joy works – at least until it goes wrong. Some cars are referred to as being four- or all-wheel drive, but what does this mean exactly?
There are two main methods of powering all four wheels at the same time.
- Permanent systems that constantly power all the wheels, all the time
- Temporary systems that operate as two-wheel drive until a loss of traction is detected
Magazines and websites cannot agree which is four- and which is all-wheel drive but, for the sake of simplicity, we’ll use the overarching term all-wheel drive. Where we’re discussing differences between the two, we’ll call permanent systems all-wheel drive (AWD) and temporary systems four-wheel drive (4WD). 4×4 means four-by-four i.e. four wheels, all four of which are powered, and is simply another way of referring to all-wheel drive.
How it works
The most common layout for modern road cars includes an engine mounted in the front of the car powering just the front wheels. The engine’s power is sent through the gearbox and onto the differential. The torque (turning force) applied to the differential is sent along two drive shafts, which are joined to each wheel, causing them to rotate in the direction of travel.
In a permanent all-wheel-drive car, power is sent through the gearbox to a centre differential. This splits the power to the front and back axles where it meets differentials at the front and rear. These distribute power between the rear wheels as well as the fronts, hence ‘all-wheel drive’.
Why is it useful?
Take a car which produces 100 horsepower. In a two-wheel drive car, the power is split evenly between both wheels so each wheel has to deal with 50hp when going in a straight line. In a four-wheel drive car each wheel will only be dealing with 25hp. With a lower amount of torque sent to each wheel, the chances of the grip being overcome by the torque (wheelspin) is vastly reduced, resulting in a more secure, stable feel.
In regular driving, a 100hp car will be relatively untroubled by sending its power through just two wheels, which is why the vast majority of smaller cars are two-wheel drive. Two-wheel drive is more fuel efficient when used on-road and cheaper to produce, making for a cheaper car. Enthusiasts also feel two-wheel drive cars are more ‘playful’ – though, for most normal drivers, this just means the car is more likely to spin out and crash.
Different types of all-wheel drive
Some cars are referred to as four-wheel drive, with others dubbed all-wheel drive, so what’s the difference? All-wheel drive cars permanently send power to all four wheels, while four-wheel drive cars only power all four on occasion. The Haldex-developed four-wheel-drive system found in many Volkswagen/Audi products (such as the VW Golf R) will default to front-wheel drive when all is well, only sending power towards the back wheels when slip is detected.
All-wheel drive cars tend to be suited to more specialised uses. Having all four wheels permanently linked is useful when off-roading because you can’t be sure which wheels are going to be able to grip so, by forcing all four to spin, you’re more likely to be able to pull yourself out of a rut. Some road cars, like the rally-derived Subaru WRX STI, use permanent all-wheel drive to deliver an ‘immovable object’ feel when driving. It’s worth mentioning that, although permanent, lots of all-wheel-drive systems allow the driver to switch between two- and all-wheel drive – typically using another controller next to the gearstick.
Several modern hybrid cars feature an all-wheel-drive layout. For example, the BMW i8 and Porsche 918 use a traditional combustion engine to drive the rear wheels, while electric motors power the fronts. Some cars even offer the option of switchable four-wheel drive – effectively overriding the computer controller. This allows the driver to manually select the drive mode that they prefer. This mainly features on more rugged SUVs like the Mitsubishi ASX.
Applications of all-wheel drive
The major use for all-wheel drive is while off-roading. Vehicles like the original Jeep and Land Rover were ideal in rugged terrain. If one wheel became stuck then, rather than just the other wheel moving things along, all three could help out.
The same benefits could be applied to motorsport. The World Rally Championship was transformed when the Audi Quattro stormed onto the scene in 1980. The added traction from its all-wheel-drive system made it much more manageable on the snow, mud and gravel of rally stages, helping it to 23 victories and two world championships over five years. The top rally cars have been all-wheel drive ever since.
Today, similar gains can be made in road cars. Of course, many SUVs like the BMW X5 are still all-wheel drive, but the performance-oriented end of the market has seen benefits too. Cars like the Lamborghini Huracan drive both axles in order to allow for traction and stability that many competitors can only dream of, with the added benefit of allowing for extremely rapid launches from a standstill. This explains why the Nissan GTR Nismo can smash the 0-60 barrier in 2.5 seconds, while the new rear-wheel drive Ferrari 488, despite weighing 350 kilos less and producing 70 horsepower more, ‘only’ manages it in 3.0 seconds.
With varying degrees of complexity, most all-wheel drive cars are able to vary the split of power sent to all four wheels in order to generate the maximum amount of grip at any time. Computers analyse the grip available at each wheel combined with the driver inputs then determine the optimum way to split power. For example, cars at the sportier end of the scale will likely have more power sent to the rear wheels under normal conditions.
It all sounds good so far, why isn’t every car four-wheel drive?
There are two main reasons: cost and efficiency. Including extra drive shafts, differentials and electrical systems to keep everything under control adds weight, while the losses through rotating these extra masses makes the system less efficient, with the result that you burn more fuel for a given distance travelled.
As we showed when we compared two- and four-wheel drive versions of the Range Rover Evoque, an equivalent front-wheel drive model will be lighter and more economical than a four-wheel drive version. It’ll be cheaper too – both to buy and maintain – because there are fewer things to go wrong.