Introduction:
A transfer case consists of a shifting system with multi-plate clutches, which can regulate the drive to the front wheels. The system is used by BMW in the X-drive systems (which are used in, for example, the X3, X5, X6, or 330xi). Every BMW is standard rear-wheel drive. The X-drive versions are all-wheel drive. However, this is not a permanent all-wheel drive. With permanent all-wheel drive, the front and rear wheels are not disengageable and always rotate together. Unfortunately, this brings disadvantages, such as increased consumption, increased tire wear, etc.
A transfer case ensures that the rear wheels are directly connected to the transmission. This is a fixed connection and is not regulated by the transfer case. Depending on the load on the engine (sporty acceleration) and the type of road surface (with slippery, unpaved ground), the connection to the front wheels is regulated. The transfer case will have the front wheel shaft rotate at the same speed as the rear wheels using the engaged multi-plate clutch.
Operation of the transfer case:
In cars with a transfer case, the transmission is directly connected to the rear wheels. In the image below, this is clearly visible with the red line. This line runs from the transmission directly to the driveshaft of the rear wheels.
When the rear wheels are about to slip or when transferring a lot of power to the road, the electric motor is activated. The electric motor rotates a gear that then operates a control slide. The rotation of the control slide is visible with the blue arrows. This control slide presses the multi-plate clutch, creating a connection between the gears of the rear wheels and the front wheels. Once the multi-plate clutch is engaged, the force indicated by the green arrows arises. The drive shaft to the front wheel differential is now driven. The front wheels now rotate at the same speed as the rear wheels.
In vehicles from the VAG group, the 4-motion / Syncro / Quattro system is applied. This works with the help of a Haldex clutch mounted at the rear axle. Click here for information about the Haldex clutch.
Transfer case with high and low gearing:
Transfer cases on off-road vehicles, such as the Chevrolet K30 and the Jeep CJ7, are equipped with a gearbox that allows for high or low gearing (in English: high gear and low gear). This provides a different gear ratio between the transmission and the wheels.
In off-road vehicles, the low gearing is used when driving over terrain with sand, stones, and hills. When switching from high to low gearing, the engine RPM increases, with the advantage that the traction at the wheels becomes significantly higher.
The following image shows the drivetrain of a four-wheel drive vehicle (left) with an arrow in the driving direction, and on the right, the different positions of the transfer case. The yellow color of the gears and axles indicates that these parts are engaged or driven.
- 2-high: the transmission drives the driveshaft for the rear wheels. In this position, one generally drives over flat terrain;
- 4-high: the four-wheel drive is engaged;
- neutral: the drive between the transmission and the wheels is disconnected;
- 2-low: between the input shaft of the transmission and the output shaft to the differential is connected to an extra shaft with gears. The power flow goes from the large gear (top) to the small one. The input shaft speed increases, as does the torque of the output shaft;
- 4-low: again, the four-wheel drive is engaged. Just like in 2-low, the middle shaft with gears is used for low gearing.
Selecting the different positions is done by operating the extra lever in the interior. In most off-road vehicles, this lever is located near the “normal” gear lever of the transmission. The image shows the shift pattern of a Dodge Ram. It can be seen that the 2L position is missing: in this vehicle, driving in low gearing is only possible in combination with all-wheel drive.
To switch to a different position, the clutch must be operated so that during the coupling of another gear set, the drivetrain is unloaded.
The principle of the transfer case in off-road vehicles resembles a splitter box in (heavy) commercial vehicles. In trucks, the extra reduction is found in the transmission. When driving with a heavy load, the transmission with eight speeds is doubled to sixteen.