Everything you need to know about all-wheel drive

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All-wheel drive can be nice to have and, for some drivers, essential, but do you really need it? And if you do, what should you look for?

There are downsides to AWD. It’s more expensive to buy, there’s more to fix if something goes wrong, and it’s heavier, using more fuel even when it’s not activated. The average AWD system adds about 60-70 kilograms to a vehicle, about the same as an adult passenger.

However, all-wheel drive provides torque to all four of your vehicle’s wheels, not just those at the front or the back. Some sports cars use AWD for better grip around corners when the tires are being pushed hard; it can also give better grip on wet, slippery roads. Most drivers, however, want AWD for its better traction in snow or on ice, or for driving off-road. It’s a popular option for Canadian vehicles.

Each manufacturer has its own all-wheel-drive system, and there can be differences even between one maker’s models. Honda, for example, has four different versions of AWD in its Honda and Acura lineup, depending on the level of off-road or sport ability that the vehicle is designed for.

Most AWD vehicles are designed to run normally in two-wheel drive, disengaging one of the axles to use less fuel; you only need to drive one pair of wheels when paved roads are dry. For the majority of drivers, this is just fine. They save fuel and have the comfort of knowing their vehicle will switch itself automatically into all-wheel drive if one of the tires should slip. This process is almost instant because sensors check for traction hundreds of times a second and activate the system long before the wheel’s finished turning.

When this happens, the vehicle’s computer will monitor the amount of slippage and send the appropriate amount of drive power to whichever wheel it considers most in need of it. Some systems can move up to 100 per cent of their power to either axle, while others limit the variance.

Electric vehicles with AWD have separate electric motors to drive the second pair of wheels, which are activated electronically. Again, there’s no point in using the power of their batteries and limiting the range by driving extra wheels when they’re not needed.

Some vehicles, notably off-roaders and almost all Subarus, have permanent all-wheel drive, in which power is always sent to both axles, in varying degrees. This uses more fuel, and you probably don’t need it, but there’s less opportunity to lose traction. After all, a good AWD system will keep you out of trouble without you even noticing it’s working.

Better systems include torque vectoring, which supplies differing amounts of power to either wheel on a driven axle. All vehicles have a differential unit on their axles that regulate this; when you turn a corner, the wheel on the inside will turn less than the wheel on the outside, and the differential allows for it. When one of the tires begins to slip, even if it’s not being driven by the powertrain, most systems will apply the brakes to that wheel to keep power constant to the gripping tire, though some will also apply additional power to that wheel.

An off-road vehicle can lock that differential, so if you find yourself in mud or deep snow with one tire spinning, the axle will continue to turn the other wheel to pull the vehicle out. If your SUV has this as an option, use it sparingly and only on slippery surfaces, or you’ll wreck your tires and drivetrain by forcing them to slide on a dry surface.

All AWD vehicles also have a differential between the front and rear axles because, in a corner, all four wheels will turn at different speeds. The amount of power to each axle can be set electronically to provide a rear-wheel or front-wheel bias. Some vehicles can lock the differential on those two axles as well, and these are generally called four-wheel drive, or 4WD, but the distinction is blurring these days as systems become more advanced.

Truly off-road-capable vehicles, such as Jeeps and Land Rovers, also offer transfer cases that provide a range of lower gearing. If you’re venturing over very slippery terrain, slow and steady often wins the day and will help get you through to the other side.

For almost all drivers, all-wheel drive is something that’s only used for a very short time when on the road. Two-wheel-drive vehicles all have some form of traction control that limits the amount of tire slippage around corners or on poor surfaces, and more advanced systems provide the torque vectoring that will control oversteer or understeer around corners at speed.

A budget-conscious driver who wants to beat snow and ice should buy good quality winter tires before anything else because they’re where the rubber literally meets the road. There’s little point in investing in all-wheel drive if none of your tires is going to grip properly.

But if you do find yourself with your driving wheels in a mud hole, stuck in a deep pile of snow or trying to drive up a steep and icy slope, that’s when you’ll be grateful you have all-wheel drive. If it saves you being rescued by a tow truck or gets you where you need to be on time, then it could be money well spent.

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