The terms "four-wheel drive (4WD)" and "all-wheel drive (AWD)" are often confused; even by those in the car business. If you are a member of the group that can raise your hand and confess that these terms are confusing, this article is for you. Let's jump right in.

Four-Wheel Drive (4WD)

4WD systems are typically seen on trucks and large SUVs. These systems divide up the power coming from the vehicle's engine and transfer a fixed amount to each of the four wheels. Here's the key concept: each wheel receives approximately the same amount of power.

The device that does the splitting of the power is called the transfer case. Transfer cases are usually mounted in the center of 4WD vehicles and they power two small drive shafts that connect to front and rear differentials. Typically, these 4WD are rugged systems. Another thing you should know is that the transfer cases in four-wheel drive vehicles usually provide high- and low-speed gearing. Also, one of the key points with four-wheel drive is that it can be turned on and off by the driver.

All-Wheel Drive

All-wheel drive systems are typically seen on smaller SUV and sporty model cars. The way they work is that they send a variable amount of power to each of the four wheels depending on driving conditions. Got that: a variable amount of power, not an evenly split amount of power like 4WD.

It is important to note that the driver doesn't typically control the power distribution, the vehicles CPU does. Instead of a transfer case, all-wheel drive systems split the engine power between all four wheels with a center mounted differential driving controllable clutch packs on each wheel.

How they compare

4WD is great for off-roading and other low-traction situations because 4WD systems send a fixed amount of power to each tire. This is ideal for low-speed or off-road conditions but not so good for highway speeds. Typically, 4WD systems are shut off at highway speeds.

Volvo Cars of North Miami (FL) insists that AWD systems are smarter. They actively send power to the wheel (or wheels) that need it most. Because the power can be adjusted on the fly, all-wheel drive systems typically corner very well and can be used for highway driving.

Other systems

While four-wheel drive and all-wheel drive systems are common designs in today's cars, others do exist. Take the Chrysler Corporation's RAM truck line, for example. RAM trucks have typical part-time four-wheel drive drivetrains centered around two-speed transfer cases that offer three operating ranges: 2HI (two-wheel drive), 4HI (four-wheel drive) and 4LO (low-range reduction four-wheel drive).

Jeeps also offers another type of 4WD system called "Active Drive." It is a fully automatic 4WD system that seamlessly shifts in and out of four-wheel drive at any speed once slip is detected. This system requires no driver intervention. A fully variable wet clutch housed in the rear-drive module utilizes the Jeep brand's proprietary controls to provide the proper amount of torque for any driving condition, including low-traction surfaces and dynamic off-road driving.

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