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BMW X6: Perfecting xDrive AWD
With Dynamic Performance Control

Dynamic Performance Control graphic.

Dynamic Performance Control (photo) is twinned with the xDrive four-wheel drive system. It spreads drive forces from side to side at the two rear wheels, stabilising the vehicle both under power and with the engine on over-run, such as under braking or descending a hill.

APN,
19 December 2007.

Making its world debut on the new BMW X6 Sports Activity Coupé (SAC), Dynamic Performance Control is twinned with the xDrive four-wheel drive system. It works by spreading drive forces from side to side at the two rear wheels, stabilising the vehicle both under power and with the engine on over-run, such as under braking or descending a hill with the throttle pedal released. The system is supplied by ZF (see the ZF Torque Vectoring drive technology).

Permanent all-wheel drive with electronically controlled, variable distribution of drive power between the front and rear axles gives BMW X models enhanced driving dynamics. As an intelligent all-wheel-drive system, it distributes drive power front to rear by way of an electronically controlled multiple-plate clutch according to driving conditions.

In normal driving situations BMW xDrive distributes drive power in a 40 : 60 split front-to-rear, with sensors consistently measuring wheel slip. Hence, the system is able within a fraction of a second to vary the distribution of drive forces, with BMW xDrive—unlike conventional all-wheel-drive systems—anticipating the upcoming situation and not just responding once a wheel has started to spin. This serves to stabilize the vehicle even before the driver notices any need for action.

On the new X6 with Dynamic Performance Control, handling and stability are enhanced to an even higher level with the ability to multiply torque to an individual rear wheel for yaw control. With the combination of xDrive and Dynamic Performance Control, the X6 is able to correct the trend to over- or under-steer at the earliest conceivable point.

ZF Torque Vectoring drive technology.

ZF graphic of its Torque Vectoring drive technology: if the vehicle begins to understeer or push its back to the outside of a corner, additional torque will be shifted to the outside rear wheel to push the car round the corner and counteract the understeer. Major advantage: the outer wheel receives more drive than the inner wheel to counteract the understeer even when cornering with a completely released throttle pedal .

 

Dynamic Performance Control works by detecting possible over- or understeer of the vehicle in, for example, a bend. When this occurs Dynamic Performance Control will vary the distribution of drive forces between the wheels for the best possible stability, dynamic traction, and forward motion. This is achieved by transferring additional wheel speed to the outer rear wheel to push the car round the corner rather than applying brake force to the inner wheels as currently happens with traditional stability control systems.

If the vehicle begins to understeer or push to the outside of a corner, BMW xDrive and Dynamic Performance Control will shift torque to the outside rear wheel to push the car round the corner and counteract the understeer.

On the other hand, if the vehicle threatens to oversteer, xDrive reduces the transmission of power to the rear wheels, then the Dynamic Performance Control diverts torque and drive forces from the outer to the inner rear wheel.

BMW says that this intervention is so quick and finely controlled that the driver will not even notice what is happening.

Dynamic Performance Control also serves to optimise traction on a slippery surface. With its ability to vary the transmission of engine power specifically as required between the two rear wheels, it improves traction and pulling force whenever the vehicle is running on different surfaces from one side to the other, for instance, with asphalt on one side and a snowbound surface on the other. Any differences in the frictional coefficient between the wheels on the left and right (modal split) are offset by Dynamic Performance Control specifically sharing out drive forces on the rear axle.

A further particular feature of Dynamic Performance Control is that engine power and torque are split variably between the rear wheels not only when the engine is “pulling” the vehicle, but also in overrun. This is made possible by the addition of a planetary gear set and a multiple-plate clutch operated by an electric motor on both output sides of the rear differential.With this configuration Dynamic Performance Control is able to vary the distribution of drive power also when the driver takes his foot off the accelerator.

Dynamic Performance Control: viewable on the instrument cluster.

Dynamic Performance Control: on the instrument cluster.

Easily viewable on the instrument cluster, the driver receives a clear message showing the interaction by Dynamic Performance Control. By simply pressing the lever on the steering column, the driver is informed of the current distribution of drive power via a bar diagram showing how much power is going to each of the four drive wheels at any given point in time.

The difference in drive forces between the two rear wheels provided by Dynamic Performance Control can be as high as 1327 lbs-ft (1800 Newton-metres). But even then, all the driver will feel when Dynamic Performance Control intervenes is a – significant – increase in agility, traction and driving stability. As a result of Dynamic Performance Control’s (DPC) intervention, the Dynamic Stability Control (DSC) intervenes much less frequently than before. In practice, this means that the driver is able to use the full drive power and torque of the engine much longer than with a conventional system, DSC acting on individual wheels or reducing engine power only when the driver really pushes the car to the extreme limits of physical performance.

In addition, the BMW X6 offers a new range of integrated DSC functions: ABS anti-lock brakes; Automatic Stability Control; Trailer Stability Control; Hill Descent Control; Dynamic Brake Control, automatically maximizing brake pressure when required to reduce stopping distances; Cornering Brake Control; and, finally, Active Cruise Control complete with its own brake application function.

The Automatic Differential Brake serves as a transverse lock between the two wheels, and Fading Compensation increasing brake pressure as required under extremely high brake temperatures, helps to maintain the brake forces required even under extreme conditions. Regular Dry Braking, in turn, optimises brake performance in wet conditions, while the Start-Off Assistant enables the driver to set off more smoothly and easily on an uphill gradient.

The new BMW X6 features also a parking brake operating both electromechanically and hydraulically, activated or released simply by pushing a button.

DSC also comes with Dynamic Traction Control as a special function, raising the DSC response thresholds to a higher level, allowing a particularly sporting style of motoring, and enabling the driver to set off smoothly on snow or in loose sand with the drive wheels intentionally spinning slightly.

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