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Chief Executive
Drives: 2008 335xi 6spd Coupe
Join Date: Dec 2007
Location: Boston
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In case the site ever goes down:
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BMW’s xDrive All-Wheel-Drive System
More BMWs than ever before now come with the brand’s one-of-its-kind xDrive all-wheel-drive technology – no less than 20 different models in all. And indeed, BMW’s intelligent xDrive all-wheel-drive system sets new standards in both traction and agility: xDrive allows infinitely variable distribution of drive forces between the front and rear wheels, modifying the flow of power at a speed never seen before. The system immediately recognises the need to change engine power, where appropriate, and respond within fractions of a second.
xDrive also in the new BMW 3 Series
Introducing the 325xi, the 330xi, and the 330xd in the BMW 3 Series, BMW now offers the advantages of xDrive technology for the first time also in the mid*range segment. And xDrive technology is also featured in the BMW 5 Series (525xi, 530xi, 530xd) as well as the X3 and X5 Sports Activity Vehicles, thus now coming in no less than four different model series.
At the same time it goes without saying that this is not the end of the road, the new BMW model series announced at this year’s Geneva Motor Show and scheduled for introduction in 2008, for example, further enlarging the xDrive portfolio: This sporting crossover, through its coupe-like silhouette interacting with its high, commanding seating position, will indeed be predestined for xDrive with its many benefits.
Action, not reaction: xDrive “thinks” for the driver and acts instantaneously
xDrive acts faster than other systems, because it is able to think ahead and anticipate upcoming situations on the road. To offer this unique benefit, xDrive uses information from the wheel sensors showing the system wheth*er a wheel is about to spin and providing data gathered by DSC stability con*trol. The yaw rate (rotation around the car’s vertical axis) and the posi*tion of the steering wheel, for example, offer important data on current driv*ing conditions. Data of this kind is essential, for example, when taking a bend fast and dynamically, xDrive consistently spreading out drive forces perfectly between the two axles and minimising both under- and oversteer in the process.
Apart from this significant improvement of agility and driving pleasure, xDrive also ensures a substantial increase in driving safety, BMW’s DSC stability control system therefore intervening far later in many cases than would be required with a conventional all-wheel-drive concept.
Better traction, particularly on snow and loose ground.
With BMW xDrive reacting within fractions of a second, the system offers enormous advantages also in terms of traction. When setting out on slippery or loose ground, for example, the time-lag between the loss of wheel grip on the surface and the re-distribution of drive forces to wheels with grip is only a few milliseconds. So xDrive elegantly solves and eliminates the usual prob*lems otherwise encountered so often when setting off, without the driver even noticing what is happening.
Setting the Standard in Traction, Driving Dynamics, and Safety
Conventional all-wheel drive seeks in particular to improve traction on diffi*cult roads and surfaces. With xDrive offering the perfect combination of in*telli*gent 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 inno*va*tive solution, xDrive thus combines the well-known and widely lauded dy*namic 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 require*ments of the Saloon and Touring. One example is the drive chain on the pow*er 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 con**cept incorporates an electronically controlled multiple-plate clutch distribut*ing drive power instantaneously, infinitely and fully variably from the rear axle to the front axle, as required in any given situation. The system imme*diately recognises the need to change the distribution of power and re*sponds 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 op*ti*mum 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 advan*tages 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 slip*pery 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 frac*tions 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 “so*lid” 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 with*out 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 per*formance 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 deactiva*ted on the all-wheel-drive models, enabling the sports-minded driver to en*joy all the dynamic driving qualities of his BMW in a controlled power slide by deliberately oversteering the car. The xDrive all-wheel-drive sys*tem as such cannot be deactivated.
Introducing the intelligent xDrive all-wheel-drive system in the BMW 3 Se*ries with six-cylinder gasoline and diesel engines, BMW is for the first time of*fering 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 agili*ty, 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 in*telli*gent 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 Se*ries 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.
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 cur*rent driving conditions. Just consider the following examples:
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When setting off under normal conditions, the multiple-plate clutch re*mains 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.
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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 mult*i*ple-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 condi*tions. 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 push*ing out of a bend over its front wheels, this threat of understeer is de*tect*ed 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 ser*pentine 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 stabi*lising the car by intervening in the brakes.
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BMW xDrive even compensates an abrupt change in throttle without the slightest problem: While the time-lag between pressing down the gas pe*dal and building up engine power is at least 200 milliseconds, the multi*ple-plate clutch closes or opens completely within just 100 milliseconds.
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When parking the multiple-plate clutch opens completely and the power*train turns into a fully rear-wheel-drive concept, without the slightest dis*tor*tion in the drivetrain and the slightest influence on the car’s steering.
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On gradients with a slippery surface such as ice or snow, locking action between the front and rear axles prevents individual wheels from spin*ning. 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 longi*tudinal or side forces on individual wheels and offering the driver much safer and more agile driving characteristics.
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 re*cog*nises 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 exact*ly 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: Com*ing downstream of the manual or automatic transmission, the divider splits up engine power to the rear and front axles. To be specific, a firm, di*rect 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 trans*mitted 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 to*tally disconnected or linked firmly to one another, such a firm, rigid connect*ion acting in the same way as a 100 per cent longitudinal lock on a con*ven*tional all-wheel-drive system. And since torque and engine power are re-distributed within a few milliseconds, the driver normally does not even no*tice such a change in power.
When driving straight-ahead under normal conditions, 60 per cent of the en*gine 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 informa*tion and data coming from BMW’s Dynamic Stability Control. The yaw rate sensor, for example, determines rotational movement of the car and a steer*ing angle sensor detects the current lock of the steering wheel. Together with information on the car’s current road speed coming from the wheel sen*sors, 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 down*hill 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 pre*fers a very sporting and dynamic style of motoring.
HDC Hill Descent Control, in turn, provides important assistance when driv*ing 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 de*activate the HDC function simply by pressing a button in the centre console.
BMW xDrive for extra driving pleasure.
BMW xDrive allows infinite and variable division of drive forces between the front and rear axle. Immediately recognising any need to change the distri*bu*tion of power, the system responds extremely quickly, generally even be*fore one of the wheels is able to spin. Hence, xDrive is able at all times to feed exactly the right level of drive power to the respective axle when taking a bend at speed, significantly reducing under- or oversteer in the process.
On the road, therefore, xDrive ensures a significant improvement in terms of agility, driving pleasure and motoring safety compared with conventional all-wheel drive.
Safe and secure even on the most slippery surfaces.
BMW xDrive guarantees even better and smoother motoring on loose or slippery surfaces, power being diverted instantaneously to the wheels with grip as soon as one of the wheels threatens to spin and thus interrupt the flow of traction. The driver benefits from these advantages consistently and under all conditions, with the flow of power being adjusted infinitely to driving conditions and individual requirements.
The “heart” of BMW xDrive: The electronically controlled multiple-plate clutch.
The electronically controlled multiple-plate clutch within the power divider forms the very “heart” of xDrive responsible for its particular features: De*pending on current driving conditions, the multiple-plate clutch controls and masterminds the flow of power to the front axle within fractions of a second. In extreme cases the front and rear axles may even be totally disconnected or linked firmly and rigidly to one another, such a rigid connection acting in the same way as a 100 per cent longitudinal lock on conventional all-wheel drive.
The perfect team: BMW xDrive and DSC.
To a certain extent the outstanding performance of BMW xDrive is attribut*able to the fact that the system uses all the information and data supplied by the DSC dynamic control system. The yaw rate sensor, for example, deter*mines rotational movement of the car while a steering angle sensor detects the current position of the steering wheel. Together with information on the road speed of the car delivered by the wheel sensors, and covering both the car’s lateral acceleration and engine data, xDrive is able to determine cur*rent driving conditions very early on, thus ensuring optimum distribution of drive power between the front and rear axles.
Intervening electronically in the brakes, DSC Dynamic Stability Control of*fers the function of a transverse wheel lock with forced distribution of engine power and torque to the two front or rear wheels: Once a wheel starts to spin without conveying power to the road, the brakes automatically slow down the wheel, with the differential in the final drive automatically feeding more power to the wheel on the opposite side in the interest of optimum traction.
DTC for sports motoring and snowbound roads, HDC for driving down*hill in a steady, controlled process
Like on the rear-wheel-drive 5 Series, BMW’s DSC Dynamic Stability Con*trol comes as standard with DTC Dynamic Traction Control allowing more slip and larger drift angles for extreme requirements or a very sporting style of motoring. And like on all BMWs, the driver may deactivate DSC com*plete*ly whenever he wishes.
HDC Hill Descent Control, in turn, provides important assistance when driv*ing downhill under difficult conditions: Without the driver having to intervene, the BMW 5 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 de*activate the HDC function simply by pressing a button on the multifunction steering wheel.
BMW xDrive distributes drive power infinitely and fully variably between the front and rear axle. The system immediately recognises the need to change the distribution of engine power and responds extremely quickly, usually be*fore a wheel even loses grip when driving onroad. As a result, xDrive feeds exactly the right drive power to each axle when, for example, taking a bend fast and dynamically, in the process significantly reducing both under- and oversteer.
Proactive distribution of power for maximum all-wheel-drive per*formance
On the road, BMW xDrive offers a significant improvement in terms of agi*lity, driving pleasure, and motoring safety. In addition, this proactive system en*sures better traction on rough or slippery terrain, with engine power be*ing immediately fed to the wheels with grip as soon as a wheel threatens to spin.
Contrary to the version featured on BMW’s passenger cars, xDrive in the BMW 5 and 3 Series incorporates a chain-driven driveshaft feeding power to the axles from the electronically controlled multiple-plate clutch. The con*nection shaft to the right front wheel, in turn, leads through the oil sump, al*lowing particularly low installation of the engine and thus ensuring a low centre of gravity together with enhanced ground clearance. This benefits both the car’s driving dynamics and rough terrain driving qualities, while at the same time reducing body sway in fast bends.
A bearing block is flanged on to the oil sump on the right-hand side in order to give both drive shafts exactly the same length. This keeps the steering free of drive forces, since the two driveshafts have the same deflection angles right and left. A further advantage is that this allows maximum steer*ing lock and, as a result, a smaller turning circle than on a conven*tional off*roader.
The electronically controlled multiple-plate clutch in the power divider con*trols the flow of power to the front axle within milliseconds as a function of individual requirements. In extreme cases the front and rear axles are even fully disconnected from one another or rigidly, firmly connected to form one unit.
Intervening electronically in the brakes, DSC provides the same function as a transverse wheel lock with forced distribution of power to the front and rear wheels: As soon as a wheel starts to spin without conveying power, the brakes cut in to slow the wheel down, the differential in the final drive thus feeding more power to the opposite wheel.
Advantage xDrive: Quicker information for faster response.
Unlike other four-wheel-drive systems which do not respond until the wheels start to spin, BMW’s proactive xDrive evaluates the driver’s com*mands – for example the current position of the gas pedal – to recognise a driving situation in which all-wheel drive can provide valuable assistance, thus ensuring all-wheel-drive performance before the wheels start to spin and develop slip.
A further forte of xDrive is the use of information and data collected by the DSC driving stability system. The yaw rate sensor, for example, serves to determine rotational movement of the vehicle around its own axis, while a steering wheel sensor detects the current angle of the steering wheel. To*gether with information on road speed coming from the wheel sensors, the lateral acceleration of the vehicle and engine data, xDrive is also able to de*termine at an early point how the Sports Utility Vehicle is currently perform*ing and what driving conditions are currently prevailing, thus ensuring opti*mum distribution of drive forces between the front and rear axle. The result is a significant improvement of agility giving the driver a particular experi*ence above all on winding mountain roads.
Like on every other BMW, DSC Dynamic Stability Control can be deactiva*ted in the X3 and X5 Sports Activity Vehicles, enabling the particularly sports-minded driver to enjoy the dynamic driving qualities of his – or her – BMW while power-sliding in a controlled oversteer. xDrive as such cannot be deactivated.
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my xdrive... 
