Transfer case

A transfer case is a part of the drivetrain of four-wheel-drive, all-wheel-drive, and other multiple powered axle vehicles. The transfer case transfers power from the transmission to the front and rear axles by means of drive shafts. It also synchronizes the difference between the rotation of the front and rear wheels, and may contain one or more sets of low range gears for off-road use.

Transfer cases used on "part-time" four-wheel-drive off-road vehicles such as trucks, truggies, rock-crawling vehicles, and some military vehicles generally allow the driver to select 2WD or 4WD, as well as high or low gear ranges. Those used in sports cars and performance sedans are usually "transparent" to the driver; there is no shifter or select lever.

There are two different types of internal power-transfer mechanism found in most transfer cases. Gear-driven transfer cases use sets of gears to drive either the front or both the front and rear driveshafts. These are generally strong, heavy units that are used in large trucks, but there are currently several gear drive cases in production for passenger cars.

Chain-driven transfer cases use a chain to drive most often only one axle, but can drive both axles. Chain-driven transfer cases are quieter and lighter than gear-driven ones. They are used in vehicles such as compact trucks, full size trucks, Jeeps and SUVs. Some off-road driving enthusiasts modify their vehicles to use gear-driven transfer cases, accepting the additional weight and noise to gain the extra strength they generally provide.

Transfer cases are also classified as either "divorced"/independent or "married". Married transfer cases are bolted directly to the transmission, usually between the transmission's output shaft and the rear or main driveshaft. Sometimes a married transfer case is an integral part of the transmission and the two components share the same housing or "case", as is commonly found on recent Subaru products and some other all-wheel-drive cars.

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Wikipedia