Vehicle drives come in two basic classes, follow by two sub-divisions. Most cars are two-wheel-drive, with either the front or rear axles receiving the power. Four-wheel-drive vehicles come in two basic varieties, either true four-wheel-drive (annotated as 4WD) and all-wheel-drive (AWD). While similar in principal, 4WD and AWD systems are very different where dynamics and application are concerned.
Most 4WD vehicles (particularly trucks) start out as rear-drive, but use a transfer case bolted to the back of the transmission to send power to the front axle. The transfer case works something like a two-speed manual transmission. The primary shaft runs straight through the case from the transmission output to the rear-axle driveshaft. A set of gears connect the primary shaft to a secondary shaft, which drives the front wheels. The driver engages the second shaft through either a high-range (1-to-1 ratio) or low-range (2-to-1 or numerically higher ratio) gear set.
4WD vs. AWD
An AWD system uses a center differential in lieu of the 4WD truck's transfer case. The center differential allows the front and rear axles to turn at different speeds when need be, which allows the vehicle to turn more easily on the street. In these systems, all four wheels are driven all the time. By contrast, a 4WD vehicle's transfer case is a strong, solid mechanical connection, so power always splits 50/50 from front to rear. This configuration is ideal for most types of off-roading, particularly low-speed events like rock climbing that require massive amounts of consistent torque.
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- "Race Car Engineering & Mechanics"; Paul Van Valkenburgh; 2004
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