(continued)
Housed in a transfer case behind the transmission, xDrive operates as follows:
- Driving torque is always transmitted to the rear wheels, and under most
conditions to all four wheels.
- The portion of torque transmitted to the front wheels is controlled by a
multi-disc clutch that can be fully open, fully engaged or at any level of
partial engagement in between.
- Engagement pressure on the multi-disc clutch is applied by a servo motor,
which in turn is directed by an electronic control system with inputs from the
Dynamic Stability Control (DSC) system via sensors for:
- Rotational speed of each wheel
- Steering angle
- Brakelight switch
- Vehicle yaw and lateral acceleration.
- The servo motor exerts engagement pressure on the multi-disc clutch via a
linkage consisting of levers, a ball ramp and a disc cam.
Considering the front and rear wheels as two pairs, the rears always drive;
the proportion of the total driving torque transferred to the front pair at any
given time depends upon the degree to which the multi-disc clutch is engaged
(i.e. the engagement pressure exerted by the servo motor). As more torque is
transferred to the front wheels, a smaller proportion goes to the rear. With
full traction at all wheels, the maximum proportion to the front is 50%; in a
situation where both rear wheels are on slick ice and the fronts are getting
traction, the proportion going to the rears can approach zero.
In normal driving, the clutch operates with minimum slip, resulting in a torque
split around 40% front/60% rear; this is consistent with the “basis” front/rear
split of BMW all-wheel drive over the years. Under conditions of uneven
traction, the control system interprets wheel-speed data to determine the
front/rear driving-torque proportions; additionally, the DSC traction-control
function can intervene to reduce engine torque and apply individual wheel
brakes, thus helping achieve maximum possible usable force to move the vehicle.
Via the DSC switch on the console, the driver can also de-activate engine
intervention, leaving only the brakes; this function is referred to as ADB-X
(Automatic Differential Brake for all-wheel drive).
But xDrive does not optimize traction alone; it also optimizes handling
characteristics, enhancing both agility and stability on non-slippery as well as
slippery road surfaces. Via the same type of logic DSC employs to recognize and
correct for excessive over- or understeer, xDrive adjusts the front/rear torque
split to avoid these tendencies. If undesirable oversteer is sensed, the
multi-disc clutch is further closed, sending additional torque to the front
wheels. If excess understeer is detected, xDrive opens the multi-disc clutch
completely, leaving no driving torque at the front wheels. All this occurs
without any overt indications to the driver; the driver merely enjoys optimum
vehicle dynamics under a wide range of driving and road conditions.
xDrive even varies the front/rear torque split according to acceleration on dry
pavement, increasing the proportion to the rear wheels under heavier
acceleration and decreasing it under lighter acceleration. xDrive makes all
these adjustments to the front/rear torque split ultra-quickly; reaction times
(from sensing to control actions) are so small that the system almost seems to
be pro-active.
The Engine
The X5 4.8is takes the Valvetronic power concept to an even higher level,
employing a larger bore and stroke to obtain a displacement of 4.8 liters. In
turn, this means even greater power and torque: 355 hp, and 360 lb-ft of torque
vs. the 4.4i’s 324 lb-ft. So motivated, the 4.8is model reaches 60 mph from a
standstill in just 5.9 seconds. (4.4i: an also lively 6.8 sec.)
Teamed with both V-8 engines is a 6-speed version of the STEPTRONIC automatic
that fully exploits the engine’s performance capacity. Its 6th gear smoothes the
flow of power from a standstill to high speeds with more, smaller steps between
the 1st and top gears, which are further apart than in the 5-speed. Because the
4.8is is the highest-performing X5 model, its transmission is calibrated in an
especially sporty way.
Subtly Updated Exterior
Design
Many consider the X5 the most handsome vehicle of its type; for the ’04 model
year BMW gave it a discreet updating. This included all-new front sheet metal
and details, such as wider grilles and new headlight clusters. Adaptive Light
Control, which “steers” the headlights to help the driver see around curves and
corners, was added to the Xenon headlights that are standard on the two V-8
models and optional on the 6-cylinder 3.0i. Rain-sensing windshield wipers and
automatic headlight control (which switches on the headlights and related
lighting at dusk and in low-light conditions) became standard on all models.
Adaptive brakelights, which illuminate more strongly under hard braking, were
also introduced in ’04.
Safety and Security: Exemplifying
BMW’s Exacting Standards
The X5’s unit body/chassis structure implies advantages in terms of passive
safety. Beyond this fundamental edge, the X5 also includes an extensive array of
BMW safety features, including the Head Protection System (HPS) front and rear,
“smart” front-impact airbags, standard front- and optional rear-seat side-impact
airbags, 3-point safety belts at all seating positions, front belts with
automatic tensioners and force limiters, and the Battery Safety Terminal to
diminish the likelihood of a fire after a severe crash impact.
In numerous stringent crash tests, the X5 has proved its mettle with regard to
safety. Indeed, the X5 performed better than any SUV ever tested by the
Insurance Institute for Highway Safety in its brutal 40-mph offset crash test.
On the basis of this result, the Institute rated the X5 a Best Pick – the third
BMW to earn this distinction. Not only was the X5 the best SUV ever tested; in
fact, it achieved the best performance ever of any vehicle in this severe IIHS
test. More recently, the X5 added to its safety laurels with 5 stars – the
highest possible rating – in the Euro-NCAP test, Europe’s equivalent of the
widely watched U.S. New Car Assessment Program safety evaluation.
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