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.
- The transfer case receives power from the transmission and sends it to both the front and rear axles. This can be done with gears, hydraulics, or chain drive. 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.
- Transfer cases that are designed to allow for normal road use synchronize 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. Different rates of tire rotation are generally due to different tire diameters (since front and rear tires inevitably wear at different rates) and different gear ratios in the front and rear differentials since manufacturers will often have a slightly lower ratio in the front vs. the rear to help with control (such as a 3.55:1 in the rear differential and a 3.54:1 in the front differential). If the transfer case did not make up the difference between the two different rates of rotation, binding would occur and the transfer case could become damaged. This is also why a transfer case that is not designed for on-road use must never be driven on dry pavement.
- 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 for off-road use. 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 allow the vehicle to drive at much slower speeds while still operating within the usable power band / RPM range of the engine. This also increases the torque available at the axles. Low-range gears are used during slow-speed or extreme off-road maneuvers, such as rockcrawling, navigating dangerous roads, 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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