WHAT IS A BMW TRANSFER BOX – xDrive

WHAT IS A BMW TRANSFER BOX – xDrive

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What is a transfer box?

The transfer case is a component of the transmission system found in 4×4 cars. The name even says what it does: it transfers power from the gearbox to both axles. That is, while a car with front or only rear-wheel drive has direct transmission, the engine sends power to the gearbox, then directly to the wheels by cardan and differential, to 4×4, 4WD or AWD cars, somehow the power must be transmitted and to the other deck. That’s what the transfer box does.

cutie de transfer este un alt element de transmisie care vine imediat dupa cutia de viteze. It can be a simple transfer box or with a reducer. In this transfer box, the 4×4 transmission is blocked from the button, which travels all the time with front-wheel drive, so that 4×4 can work in off-road conditions. But the transfer case can be even more complex, computer-controlled, so that it automatically sends power to the front, rear or both axles.

Cars equipped with all-wheel drive system, compared to one with single-wheel drive (front or rear), is more stable while driving and has better dynamic performance because:

  1. Acceleration is improved because, being distributed on all wheels, the total traction force is higher;
  2. Thanks to the control of the traction force on each axle, the stability while driving the car is improved.

Compared to a rear-wheel drive car, an xDrive car also has a transfer case, a longitudinal shaft and a differential. The transfer case divides the torque resulting from exiting the gearbox between the two axles of the car.

The transfer boxes are produced by Magna Powertrain. In terms of how to transfer torque to the front axle, there are two variants of transfer boxes:

  • with gears
  • with metal chain

The xDrive all-wheel drive system stands out primarily for its simplicity, efficiency but also a very short response time. In addition, working with the Electronic Stability Control System (DSC), the xDrive system contributes decisively to both active safety and the dynamic performance of the car.

What is xDrive?

BMW xDrive is the trade name for the all-wheel drive system in BMW X1, X3, X4, X5 and X6 sports vehicles. It is also optional on Series 1 (2012-present), 2 series (2015-present), Series 3 (2000-present), 4 Series (2014-present), Series 5 (2005-present), Series 6 (2012 -present) and series 7 (2010-present).

Instead of a permanent split torque (which is shown in previous systems), xDrive ensures a variable torque distribution between the front and rear axles, by using a wet multi-plate clutch, located in the gearbox on the output to the front drive shaft. This configuration allows the xDrive system to modulate the torque split between the front and rear axles, which is normally split at 40:60. If the ABS / DSC system detects a wheel slip, xDrive can react within a tenth of a second to redistribute up to 100% of engine power on the front or rear axle. The wet clutch is applied by a high-speed electric servo motor, transforming a cam-shaped drive disc. As the rear drive shaft is engaged at the output of the transmission, full torque transfer to the front axle can only be achieved if the rear wheels have no traction and both are slippery.

xDrive is connected to ABS and DSC systems. If instability on the wheels or in the direction still occurs while xDrive changes or has changed the breaking moment, the DSC will break the independent wheels to regain traction and improve directional stability without driver intervention.

The front and rear differences in xDrive vehicles are typically an open differential design, thus relying on the application of the brakes by the DSC system to transfer power from the sliding wheel to the traction wheel.

A variant of the xDrive system is present in the 2015 BMW X1 (F48) model, which is based on a front-wheel drive design and a transversely mounted engine. In the FDD-derived xDrive version, the front wheels receive 100% of the torque when the xDrive clutch is open, giving it a front bias instead of the regular rear xDrive.

BMW has gradually expanded the availability of xDrive to its full range to better compete with Mercedes-Benz’s Audi quattro and 4Matic, the latter two being widely available in their offerings. Since the 2010-12 model years, BMW Canada has even stopped offering RWD models if an xDrive counterpart is available for the 5 Series 6 and 7 Series, a major reversal from the 2009 and earlier models, only if the BMW 528i and 535i were available with xDrive. The change in BMW Canada surprised some observers, because the AWD was “once when Audi had a single opportunity with Mercedes with a strange mix, the switch to AWD is so strong that many luxury models are now predominantly AWD, although 650 is the first is equally surprising that the Coupe will only be available in the AWD style, if you want a 650 model that only acts on the rear wheels, you will have to choose the ragtop version.

Components of the transfer case

Compared to a rear-wheel drive car, an xDrive car also has a longitudinal shaft and a differential transfer case. The transfer case divides the torque resulting from exiting the gearbox between the two axles of the car.

The transfer boxes are produced by Magna Powertrain. In terms of how to transfer torque to the front axle, there are two variants of transfer boxes:

  • with gears
  • with metal chain

The sprocket version (ATC350) is used to equip BMW 1 Series to 7 Series. The metal chain variant (ATC450) is equipped with BMW X models.

Sistemul de tracțiune integrală xDrive BMW – componente cutie de transfer cu roți dințate

Photo: BMW xDrive all-wheel drive system – gearbox components
Source: BMW

  1. gearbox input
  2. exit to the rear axle
  3. exit to the front axle
  4. electric motor clutch control
  5. multi-disc clutch
  6. clutch lever coupling / disengagement
  7. intermediate gear
  8. lever drive cam
  9. oil pump

Sistemul de tracțiune integrală xDrive BMW – cutie de transfer cu lanț metalic

Photo: BMW xDrive all-wheel drive system – metal chain transfer case
Source: BMW

Sistemul de tracțiune integrală xDrive BMW – componente cutie de transfer cu roți dințate

Photo: BMW xDrive all-wheel drive system – gearbox components
Source: BMW

  1. gearbox input
  2. exit to the rear axle
  3. exit to the front axle
  4. electric motor clutch control
  5. multi-disc clutch
  6. clutch lever coupling / disengagement
  7. metal chain
  8. lever drive cam

xDrive la BMW

It is the marketing name for all-wheel drive, a system found in the BMW X1, X3, X5 and X6 sport crossover. It is also optional on Series 1 (2012-present) Series 3 (2000-present), 4 Series (2014), Series 5 (2005-present), Series 6 (2012-present), and Series 7

About xDrive

Instead of a permanent torque fraction (with which they are equipped with previous systems), xDrive ensures the division of the variable torque between the front and rear axles, using a wet multi-disc clutch located in the gearbox at the front output of the drive shaft . This configuration allows the xDrive system to modulate the torque between the front and rear axle, which is normally split at a ratio of 40:60. If wheel slip is detected by the ABS / DSC system, xDrive can react in a tenth of a second to redistribute up to 100% of engine power to the front or rear axle. The wet clutch is applied quickly, being electrically driven by the servo motor, a cam-shaped drive disc.

xDrive is connected to ABS and DSC systems. If wheel skidding or steering instability occurs while xDrive is on or off, the DSC will brake the wheels independently to regain traction and improve steering stability without driver intervention.

The front and rear differentials in xDrive vehicles are an open differential design, thus relying on the DSC system to transfer power from the sliding wheel to the traction wheel.

For BMW, 1985 marked the beginning of the use of all-wheel drive systems. The first all-wheel drive car was a BMW 3 Series. The system used a permanent distribution transfer case: 37% of the traction force on the front axle and 63% on the rear axle.

The modern all-wheel drive system used by BMW, called xDrive, equipped the first BMW X5 in 2004. This system is currently present in the basic equipment for the X1, X3, X4, X5 and X6 models and is offered as an option for the rest of the models.

BMW cu sistem de tracțiune integrală xDrive

Photo: BMW with xDrive all-wheel drive system
Source: BMW

A car equipped with an all-wheel drive system, compared to one with a single-wheel drive (front or rear), is more stable while driving and has better dynamic performance. Acceleration is improved because, being distributed on all wheels, the total traction force is higher. Also, thanks to the control of the traction force on each axle, the stability while driving the car is improved.

Sistemul de tracțiune integrală xDrive de la BMW

Photo: xDrive all-wheel drive system
Source: BMW

  1. gearbox (automatic)
  2. transfer box
  3. longitudinal shaft (towards the front axle)
  4. differential face
  5. longitudinal shaft (towards the rear axle)
  6. rear differential

Compared to a rear-wheel drive car, an xDrive car also has a transfer case (2), a longitudinal shaft (3) and a differential (4). The transfer case divides the torque resulting from exiting the gearbox between the two axles of the car.

The transfer boxes are produced by Magna Powertrain. In terms of how to transfer torque to the front axle, there are two variants of transfer boxes:

  • with gears
  • with metal chain

The sprocket version (ATC350) is used to equip BMW 1 Series to 7 Series models. The metal chain variant (ATC450) is equipped with BMW X models.

Sistemul de tracțiune integrală xDrive BMW – componente cutie de transfer cu roți dințate

Photo: BMW xDrive all-wheel drive system – gearbox components
Source: BMW

  1. gearbox input
  2. exit to the rear axle
  3. exit to the front axle
  4. electric motor clutch control
  5. multi-disc clutch
  6. clutch lever coupling / disengagement
  7. intermediate gear
  8. lever drive cam
  9. oil pump

Sistemul de tracțiune integrală xDrive BMW – cutie de transfer cu lanț metalic

Photo: BMW xDrive all-wheel drive system – metal chain transfer case
Source: BMW

Sistemul de tracțiune integrală xDrive BMW – componente cutie de transfer cu roți dințate

Photo: BMW xDrive all-wheel drive system – gearbox components
Source: BMW

  1. gearbox input
  2. exit to the rear axle
  3. exit to the front axle
  4. electric motor clutch control
  5. multi-disc clutch
  6. clutch lever coupling / disengagement
  7. metal chain
  8. lever drive cam

The principle of operation is relatively simple and applies to both versions of transfer boxes. When the clutch is fully open all engine torque is transmitted to the rear axle. When the clutch is completely closed and both axles have the same load, the torque is transmitted 50:50 between the two axles.

In the case of acceleration, starting from the spot, even if the clutch of the transfer case is completely closed, the rear axle transmits more torque because it is loaded more dynamically. The transfer of engine torque between the two axles is done automatically depending on the adhesion available at each axle.

Sistemul de tracțiune integrală BMW xDrive – transferul cuplului motor între cele două punți

Photo: BMW xDrive all-wheel drive system – transfer of engine torque