BMW's xDrive All-Wheel-Drive System

Press Release

xDrive in the New BMW 3 Series

 

Introducing the intelligent xDrive all-wheel-drive system in the BMW 3 Se­ries with six-cylinder gasoline and diesel engines, BMW is for the first time of­fering an intelligent drive concept of this kind in the midrange segment. This combines the proverbial sportiness of the 3 Series with an even higher standard of superior – if not to say, supreme – traction for even greater agili­ty, driving safety, and stability on the road.

 

xDrive in the BMW 3 Series is both innovative and convincing: Benefiting from the system’s inherent intelligence, the driver uses the advantages of four drive wheels whenever they really count. And at the same time the in­telli­gent system avoids the disadvantages of conventional all-wheel drive, the 3 Series with xDrive offering virtually the same agility and dynamic performance under everyday driving conditions as a 3 Series with rear-wheel drive.

 

With the exception of its modifications for all-wheel drive, the chassis is the same as on the “basic� model. In its basic principle, xDrive in the BMW 3 Se­ries is the same as in the X3 and X5, for which it was originally developed. Both the hardware and software, however, have been adapted to the specific requirements of the 3 Series.

 

xDrive thinking for the driver and acting by itself.

In the compact BMW 3 Series, xDrive allows infinite, variable distribution of drive forces between the front and rear axles: The system immediately re­cog­nises any need to change the distribution of engine power and responds extremely quickly, usually before a wheel is even able to spin. When taking a fast bend in dynamic style, for example, the system consistently feeds exact­ly the right level of drive power to the respective axle, significantly reducing the risk of under- or oversteer. The advantages for the driver are a significant improvement of agility, even greater driving pleasure, and at the same time a supreme standard of safety on the road.

 

The power divider forms the very “heart� of xDrive in the BMW 3 Series:
Com­ing downstream of the manual or automatic transmission, the divider splits up engine power to the rear and front axles. To be specific, a firm, di­rect drive shaft leads out of the transmission straight to the rear axle. From there, via the electronically controlled multiple-plate clutch with infinite power distribution, the appropriate level of power is fed to the front axle and trans­mitted via a two-stage set of spur gears.

 

This division of power covers a range extending infinitely from 50:50 all the way to 0:100. So in an extreme case the front and rear axles are either to­tally disconnected or linked firmly to one another, such a firm, rigid connect­ion acting in the same way as a 100 per cent longitudinal lock on a con­ven­tional all-wheel-drive system. And since torque and engine power are re-distributed within a few milliseconds, the driver normally does not even no­tice such a change in power.

 

When driving straight-ahead under normal conditions, 60 per cent of the en­gine power goes to the rear axle, 40 per cent to the front.

 

Interacting closely with DSC, xDrive even looks ahead.

Fully networked with the DSC stability control system, xDrive uses informa­tion and data coming from BMW’s Dynamic Stability Control. The yaw rate sensor, for example, determines rotational movement of the car and a steer­ing angle sensor detects the current lock of the steering wheel. Together with information on the car’s current road speed coming from the wheel sen­sors, the lateral acceleration of the vehicle, and engine data, xDrive detects the car’s current driving conditions almost immediately, with virtually no delay. This enables the system to even look ahead, distributing drive power perfectly between the front and rear wheels.

 

This interaction with DSC also provides a transverse lock function, forced distribution of power to the two front and, respectively, rear wheels by means of electronic intervention in the brakes: As soon as a wheel starts to spin without transmitting power, the brakes are applied specifically on that wheel, the differential in the final drive automatically feeding more power to the opposite wheel still maintaining its grip.

 

DTC for sports motoring and snowbound roads, HDC for driving down­hill in a smooth, controlled process.

On a BMW equipped with xDrive, DSC Dynamic Stability Control still retains its DTC Dynamic Traction Control function. The big advantage of DTC is that the system allows greater slip and larger drift angles under extreme conditions, for example when driving on loose snow or when the driver pre­fers a very sporting and dynamic style of motoring.

 

HDC Hill Descent Control, in turn, provides important assistance when driv­ing downhill under difficult conditions: Without the driver having to intervene, the BMW 3 Series equipped with xDrive applies the brakes on all four wheels, allowing the car to proceed at a steady, consistent walking pace. And on the BMW 3 Series with xDrive, the driver is able to activate and de­activate the HDC function simply by pressing a button in the centre console.