transfer case


  • General
  • Operation of the transfer case
  • Transfer case with high and low range

A transfer box consists of a shifting system with multi-plate clutches, which can control the drive of the front wheels. The system is used by BMW in the X-drive systems (which are used e.g. on an X3, X5, X6 or 330xi. Every BMW is standard rear-wheel drive. The X-drive versions are four-wheel drive. However, this is not permanent four-wheel drive. With permanent four-wheel drive, the front and rear wheels cannot be separated from each other and they always rotate with each other, which unfortunately entails disadvantages such as increased consumption, increased tire wear, etc.

A transfer case ensures that the rear wheels are directly connected to the gearbox. This is a fixed connection and is not controlled by the transfer case. The connection to the front wheels is regulated depending on the load on the engine (sporty acceleration) and the type of road surface (with smooth, unpaved surfaces). The transfer case will rotate the axle of the front wheels at the same speed as the rear wheels with the aid of the multi-disc clutch engaged.

Operation of the transfer case:
On cars with a transfer case, the gearbox is directly connected to the rear wheels. This can be clearly seen in the image below by the red line. This runs from the gearbox directly to the cardan shaft of the rear wheels.
When the rear wheels threaten to slip, or when transferring a lot of power to the road surface, the electric motor is controlled. The electric motor turns a gear which then operates a control slide. The rotation of the control slide is indicated by the blue arrows. This control slide presses on the multi-disc clutch, creating a connection between the sprockets of the rear wheels and the front wheels. As soon as the multi-disc clutch is energized, the force indicated by the green arrows is created. The drive shaft to the front wheel differential is now driven. The front wheels now rotate at the same speed as the rear wheels.

The 4-motion / Syncro / Quattro system is used on vehicles from the VAG group. This works with the help of a haldex coupling that is mounted at the rear axle. Click here for information about the Haldex coupling.

Transfer case with high and low range:
Transfer cases of off-road vehicles such as the Chevrolet K30 and the Jeep CJ7 are equipped with a gearbox with which a choice can be made for a high or low gearing. This results in a different transmission ratio between the gearbox and the wheels.

On off-road vehicles, the low gear is used when driving over a landscape with sand, rocks and hills. When switching from high to low range, the engine speed increases, with the advantage that the pulling force at the wheels is considerably higher.

The following image shows the powertrain of a four-wheel drive vehicle (left) with an arrow in the direction of travel, and on the right the different positions of the transfer case. The yellow color of the gears and shafts indicates that these parts are switched on or off. powered.

  • 2-high: the gearbox drives the cardan shaft for the rear wheels. In this position, people generally drive on flat road surfaces;
  • 4-high: the four-wheel drive is engaged;
  • neutral: the drive between the gearbox and the wheels is interrupted;
  • 2 layer: between the input shaft of the gearbox and the output shaft to the differential is coupled to an additional shaft with gears. The power distribution goes from the large gear (top) to the small one. The speed of the input shaft increases, as does the torque of the output shaft;
  • 4 layer: once again the four-wheel drive is engaged. As with 2-layer, the middle geared shaft serves for the low gearing.

The various positions are selected by operating the extra lever in the interior. On most off-road vehicles, this lever is located near the “normal” gear lever of the gearbox. The image shows the shift pattern of a Dodge Ram. It can be seen that the 2L position is missing: with this vehicle, driving in low gear is only possible in combination with four-wheel drive.

To switch to another position, the clutch must be actuated so that the drive train is unloaded when connecting another gear group.

The principle of the transfer case in off-road vehicles resembles a splitter box in (heavy) commercial vehicles. In trucks we find the extra reduction in the gearbox. When driving with a heavy load, the eight-speed gearbox is doubled to sixteen.

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