Four-Wheel Drive (4WD)
Definition:
A method of driving a vehicle by applying engine torque to all four wheels. Various schemes are used for 4WD including part-time, full-time and variable four-wheel drive. The primary advantage of four-wheel drive is increased traction-which is especially useful for off-road excursions or severe weather driving, but is of little practical value for normal driving. Because of the added friction in the drivetrain, a four-wheel drive vehicle typically gets significantly lower fuel mileage than a front- or rear-wheel drive vehicle. To help cut the drag, most 4WD drivetrains have a transfer case that allows the driver to select either two- or four-wheel drive depending on driving conditions. In trucks, you'll often find locking hubs on the front wheels that can be locked in the "on" or free-wheeling position as needed. Some performance cars have full-time variable four-wheel drive and use a computer-controlled transfer case to route power between the wheels.
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