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There are a number of different ways to implement AWD. Most automakers define the difference between all-wheel-drive and four-wheel drive by the mechanism used.


This month's weather was unusual, but not unheard-of, for Portland. Every three or four years, we get a solid snowfall that sticks around for several days. Within hours, the snow on many of our roads is packed down into a solid sheet of ice. When that happens, we see the same results every time. Cars and trucks go sliding, unable to climb even the most modest hills, and fender-bender accidents happen at every intersection.

During and after a snow event, interest in all-wheel-drive vehicles goes up, as people notice that those vehicles tend to drive along with much less difficulty than ordinary 2WD vehicles. There are a number of reasons for that, and there is also wide variation in the way that AWD systems operate. So a little explanation of how AWD works is timely right now.

First things first. The most important thing to know about AWD is that it helps your car get going and stay moving. To an extent, it also helps your car turn. But AWD does not help your car stop in any way. So drivers of AWD vehicles need to be just as careful as anyone else when it comes to following distance.

There are a number of different ways to implement AWD. Most automakers define the difference between all-wheel-drive and four-wheel drive by the mechanism used.

In four-wheel-drive (4WD or 4X4) the front wheels are not driven unless the driver specifically engages them through an extra gearbox called a transfer case. Whether you press a button or move a lever, a 4WD system depends on a transfer case. Most 4WD implementations also offer a high-range and low-range inside the transfer case, to gear down for extremely low speeds in order to maximize torque. This kind of design is really best for off-road driving and climbing steep inclines, rather than dealing with ice and snow.

In contrast, AWD is typically always working, and there's only one standard set of gear ratios available. Instead of a transfer case, there's an extra differential that divides power between the front and rear wheels. Then front and a rear differentials allow the wheels to turn at slightly different speeds to allow smooth cornering. The key thing to remember is that you don't have to do anything to engage most AWD systems.

Some AWD systems always direct power to all four wheels more or less equally, while others engage only when needed for traction. Most recently developed systems offer "on-demand" AWD, because powering all four wheels all the time reduces fuel economy. An on-demand system engages the rear wheels only when the front wheels start to slip. But while that might seem less effective, modern on-demand AWD systems are sophisticated enough to engage in a fraction of a second whenever wheel spin is detected. These systems engage so quickly and so smoothly, you'll rarely be able to tell when they're working and when they're saving fuel.

If you buy a new car today, it will come with advanced traction and stability controls, and with anti-lock brakes as well. If your new car or SUV comes with AWD, chances are good that you won't have any trouble the next time it snows.

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