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BMW improves xDrive All-Wheel-Drive System

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BMW’s xDrive All-Wheel-Drive System with the Intelligence to Look Ahead
Setting the Standard in Traction, Driving Dynamics, and Safety.

Conventional all-wheel drive seeks in particular to improve traction on difficult roads and surfaces. With xDrive offering the perfect combination of intelligent all-wheel drive and active dynamic drive control, BMW is changing the usual priorities, setting a new benchmark in all-wheel-drive technology: Retaining optimum traction on every surface, xDrive focuses in particular on the improvement of agility, driving dynamics and stability. Offering this innovative solution, xDrive thus combines the well-known and widely lauded dynamic advantages of BMW standard drive with the enhanced traction of all-wheel drive, quite literally offering the best of both worlds.

xDrive also in the new BMW 3 Series.
Following the BMW X3 and X5 Sports Activity Vehicles (SAVs) as well as
the 5 Series, the new BMW 3 Series is now also available with all the
benefits of BMW xDrive. This is indeed the first time for intelligent all-wheel drive of this calibre to make its appearance in a midrange car.

In principle, the xDrive concepts in the 3 and 5 Series come with the same standards and features as in the all-wheel-drive X3 and X5. In terms of their hardware and software they have however adapted to the specific requirements of the Saloon and Touring. One example is the drive chain on the power divider replaced in this case by a spur gear for optimum integration of the entire package.

xDrive: even faster than wheel slip.
Unlike BMW’s previous all-wheel-drive system with the same consistent distribution of power to the rear and front axles at all times, the xDrive concept incorporates an electronically controlled multiple-plate clutch distributing drive power instantaneously, infinitely and fully variably from the rear axle to the front axle, as required in any given situation. The system immediately recognises the need to change the distribution of power and responds extremely quickly, usually before one of the wheels is even able to spin. As a result, each wheel receives exactly the right drive power required and possible at any given point in time for maximum traction and dynamic performance.
The result is a significant, clearly noticeable improvement of driving safety and stability as well as agility and traction, particularly on winding roads.

When taking a bend dynamically and at high speed, xDrive delivers optimum drive power to the respective axle at all times, significantly reducing both under- and oversteer in the process. And when driving straight ahead under normal conditions, xDrive in general splits up drive power between the front and rear axles in a 40:60 ratio, thus maintaining that typical BMW style of rear-wheel drive and power.

“Intelligent” means using all-wheel drive when you really need it.
Benefiting from the intelligence of BMW xDrive, the driver uses the advantages of four drive wheels only when he really needs them. In other words, BMW’s all-wheel-drive system prevents the disadvantages of conventional systems, making the xDrive models just as agile and dynamic in everyday motoring as BMW’s rear-wheel-drive models. So it is fair to say that BMW xDrive sets the standard in the all-wheel-drive segment.

On the road BMW xDrive offers significant improvements in terms of agility, driving pleasure and safety over conventional all-wheel drive, the DSC dynamic control system intervening only when required at a far later point than usual. A further advantage of xDrive is better traction on loose or slippery surfaces, power being fed directly to the wheels with grip as soon as one of the wheels threatens to spin and thus loose traction.

The “heart” of BMW xDrive: the electronically controlled multiple-plate clutch.
The “heart” of BMW xDrive is the electronically controlled multiple-plate clutch within the power divider ensuring a smooth flow of power within fractions of a second. In an extreme case the front and rear axles may indeed be completely separated from one another or firmly connected as one “solid” unit acting just like a 100 per cent longitudinal lock on conventional all-wheel drive.

Offering electronic brake intervention, DSC Dynamic Stability Control acts in the same way as a transverse lock distributing engine power to the two front and, respectively, rear wheels: As soon as a wheel starts to spin without conveying engine power, the brakes are applied automatically to slow down the wheel as required. In that case the axle differential automatically conveys more power to the wheel on the other side of the axle. Networked with DSC for proactive control.
One of the particular features of BMW xDrive is that it is directly connected to the DSC dynamic control system. The big advantage is that this makes xDrive a proactive system quite different from conventional four-wheel drive which only responds once the wheels have started spinning. By contrast, xDrive, evaluating all driving data, detects driving situations which would benefit from all-wheel drive in advance, thus providing all-wheel-drive performance before the wheels are even able to slip.

Much higher standard of agility.
BMW xDrive also uses information provided by the DSC stability control system: The yaw rate sensor, for example, determines rotational movement of the car while a steering angle sensor detects the position and lock of the steering wheel. Together with information provided by the wheel sensors on the speed and lateral acceleration of the car, as well as engine data, xDrive is able to detect current driving conditions reliably at an early point in time, sharing out wheel torque accordingly between the front and rear axles.

Like on every BMW, DSC Dynamic Stability Control may also be deactivated on the all-wheel-drive models, enabling the sports-minded driver to enjoy
all the dynamic driving qualities of his BMW in a controlled power slide by deliberately oversteering the car. The xDrive all-wheel-drive system as such cannot be deactivated.

The driver benefits all the time.
The driver benefits consistently from the outstanding advantages of BMW’s xDrive system, with the flow of power being continuously adjusted to current driving conditions. Just consider the following examples:
• When setting off under normal conditions, the multiple-plate clutch remains locked up to a speed of approximately 20 km/h or 12 mph in the interest of maximum traction. Then the system spreads out drive power variably between the rear and front axle, depending on road conditions and the quality of the surface.
• Re-directing the flow of power within fractions of a second, the system minimises under- or oversteer in bends: As soon as the rear end of the car pushes to the outside in a bend (oversteer), xDrive closes the multiple-plate clutch even tighter and guides even more drive power to the front wheels. This allows the rear wheels to build up more lateral force and helps to stabilise the car. Being combined with DSC stability control, the system detects any tendency to understeer right from the start and intervenes before the driver even notices any change in driving conditions. It is fair to say, therefore, that a BMW with xDrive guides you round corners just like a vehicle running on rails.
The same applies to strong understeer: As soon as the car starts pushing out of a bend over its front wheels, this threat of understeer is detected by information from DSC stability control and torque is reduced on the front wheels – in an extreme case feeding up to 100 per cent of the engine’s power to the rear. As a result, the driver taking, say, a serpentine route is able to handle a BMW equipped with xDrive with almost the same agility as a BMW with rear-wheel drive. Only when the risk of over- or understeer can no longer be compensated by variable power distribution alone will DSC Dynamic Stability Control cut in, safely stabilising the car by intervening in the brakes.
• BMW xDrive even compensates an abrupt change in throttle without the slightest problem: While the time-lag between pressing down the gas pedal and building up engine power is at least 200 milliseconds, the multiple-plate clutch closes or opens completely within just 100 milliseconds.
• When parking the multiple-plate clutch opens completely and the powertrain turns into a fully rear-wheel-drive concept, without the slightest distortion in the drivetrain and the slightest influence on the car’s steering.
• On gradients with a slippery surface such as ice or snow, locking action between the front and rear axles prevents individual wheels from spinning. Hence, DSC is required to cut in much later than usual under far more difficult road conditions, taking back the throttle or applying the brakes on the wheels. And then, continuing the drive, the driver benefits from the same locking action significantly reducing the risk of losing longitudinal or side forces on individual wheels and offering the driver much safer and more agile driving characteristics.

Last Updated on Thursday, 11 August 2005 12:11  

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