Functions
- The transfer case receives power from the transmission and sends it to both the front and rear axles. This can be done with a set of gears, but the majority of transfer cases manufactured today are chain driven. On some vehicles, such as four-wheel-drive trucks or vehicles intended for off-road use, this feature is controlled by the driver. The driver can put the transfer case into either "two-wheel-drive" or "four-wheel-drive" mode. This is sometimes accomplished by means of a shifter, similar to that in a manual transmission. On some vehicles this may be electronically operated by a switch instead. Some vehicles, such as all-wheel-drive sports cars, have transfer cases that are not selectable. Such a transfer case is permanently "locked" into all-wheel-drive mode.
- An on-road transfer case synchronizes the difference between the rotation of the front and rear wheels, in much the same way the differential acts on a given axle. This is necessary, because the front and rear tires never turn at the same speed when front and rear tire sizes differ.
- Transfer cases designed for off-road use can mechanically lock the front and rear axles when needed (e.g. when one of the axles is on a slippery surfaces or stuck in mud, whereas the other has better traction). This is the equivalent to the differential lock.
- The transfer case may contain one or more sets of low range gears (generally for off-road vehicles). Low range gears are engaged with a shifter or electronic switch. On many transfer cases, this shifter is the same as the one that selects 2WD or 4WD operation. Low range gears slow down the vehicle and increase the torque available at the axles. Low-range gears are used during slow-speed or extreme off road maneuvers, such as rockcrawling, or when pulling a heavy load. This feature is often absent on all-wheel-drive cars. Some very large vehicles, such as heavy equipment or military trucks, have more than one low-range gear.
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—Ralph Waldo Emerson (1803–1882)
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