thumb|Inside of a [[New_Venture_Gear#Transfer_cases|231 New Process Gear transfer case. Part-time/Manual, shift on the fly]]

A transfer case is an intermediate gearbox that transfers power from the transmission of a motor vehicle to the driven axles of four-wheel-drive, all-wheel-drive, and other multi-axled on- and off-road machines. A part of the vehicle's drivetrain, it employs drive shafts to mechanically deliver motive power. The transfer case also synchronizes the difference between the rotation of the front and rear wheels (only high-speed 4wd-Awd systems), and may contain one or more sets of low range gears for off-road use.

Functions

The transfer gearbox (a secondary transmission system) receives power from the transmission and sends it to both the front and rear axles, or just one (usually the rear.) 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 was traditionally accomplished by means of a shifter, similar to that in a manual transmission. Increasingly it is electronically operated by a dashboard, center console, or shift lever switch.

A transfer case that allows alternating between 2-wheel drive and 4-wheel drive modes but lacks a center differential for coordinating axle speeds is known as "part-time". Some vehicles, such as all-wheel-drive (AWD) sports cars, have transfer cases that are not selectable, known as "full-time". Such a transfer case is permanently "locked" into AWD mode. In recent decades hybrids have been developed that share properties of each.

Transfer cases also perform other functions. Some are common to all types, others vary by type:

  • 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. In 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 for very inclement road conditions, towing a heavy load slowly, driving on rough, unimproved roads slowly, and extreme off-road maneuvers such as rockcrawling. This feature is often absent on all-wheel-drive cars. Some very large vehicles, such as heavy equipment or military trucks, may have more than one set of low-range gears.
  • The transfer case may require stopping entirely to engage four-wheel drive (or just low-range 4WD), slowing to a designated speed to engage or disengage an extra axle, or may permit full "shift-on-the-fly" engagement of high-range 4WD at any speed.
  • 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 the 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 will cause problems with driveline windup if driven on dry pavement.
  • Most traditional transfer cases designed for off-road include the ability to mechanically lock the front and rear drive shafts to evenly distribute engine torque between them.