xDrive: BMW's New All-Wheel-Drive System.

An important innovation in technology made its world debut at the
Frankfurt Motor Show in conjunction with BMW's two X3 and X5 Sports
Activity Vehicles: xDrive, BMW's new, intelligent all-wheel-drive
concept with active dynamic suspension control. xDrive provides not
only optimum traction on bad roads, but also ensures an even higher
standard of driving safety and dynamic performance through its
electronically controlled, variable distribution of power
front-to-rear. BMW's new xDrive all-wheel-drive system is able to vary
drive power very quickly and fully variably between the front and rear
axle, depending on driving conditions and requirements. This ensures
the proactive distribution of power required for maximum
all-wheel-drive performance before a wheel is able to spin on the ground
beneath. To perform this function, the electronically controlled
multiple-plate clutch with an extremely short response time transmits
power via a chain-driven propeller shaft extending to the front
parallel to the engine on the left all the way to the front axle, from
where drive shafts lead to the two wheels. Particularly this latter
feature helps to lower the position of the engine, thus providing a low
centre of gravity with greater ground clearance. And this, in turn,
helps to improve driving dynamics, reducing body sway in bends and
ensuring good driving conditions on bad surfaces. The
"heart" of xDrive: the electronically controlled
multiple-plate clutch. The "heart" of BMW xDrive is housed
within the power divider: This is the electronically controlled
multiple-plate clutch masterminding the flow of power to the front axle
according to current requirements within fractions of a second. In
extreme cases the front and rear axle may indeed be totally
disconnected or firmly connected to one another, such a firm or rigid
connection then acting in the same way as a longitudinal differential on
conventional all-wheel drive. The transverse differential function, in
turn, that is the distribution of power to both of the front or,
respectively, rear wheels, is provided by DSC using the electronic
brake intervention function: Whenever a wheel is about to spin without
transmitting any power, the brakes are applied and it is slowed down
accordingly. This then allows the differential in the axle transmission
to automatically feed more power to the wheel on the other end of the
axle. Advantage xDrive: quicker information for a faster response.
All of these functions are controlled and masterminded by the new xDrive
all-wheel-drive system directly networked to BMW's DSC Dynamic Stability
Control enable to do an even better job with the help of xDrive. Indeed,
this partnership is mutual, DSC also giving xDrive a number of
important benefits: Unlike other all-wheel-drive systems, xDrive is
proactive in its effect, taking the driver's commands such as the
position of the gas pedal into account to recognise when all-wheel
drive will be helpful right from the start, while conventional
all-wheel drive systems only respond once the wheels have started to
spin. This therefore ensures superior all-wheel-drive performance prior
to the commencement of any spinning action. A further point is that
xDrive uses all the information compiled by the DSC stability control
system, taking, say, any rotating motion of the vehicle into account by
way of the yaw rate sensor and determining the lock of the steering
wheel by means of the steering angle sensor. Together with information
on road speed provided by the wheel sensors, information on lateral
acceleration of the vehicle as well as engine data, xDrive is able to
determine as an early point in time what current driving conditions are
like, thus spreading out drive forces on the X5 perfectly between the
front and rear axle. In practice, this means a considerable improvement
of agility particularly advantageous on winding mountain roads. As a
result, xDrive shifts torque in any way required from one axle to the
other within the shortest conceivable time, giving the driver the
benefit of drive power consistently adjusted to driving conditions. A
few examples: - When setting off under everyday conditions the
multiple-plate clutch closes almost completely until the vehicle
reaches approximately 20 km/h, in the interest of maximum traction in
the start-off phase. Then the system distributes engine power fully
variably between the front and rear axle, depending on driving
conditions, finally converting to rear-wheel drive alone as of 180
km/h. - In bends this instantaneous changeover of drive power prevents
any risk of under- or oversteering. Whenever the rear end of the
vehicle threatens to swerve round in a bend, xDrive closes the
multiple-plate clutch a bit further, feeding an increasing amount of
power to the front wheels. This allows the rear wheels to build up more
side forces, stabilising the vehicle in the process. And thanks to the
combination with DSC, the system detects any risk of oversteering at a
very early point and intervenes before the driver even notices any
change in driving conditions, thus enabling the vehicle to literally
run on rails with supreme directional stability. DSC Dynamic Stability
Control, in turn, will not intervene until the risk of oversteering can
no longer be eliminated by variable power distribution alone. -
Whenever the vehicle pushes out of a bend over its front axle, such
understeering is recognised with the help of information from the DSC
stability control system, and is immediately set off by reducing the
power fed to the front axle. In an extreme case, therefore, up to 100
per cent of the engine's power may go to the rear wheels - and if even
this is not enough, DSC will intervene once again, allowing the driver
to manoeuvre his X5 on serpentine roads with the same agility as a BMW
5 Series. Indeed, xDrive even copes with an abrupt change in the
position of the gas pedal without the slightest problem: While the
time-lag between the driver pressing down the gas pedal and the process
of building up engine power is at least 200 milliseconds, the
multiple-plate clutch opens or closes completely within just 100
milliseconds, that is twice as fast. - When driving with a large
steering angle and only little engine power - the typical situation
when parking - all-wheel drive turns into 100 per cent rear-wheel drive
without any unpleasant distortion of the drivetrain or any influence on
the steering. - On gradients with a slippery surface, for example on
snow, locking action between the front and rear axles prevents
individual wheels from spinning, meaning that DSC is only required to
take back the position of the gas pedal or apply the brakes on the
wheels under far more difficult conditions in order to modify the flow
of power. And then, when proceeding, the locking action significantly
reduces the risk of individual wheels losing their longitudinal or side
forces, offering the driver much safer and more agile behaviour. To
give the two drive shafts exactly the same length, a bearing block is
connected to the oil sump on the right-hand side. The big advantage is
that this keeps the steering free of drive forces, since the drive
shaft deflection angle is the same both right and left. A further
advantage of this concept is that it allows maximum steering wheel lock
with a turning circle of just 12,2m - much less than with a
conventional off-roader. And this, in turn, is a significant advantage
of the X5 in everyday traffic, for example when manoeuvring or in a
confined car park. The X5 consciously does without an additional
reduction gear as used under extreme off-road conditions. First, in
order to keep the centre of gravity as low as possible despite generous
ground clearance; second, because such a reduction gear is not
required, anyway, on a Sports Activity Vehicle; and third, because the
weight saved in this way gives the customer a significant advantage.
Like every other BMW, DSC Dynamic Stability Control on the X5 is an
on-demand system giving the very sporting driver the option to enjoy
the dynamic driving quality of the X5 in a controlled drift by
intentionally oversteering. The xDrive all-wheel-drive system, in turn,
always remains operative and cannot be switched off.