xDrive

The German car manufacturer BMW has developed the xDrive system as a response to competing intelligent all-wheel drive systems (typically Torsen or Haldex) that have started to appear on the market. The general principle of intelligent all-wheel drive systems is the use of inter-axle couplings. These clutches allow the individual drive components to be connected or disconnected according to the current conditions and traction requirements of the vehicle.

BMW has been developing all-wheel drive for various model series for many years, which is why there are several types and generations of drive called xDrive. These range from permanent all-wheel drive with the ability to regulate the ratio of driving forces between the axles, to full torque vectoring of the driven wheels. All of these drive variations fall under the umbrella term xDrive and vary according to specific models and generations. A brief overview of the development of xDrive can be found at the end of this article.

 

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How BMW’s xDrive works:

Each new generation of xDrive all-wheel drive is a little more sophisticated and faster. Today, xDrive routinely uses information from the DSC driving stability system to react in advance to a driving situation, for example by shifting power to the outside rear wheel. Reaction times for multi-link clutches are now below 100 milliseconds. And the whole process of directing the torque flow is controlled by powerful electronics and is technically called – torque vectoring. In practice, this means that the amount of torque applied to the individual wheels of all driven axles can be varied. The main benefit of such a system is the ability to significantly influence the driving behaviour of the vehicle in different situations.

With the advent and development of electrification in automotive transport, the concept of a purely electrically driven secondary axle is becoming increasingly popular. The primary drive is therefore solved by a conventional internal combustion engine. In addition, the electrically driven axle is activated when increased traction is required. BMW has named this technical solution eDrive.

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History:

In 1985, the first BMW model with all-wheel drive was introduced – the BMW 325i Allrad in sedan body style. The chosen all-wheel drive technology used automatic viscous couplings located directly in the gearbox and rear differential. These viscous couplings distributed the driving force in a 37:63 ratio in favour of the rear axle in order to maintain the classic BMW driving feel.

From 1988 onwards, the same model was also offered in “touring” or station wagon form. At the same time, this model was given a new name with the letter X to represent all-wheel drive – BMW 325iX. This system was still without electronic steering with automatic viscous couplings.

In 1991, all-wheel drive with electronic steering was introduced – the BMW 525iX. The upgraded system already featured multiplate clutches that allowed the power distribution between the front and rear axles to be changed continuously and fully automatically. The standard distribution was 36:64. Initially, a hydraulically operated multiplate clutch was used on the rear axle, but this was later replaced by electronically controlled selective braking. The system only evaluated any loss of adhesion, it had not intervened before. It used information on wheel speed, brake condition, engine speed and throttle position.

In 2003, a new intelligent all-wheel-drive system called xDrive was introduced on the BMW X3 and X5. It was the first system to proactively monitor the driving situation by working in conjunction with the DSC driving stability system. Using data on steering wheel angle, accelerator pedal position and lateral acceleration value, xDrive detected the risk of oversteer or understeer before it occurred. By redirecting the flow of drive power in a timely manner, xDrive was able to support balanced driving performance in all situations. Later, the all-wheel drive was further enhanced by the addition of an active rear differential (DPC), which could even distribute power to the individual wheels of the axle.

In 2009, for the first time, the BMW brand combined all-wheel drive with a hybrid powertrain – the BMW ActiveHybrid X6. In this new combination, the all-wheel drive helped to better handle the dynamics of electric driving in particular.

For 2014, the BMW 2 Series Active Tourer combines xDrive with a new front-wheel drive system and a front-mounted engine. This evolutionary step was met with a certain amount of skepticism by fans of the brand. Since it made the vehicle a BMW branded pre-wheel drive, and still with the engine mounted across the front. The rear axle could be driven via a Haldex multiplate clutch.

For 2015, the latest significant evolution comes in the form of the BMW X5 xDrive40e, which offers electrified all-wheel drive. The intelligent hybrid control of the eDrive system synchronises the operation of the combustion engine and the electric motor.

M xDrive

It remains to add that BMW offers a completely different xDrive system for its high-performance “M” division models – it calls it M xDrive. This system can be set up in much greater detail and is also physically a completely different system. In the BMW M5, for example, the infotainment system can be used to make the car send power to the rear wheels only.

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You know that…?

BMW has a special designation for vehicles using torque vectoring and single-axle drive – sDrive.