All-Wheel Drive is a much more recent innovation, and, as you might expect, much more complicated. It crops up on everything from supercars like the Audi R8 to grocery-getters like the Buick Encore. In fact, a good rule of thumb might be to think of AWD as the “car” system while 4WD is the “truck” system. In this rule, consider crossovers like the Honda CR-V, Toyota RAV-4, Mazda CX-3, etc. to fall under the “car” category while SUVs like the Chevrolet Tahoe and Toyota 4Runner fall under the “truck” category. The biggest difference between 4WD and AWD is that an AWD drive system is on all the time. Well, mostly. But we’ll get to that, as there are two types of all-wheel drive: mechanical and electronic.
The most common way of accomplishing a capable, mechanical AWD system is by using three differentials. A differential is a box of gears, a.k.a. engineering magic, that can take power from the transmission and split it at different levels between two wheels or the front and rear axles (four wheels).
In AWD, this system works to get power to the wheels with the most traction by splitting power between the front and rear axles on the center differential, and the individual wheels by way of the front and rear differentials. This is useful either in slippery conditions when different wheels might be getting different amounts of grip from moment to moment. The Mercedes-Benz E63 AMG is a perfect example. It is now sold only in AWD in the United States because its power can overwhelm the traction of the rear wheels alone. But even when we aren’t talking about 500+ horsepower cars, splitting power evenly means added stability in all types of weather.
AWD isn’t quite as robust as 4WD and it can’t match the same levels of traction in extremely low-speed off-roading that the older 4WD systems provide. But AWD does have some clear advantages.
The pioneer and industry standard for AWD systems, Audi Quattro, distributed torque mechanically. Quattro allowed Audi to dominate rallying for nearly a decade in the 1980s, but heaven help you and your bank account if it went wrong.
These days, computers are involved in most AWD systems. Sensors on each wheel monitor traction, wheel speed, and several other data points hundreds of times a second. An ECU (engine control unit) dictates where power is sent and to which individual wheel depending on whichever has the most grip.
This type of system, usually called torque vectoring, appears on everything from the Subaru WRX to the Dodge Charger these days. Torque vectoring has allowed massive improvements in handling and all-weather capability.
