How It Works: Variable All-Wheel Drive

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Thanks to Canada’s wintery winters, all-wheel drive (AWD) is a very popular feature. But while it sounds like all wheels driving all the time, that isn’t necessarily the case.

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Most AWD cars and sport utilities use some form of a variable system, able to distribute power as needed for extra traction, including on slippery surfaces, and on curves and acceleration.

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How It Works: Variable All-Wheel Drive Back to video

Four-wheel drive (4WD) isn’t the same thing. Most 4WD pickup trucks and SUVs use a “part-time” system, which requires you to engage four-wheel when needed. On an AWD car or sport-utility, the system is active all the time.

All vehicles use differentials. These are units with gears, connected to the output shafts that turn the wheels, which let the wheels turn at different speeds. Whenever you turn a corner, the outside wheel has to travel farther than the inside wheel. The differential allows it to turn faster to make it around.

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However, there’s always a limit as to exactly how fast one wheel can turn. If one tire’s spinning on ice, for example, you want the wheel on the other side to get most of the power, or you won’t be going anywhere. A limited-slip differential transfers power to the wheel that has traction.

With all-wheel drive, there’s a centre differential that transfers power between the front and rear wheels as needed. Exactly how much engine power is split between them depends on the vehicle. Generally, under normal driving, most mainstream AWD puts more power to the front wheels, while sportier models will usually emphasize the rear wheels. If a system is described as 60/40, for example, it means 60 per cent of power goes to the front wheels, and 40 per cent to the rear.

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AWD vehicles that direct more power to the front wheels include the Toyota Highlander and Ford Escape, while examples of rear-bias vehicles include the BMW X5 and Dodge Charger AWD.

Subaru’s “symmetrical” all-wheel drive sounds like each wheel gets the same amount of power, but the name actually refers to the way the driveline is laid out symmetrically along the car’s axis. While some Subaru models do divide power 50/50 front to back, most are 60/40.

On any AWD vehicle, when power needs to be moved around, how much is available depends on the system. Many only go as high as 50/50 front to rear, but some can transfer as much as 100 per cent to the front or back as necessary. From there, some of them – mainly sportier vehicles – can also distribute power between the left and right wheels.

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In a few cases, that can be most or all of the engine’s power directed to just one wheel, if that’s what’s best for the driving conditions. Some less-complex systems provide this type of “torque distribution” by applying the brake on the wheel that’s spinning, so the wheel on the other side takes over the task of moving the vehicle forward.

The movement of power is done automatically, but on some crossovers and SUVs, there’s a “Lock” button on the dash. When it’s activated, it locks the differential so the front and rear wheels turn together at the same speed. It’s only meant for getting out of snow or mud at very low speeds, and it shuts itself off once you get moving above that speed limit.

There are different ways to move that power around. Some centre differentials use a viscous coupling. This unit uses plates that are connected to the output shafts, and which are submerged in thick fluid. If one wheel slips, its plate turns faster. That gets the fluid circulating, and it’s thick enough to pull the other plate and get that wheel – the one with traction – moving too.

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Multi-plate hydraulic clutch differentials react faster to wheel slip, but they’re more complex and, correspondingly, more expensive. These use clutch plates that are activated by hydraulic pressure, sending power to a wheel when needed. The hydraulic pumps are activated electronically, and the system can gather information from the vehicle’s various sensors – such as speed and how far the steering wheel is turned – to determine the potential for traction loss, and activate the all-wheel system as needed.

A few vehicles use Torsen differentials (it’s a brand name), which are mechanical and use pairs of small gears meshed to larger gears on the shafts. When one wheel slips, the gears transfer power to the other.

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All of these AWD systems use a drive shaft that connects the front and rear axles, so the engine’s power can be distributed between them. It takes more fuel to drive an extra set of wheels, though, and a few vehicles can electronically disconnect one axle when all-wheel isn’t needed. When it is, they reconnect in a split second to transfer power.

Some hybrids and electrics offer all-wheel drive, including the 2019 Prius AWD-e and Tesla’s Model 3, but they don’t use a connecting shaft. Instead, the engine/electric motor powers the front wheels, while a separate electric motor (or motors) kicks in to power the rear wheels when they’re needed. It’s whatever it takes to get you where you’re going.