If you are in need of a vehicle with the added safety and go-anywhere potential provided by four-wheel drive, you may be wondering which type is best suited to you, and probably what all the jargon means. Not only are there several different types of four-wheel drive technology, marketing departments love to confuse buyers by coming up with exotic new names and dazzling new features to entice buyers. But donít get stuck in a research rut, most four-wheel drive systems actually fall into a few basic categories, and here’s Diesel Car’s essential guide to debunking all the jargon and mystery.
Often called 4WD or 4×4, think of this as ëtraditionalí four-wheel drive, the hardware youíre most likely to find in a rugged SUV like the Jeep Wrangler or a pick-up truck. At the heart of this system, thereís a transfer case which receives drive from the engine and can output it to either two wheels (usually the rear axle) or all four. Most of these systems also feature a set of ‘low’ and ‘high’ range gears, and you choose between two- and four-wheel drive using a mechanical lever or electric switch. It used to be the case that you could only switch between two- and four-wheel drive while stationary, but most vehicles now offer what is termed ëshift-on-the-flyí via a dashboard mounted button for greater convenience. This is usually operable at speeds below 60mph and some vehicles also stipulate you should have the wheels in the straight-ahead position. In most cases youíll need to stop to shift into ‘low-range’ mode.
Select four-wheel drive and the advantages are excellent off-road performance and durability. With all four wheels driven at the same speed, most obstacles can be driven over without getting stuck. Select the low range gears and engine torque is maximised at far slower speeds, making it possible to crawl up or descend steep slopes without needing to utilise the brakes. Rugged off-roaders may also have the facility of a locking differential, which sacrifices the need for wheels to rotate at different speeds when cornering, to maximise your forward progress.
There are negatives though. Leaving such vehicles in four-wheel drive isnít usually suitable for prolonged or high-speed road use, causing more mechanical wear and tear to the drivetrain and possibly the tyres too. Lastly, heavy components with increased friction mean four-wheel drive vehicles tend to be considerably more expensive to run than two-wheel drive versions, which are simpler to build and lighter, with improved fuel economy as a result.
Permanent four-wheel drive
Perhaps itís surprising how few ëfour-wheel driveí cars actually drive all four wheels all the time, but the simple fact is, permanent four-wheel drive will always reduce fuel economy and ramp up CO2 emissions, to a certain extent. Thatís why it has become the preserve of models with a particular focus on either off-road ability or on-road handling, where the increased running costs are seen as an acceptable trade-off. How does it work? Well, engine power is sent to the front and rear axles, with either a front, neutral or rear bias, depending on the design brief of the vehicle and the kind of customers the vehicle needs to appeal to.
The Range Rover Sport has a rearwards bias (around 42:58 per cent front-to-rear) in road driving, giving the Sport a less nose-heavy feel in corners, while the Mercedes-Benz GLS-Class and GLE-Class get a neutral 50:50 split. Modern permanent four-wheel drive systems can also vary the amount of torque sent to each axle using a centre differential or clutch, apportioning it to the wheels with most grip. Off-road biased vehicles tend to be equipped with a low range mode too, allowing for steeper climbing abilities of up to a staggering 80 per cent gradient in the case of the Mercedes-Benz G-Class.
One crucial difference between permanent four-wheel drive models and part-time systems is the presence of differentials designed to allow the inside and outside wheels to travel at different speeds around a corner, without causing excessive wear to the drivetrain (often referred to as ëtransmission wind-upí) or to the tyres.
Another stalwart of permanent four-wheel drive is Subaru, which comes as standard on virtually every model, except the BRZ sports car. Its Symmetrical All-Wheel Drive system gets its name from the fact that the gearbox and propshaft are positioned along the centre line of the vehicle, with equal length driveshafts providing each wheel with power.
In motoring literature and showrooms, thereís little consistency around the naming of the four-wheel drive systems fitted to most modern crossovers, hatchbacks, saloon cars and performance cars, but letís stick with ëall-wheel driveí here. Increasingly common, you can think of all-wheel drive as ëintelligentí or ëadaptiveí four-wheel drive. This type of system is fitted to everything from a MINI Clubman to a Range Rover Evoque.
How does it operate? Here power is sent predominantly to either the front or rear wheels to improve efficiency and reduce running costs, but the vehicleís electronic brain constantly monitor sensors, harvesting data from everything from the wheels to the steering, and can send power to all four wheels in the blink of an eye. This usually happens when wheel slip is detected, which triggers a centrally-mounted clutch to lock and engage a driveshaft to send power to the wheels which are usually coasting.
Intelligent all-wheel drive has become incredibly popular with car manufacturers, because the fact most of us spend almost all of our time on tarmac makes full-time four-wheel-drive an unnecessary drain on fuel economy and increases CO2 emissions. Because all-wheel drive engages relatively infrequently, drivetrain components can also be lighter and more compact, further improving efficiency and cabin space. Safety gets a boost, too, because modern all-wheel drive systems can redirect power so quickly (more on torque vectoring later), which can help improve agility and neutralise skids which could otherwise cause a crash. The disadvantages? All-wheel drive systems are not designed for heavy, frequent use off-road, so they are less rugged as a result, and if youíre looking to do serious off-roading, then youíll need to look elsewhere at something tougher.
With the increasing popularity of hybrid and electric cars and SUVs, engineers have a new toolkit at their disposal. In a hybrid vehicle incorporating a traditional internal combustion engine (ICE) with one or more electric motors, whatís to say the electric motor needs to power the same wheels as the combustion engine? In vehicles with enough space, designers can position an electric motor close to the rear axle, giving a front-wheel drive model the automotive equivalent of ëplug and playí all-wheel drive. Hybrid SUVs from Toyota, Lexus and Porsche, along with the Peugeot 3008 HYbrid4 and 508 RXH are just a few of the models on the market, and while none are marketed as hardcore off-roaders, the extra traction is enough to improve all-weather safety and get you out of a muddy field. As an example, the electric motor in the rear of the 508 RXH might only have 36bhp, but its 148lb ft of instant torque can offer an impressive helping hand.
Of course, all-electric cars have the potential to be even more flexible in their layout. So, build a car with one or more electric motors powering the front wheels, while further motors also turn the rear wheels, and you have the ability to send torque to all four tyres almost instantly. Tesla has harnessed this technology in the Model S, creating a dual-motor electric all-wheel drive car which can accelerate from zero to 62mph in just 2.5 seconds in the P100D, and yet still tackle snow and ice when the weather conditions become more hostile.
Do you even need
While four-wheel drive will always be preferable when your SUV is up to its lug nuts in mud, or traversing a slope steeper than a roller coaster, two-wheel drive cars can be capable enough for most drivers. Tyres are one of the most important features, of course, with front-wheel drive cars equipped with mud and snow, or winter tyres, often able to outperform all-wheel drive cars equipped with normal road tyres once the temperature dips below seven degrees Celsius.
Thereís also a new breed of two-wheel drive crossovers fitted with heavily grooved all-season tyres and traction aids to help you keep going in difficult conditions. Models like the CitroÎn C4 Cactus Rip Curl and Peugeot 2008 with Grip Control, and the Fiat Panda Trekking employ a locking electronic front differential, applying braking force to a spinning front wheel to transfer torque across the axle to the tyre with more grip.
What else can help set four-
wheel drive vehicles apart?
Of course, being able to send power to all four wheels is just the beginning. For added safety and performance, there are other pieces of tech you might want to consider.
Hill descent control
If youíve ever been off-roading, youíll know the feeling of teetering on the edge of a precipice, with the nagging thought your two-tonne SUV is about to become a very expensive crushed piece of metal at the bottom of the ravine. When Land Rover introduced the Freelander without any low range gears, it pioneered Hill Descent Control to make such antics less fraught. Using the anti-lock braking system, it brakes individual wheels to control the speed of descent, with no input from the driver. Recent models have also included a variable speed of descent, all handily controlled by the driver via the cruise control buttons on the steering wheel. At first itís quite disconcerting, but once you take the leap of faith, youíll marvel at how easy the system makes traversing down a steep hill feel.
Vehicles with a locking differential can prevent a spinning wheel from sapping power, by redistributing it to the opposite wheel. In more serious, dedicated off-road vehicles, the driver will need to choose when to lock the mechanical differentials, and unlock them. In more road-biased vehicles this is taken care of automatically as anti-lock-braking sensors detect slippage, either locking the differentials as needed, or mimicking their effects with braking of individual wheels.
Torque vectoring control
You may have heard more about torque vectoring recently, because several manufacturers are citing the technology as dramatically improving the way their new models handle. But what is torque vectoring? Well, itís the next step beyond an open or limited slip differential, because torque vectoring adds computer control to the differential, so it can actively redistribute power, even if thereís no loss of grip at all.
Not only does this mean that manufacturers can programme the four-wheel drive system to respond in a certain way for varying off-road conditions (mud, snow, rocks etc), but they can also boost agility and handling. Take a sharp bend in a Range Rover Sport and more torque can be sent to the outside rear wheel, pushing the vehicle around the corner and neutralising understeer. In smaller, less expensive models, torque vectoring can also be achieved with braking, so in a similar example, a Volkswagen fitted with the manufacturerís XDS electronic differential lock system may apply a braking force to the inside front wheel to quell understeer.
Around view cameras
Engineers realised that by placing fish-eye cameras beneath the door mirrors and front bumper, a 360-degree image could be sewn together and beamed live to the infotainment screen. For those of us who donít have an off-road expert outside of the car to provide hand gestures to tell us where to steer, this birds-eye view can be a lifesaver when negotiating tricky terrain.
This system was pioneered by Jaguar and has been recently added to Land Rover models and is fitted to both two- and four-wheel-drive vehicles with automatic transmission. The system works well in low traction conditions, particularly with wet grass and ice. The electronics works hand-in-hand with the traction control system and limits the amount of torque being transmitted to the wheels. By moderating the power, thereís less likelihood that the wheels will slip, allowing more grip and smoother progress off the line.
Trailer stability programme
If you are buying a vehicle for extra safety while towing, try and choose a model equipped with a trailer stability programme. Vehicle sensors are programmed to detect a swaying motion before you may even feel it developing, and neutralise it by braking individual wheels on the car and automatically reducing engine power until the trailer and vehicle regain composure.