Do you Really Need All-Wheel Drive?

It seems like only yesterday when all-wheel drive was still the reserve of only a few special vehicles. Proper handling champions like the Porsche 911 Turbo or the Nissan Skyline GT-R, and the pair of monsters that came from the stables of Mitsubishi and Subaru were amongst a few that worked through all four wheels, and everyone else simply managed with just two.

But things change faster than an Audi RS6 Avant at the lights in this industry. A few months ago, AMG head Tobias Moers announced that all next-generation Mercedes-AMG models are set to feature four-wheel drive as standard: apparently 90 percent of all customers opted for the all-wheel drive option on the previous generation E63s, and enquiries for an all-wheel drive option are high even on the AMG GT sports car.

This generation of Mercedes C63s could be the last to sport the nimbleness of rear-only drive. Shame.

But that’s not all. International sales of 4Matic and xDrive equipped models remain buoyant even all the way down to base models of the C-class and 3-series, while increasing sales across all SUV segments is starting to make all-wheel drive near ubiquitous even amongst non-performance vehicles.

So if you are wondering if you likewise ought to be opting for all-wheel drive in your next car, Oneshift looks at how they work as well as their pros and cons in this handy guide. Read on to find out more!

Before we can understand the complications behind driving any number of wheels we need to understand some basic car mechanics. Contrary to how many imagine cars work, any car that even occasionally spends time on the road needs to be able to have its wheels drive at different speeds. This is because as your car takes a turn, the outside wheels will need to spin faster as the radius of the circle they cover is larger. If this is hard to picture imagine a car constantly driving in a circle: the outside wheels will draw a larger circle and thus need to spin faster to cover more distance within the same time. Regular two-wheel drive cars achieve this by utilising open differentials to split power accordingly between left and right wheel, but when driving off-road this is far from ideal since once even a single driven wheel is lifted off the ground, any power sent through the drivetrain will immediately follow the path of least resistance straight and only to that same wheel, making sure that your car is unable to move anywhere.

Now this is the part that is going to be important if you are planning to buy a Subaru XV to take into some adventure trail up North: all-wheel drive (AWD) is not 4-wheel drive (4WD). The two are often conflated but actually represent very different sets of equipment under your car seat.

4WD is distinguished by the fact that it utilises a transfer case to give you a set of drive modes: typically 2-wheel drive high, 4-wheel drive high and 4-wheel drive low. When driving on the road, you put the transfer case into its 2-wheel drive setting, allowing the axles and wheels to rotate at different speeds as described above. However, once you are on a surface that will allow the wheels to slip, such as snow or gravel, you can utilise the 4WD modes which lock the centre transfer case differential and ensures that power is transferred to both the front and rear axle for maximum grip. Most trucks designed for heavy off-road duties, as well as some cars like the Land Rover Discovery will also feature another lockable rear differential which ensures that power gets split evenly again between the right and left rear wheels, for driving through really difficult undulating terrain.

In a permanent all-wheel drive vehicle such as the XV however, which does not allow switching over to 2-wheel drive only, power is still sent to all four wheels, but the differentials that divert power from engine to each axle and wheel are left open to allow the wheels to rotate at different speeds when on the road.

Does this mean that AWD is useless the moment one wheel is off the ground? On a very basic level, yes. However, an AWD vehicle with all open differentials can still resort to activating the brakes on the lifted wheel in order to force power to get redirected back to the other three wheels, moving the car along.

Don't fret. All the advantages of AWD are listed below.

The downside to utilising the brakes in this manner is that is still a crude system for managing power transfer off-road compared to 4WD locking differentials. You need to bear in mind that when off-road, the remaining wheels will also be offering various reduced levels of grip, making judging how much and when to brake each wheel difficult to get right even for the best systems.

Another factor to consider is that in an AWD car you will be lacking that low-range transfer box. If you have ever tried to drive up a kerb in a regular car you will know that it requires some power, even in first gear. When off-road, the constant revving to scramble up obstacles and low speed driving in an AWD car can make engines and transmissions throw up overheating warning lights. Proper 4WD vehicles have drivetrain components built and designed with this more abusive use in mind and the torque multiplication from the low range transfer box ensures that shocks to the drivetrain do not get transmitted back into the engine. A regular AWD system has to compromise some of its off road ability in order to display reasonable on-road smoothness and high-speed efficiency.

My advice? If you are looking for some adventure even in an AWD vehicle by all means go ahead, but be sensible and stick to the trails, avoid soft sand, as well as any continuous steep and undulating uphill climbs which can overheat the engine and clutch-based AWD systems.

Fortunately, if your flavour of AWD system is more on-road than off, the verdict is far more favourable.

Not all AWD systems are the same. At their most basic, as above, the engine is connected to a set of open differentials that are free to divert power to which ever wheel provides the least resistance. On the road where the difference between traction available at each wheel is not as great, even a basic setup like this will work smoothly and find more grip than just driving through two wheels.

However, performance oriented AWD setups also exist that transfer power through either active or limited-slip, as opposed to open differentials. As the name suggests, a limited-slip differential only allows a certain amount of slip, before locking itself and transmitting power equally to both left and right wheel. An active differential, meanwhile, is able to deliberately direct power across either the left or right wheel. Both of these differentials seek to transfer more power to the outer wheels of a car in a high speed turn, giving the car more cornering force by pushing the outer-side of the car further, and allowing for faster acceleration through a turn by delivering power to the wheels on the side of the car which will have more weight on them in a turn. Active differentials just do this better since they are able to take in electronic inputs such as the vehicle yaw rate, lateral acceleration and amount of slip experienced at each wheel, and transfer power accordingly to adjust the steering line of the car so that it stays as true as possible to your steering wheel input.

Audi’s quattro probably the most well-known AWD system.

Here is another area where permanent AWD and clutch-based systems shine. Suppose you are accelerating through a corner that then throws up an unseen slippery obstacle, or if a sudden surge of power is needed to safely merge into traffic in bad weather, with an AWD system, the car is already primed and ready to utilise any available traction from all four wheels, without requiring any intervention from the driver to change driving modes. An AWD system graces your car with a wider breath of traction to work with when accelerating through difficult conditions, without taking away from the concentration of a driver in situations where precious seconds can make the difference between relative safety and a costly repair.

Even on the roads it’s not all roses for AWD however. Forget everything the many advertisements imply when you shop for something with AWD. The difference in safety between driving an AWD car and a 2-wheel drive one is smaller than you think. You will notice that as above, AWD only provides better and safer acceleration, and has no effect on a car's ability to stop or turn in those same poor conditions. Unless you are a regular contestant at Gymkhana events, most of your accident avoiding manoeuvres are going to only involve either braking or steering, or a combination of both, so having AWD is not going to contribute to any margin of safety if you are already driving at speed. Be sensible and drive at a safe speed for the road conditions at all times.

One more thing to consider: an AWD car may be able to accelerate with all wheels, but in a hard braking maneuverer your front wheels will still be doing most of the work. When driving in truly treacherous conditions, it is important to pay attention to how your car is behaving in order to form a good gauge for how much grip is available from the surface below. With AWD, it can be easy to overestimate how much traction there is and end up driving too fast. Be vigilant of when your car starts to shimmy or cut power from the engine, since those are clear indicators that you are already demanding too much grip even from all four of your wheels.

Local price lists do not offer cost breakdowns, but looking at U.K. prices reveals that opting for xDrive typically adds close to £1,500 to the sticker price of a base spec 3-series, and BMW media usually places the penalty of xDrive at 70kg to the total weight of your car. But with more moving mechanicals also come more friction and greater fuel consumption: a G20 BMW 320i Sport equipped with xDrive will take an additional 0.5 litres / 100km against a similarly equipped one without xDrive. If you are concerned with economy, opt out of AWD.

xDrive a costly option in Europe, but still expected to be popular, even in saloons like this new 3-series.

The power outputs of even our regular mid-specification cars have grown substantially over the past decade, and their kerb weights even more so. If you must have that thrill of brutal acceleration, whether it be on the straight or in the bends, AWD is the way to go.

But is there any point to getting a car specifically equipped with AWD when opting for the smallest engine offered with your crossover? Unless you have plans to drive away from tarmac, I’d wager no. Press material for these cars often paint scenarios of unexpected obstacles and then procced to elaborate on how AWD systems offer greater grip, while casually leaving out the fact that that additional grip is only available under acceleration. But here is the real kicker I think: even if you do drive in the most severe of torrential downpours, you will still have a whole host of electronic management systems to ensure your car stays true to the direction your steering wheel is pointed. Electronic stability control and the anti-brake system specifically work to maximise your control of the car precisely where it matters most for accident avoidance, when doing hard braking or sharp direction changes, nullifying the claim that you get greater stability with AWD.

Always remember to look ahead when driving and keep to a reasonable speed.

Stay safe everyone!

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