TechSplanation: All-Wheel Drive

Published: Friday, January 20, 2012

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The Northeast, Great Lakes and even Seattle have been hit with winter storms this week. Is it time to consider whether or not you need or want an all-wheel drive-car or SUV to better handle the snow and slush?

Not surprisingly, sales of all-wheel-drive vehicles spike in winter months. But there are pros and cons to owning one. Also, before you buy you should understand how the systems work, and how they are different from four-wheel-drive systems.

Check out our new feature--TechSplanations-- to get an easy-to-digest take on how these systems work, and then check out our gallery of AWD and 4WD vehicles to get our editors' comments on each vehicle. Finally, if you are interested in pricing and more detailed information, research the car on our easy-to use shopping tool.

What is All-Wheel-Drive

All-wheel-drive vehicles have systems in which the engine drives all four wheels at the same time, as opposed to just the front wheels in most cars. By means of sensors and the vehicle's central computer power from the engine is routed to each wheel depending on your traction. If you are traveling on a dry road, your car will operate in front-drive mode or rear-drive mode, depending on its orientation. A BMW all-wheel-drive vehicle tagged with the "x" designation will travel in rear-drive on a dry road. A Subaru Impreza will move in front-drive mode in dry conditions. But if that car is in snow, wet leaves, gravel, sand, or the like, the sensors will automatically send power to each wheel in the right proportions to maximize traction.

How does it work?

An AWD system is made up of the front and rear differentials and the transfer case. The differentials are located between the front wheels and the back wheels. The differentials transfer torque (power at low engine revolutions-per-minute) from the drive shaft to the drive wheels. Sensors will shift that torque to each wheel depending on traction needs, literally putting the power to the pavement to maximize control of the vehicle. Besides providing drive power to all four wheels, an AWD system can also help your vehicle make turns--the inside wheels and outside wheels of your vehicle take different paths during a turn, and having them move at different speeds makes your vehicle corner more effectively.

The beauty of the AWD system is that it woks automatically, leaving the vehicle in two-wheel-drive when that's all that is needed, which is most of the time. If an AWD system worked full-time, the vehicle would unnecessarily burn a lot of fuel. Why would I want it?

If you live in the Northeast, Great Lakes, Pacific Northwest or any place that is going to see snow four or five months of the year, or if you live and drive on a lot of unpaved roads, an all-wheel-drive vehicle is the ticket. It's not for no reason that these are the strongest markets for Subaru, which has long offered AWD systems as standard equipment.

Is there any downside?

These systems are terrific on pavement and unpaved roads, but an AWD car or crossover is not the same as a Jeep or Range Rover, and is not designed to handle the same sorts of off-road courses. AWD systems are different than four-wheel-drive systems which have levers or buttons inside the car that enable the driver to lock the differentials, putting the car into a low-range mode where all four wheels are engaged for real off-roading and trail crawling. AWD vehicles also carry a fuel economy penalty, as the weight of the system means your vehicle will get about 5 mpg less than if it wasn't equipped with AWD.

What vehicles offer it?

All Subarus come with AWD as standard equipment, except for the new BRZ sportswear. Audi's Quattro AWD system is an option on its vehicles, but it is offered on all models. Some family sedans, such as the Ford Fusion and Suzuki Kizashi have AWD as an option. BMW AWD cars are marketed as "xDrive"; Mercedes-Benz AWD vehicles are marketed as "4matic"; Volkswagen markets its AWD vehicles as "4Motion." Suzuki markets one of the few small-car AWD systems in its SX4. Virtually all SUVs and crossover vehicles have AWD options.

Bottom line

If you drive in the snow or travel lots of unpaved roads, it's tough to find a better insurance policy than a solid AWD system. The worst vehicle we can think of to drive in snowy climes would be, say, a Ford Mustang or a light, two-wheel-drive pickup truck like a Dodge Dakota.

View Gallery: AWD Vehicles