The new xDrive system: variable torque split, quick reflexes, in adapting to changing driving conditions

The original X5 all-wheel drive system utilized a planetary center differential that apportioned driving torque to the front and rear pairs of wheels in a ratio of 38% front/62% rear. Intervention by the traction-control function of the Dynamic Stability Control (DSC) system influenced this basic torque split as required by traction conditions.

The new xDrive system, like its predecessor housed in a transfer case behind the transmission, takes an entirely different path to assigning driving torque to the front and rear wheels. In its essence, xDrive operates as follows:
Driving torque is always transmitted to the rear 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 servomotor, which in turn is directed by an electronic control system that receives inputs from the 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.

All-wheel drive is essentially permanent; under normal traction conditions, the circumstances which cause xDrive to operate in rear-wheel drive only are:

• During very tight cornering with little engine torque, to permit large wheel-speed variations between the front and rear wheels (such as when parking).
• In tight curves with low to medium torque.
• When the vehicle tends to understeer markedly.
• Anytime the antilock braking function is active.

In normal driving, the clutch is operated with minimum slip, resulting in a torque split around 40% front / 60% rear; this is similar to the 38% / 62% permanent split provided by the previous planetary center differential. 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 assisting in achieving the maximum possible usable force to move the vehicle. Via the DSC switch on the console, the driver can also elect to 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 can also optimize 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 the 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, a feat the system's algorithms are programmed to accomplish.

In connection with all these adjustments to the front / rear torque split, it's essential to note that xDrive makes them ultra-quick; reaction times (from sensing to control actions) have been reduced so much that it could almost be said that the system operates pro-actively.

Bottom line, xDrive deals optimally with a wide spectrum of traction conditions. More remarkably, the system also optimizes vehicle dynamics or, more popularly expressed, handling; it adds to the X5's inherently great agility for this type of vehicle.

xDrive will also be found in the new BMW X3; the X5 transfer case is heavier-duty than that of the X3, with larger dimensions and one more disc in its multi-disc clutch.

Upgraded power trains in both X5 models

Each '04 X5 model enjoys an upgrading of its powertrain. With its new Valvetronic V-8 engine, the X5 4.4i shows the most dramatic change. From its '00 inception through the '03 model year, the 4.4i was powered by a 4.4-liter aluminum V-8 engine with four overhead camshafts, 32 valves and VANOS variable valve timing on its intake valves; this unit delivered 290 hp and 324 lb-ft. of torque, and earned approval like this from Motor Trend in April '03: "There was no shortage of praise, nevertheless, for the smooth-yet-potent 4.4-liter V-8."

The newest X5 4.4i sports an even smoother, more potent 4.4-liter V-8. It puts out 315 hp, and also achieves better results in both portions of the Environmental Protection Agency's fuel-economy testing: 16 mpg city / 22 mpg highway, versus the previous 14 / 18. Behind these impressive new data are engineering advances:
Valvetronic - a patented BMW system that varies valve lift to control engine power. Instead of a conventional throttle, Valvetronic varies valve lift all the way from a minuscule 0.3 mm (for idling) to a high 9.7 mm (for full-power operation). Thus eliminating the "pumping losses" associated with throttles, Valvetronic plays a major role in the 25-hp gain; it also helps make the engine smoother.

Fully variable intake manifold. Many contemporary engines employ 2-stage intake manifolds that switch between two paths for air entering the engine: one for low- to medium-speed operation (improving torque and response), the other for high-speed operation (improving top-end power). BMW's engine team conceived a fully, steplessly variable, intake manifold that optimizes performance right across the entire engine-speed range.

Double VANOS. Shared with other engines, this feature applies variable valve timing to exhaust as well as intake valves.

Teamed with the Valvetronic engine is a new 6-speed automatic transmission that can more fully exploit the engine's performance capacity than the previous, already excellent 5-speed automatic. The extra forward gear also smoothes the flow of power from a standstill to high speeds with more, smaller steps between the 1st and top gears, which are almost identical in the previous and new transmissions.

No published evaluations of this powertrain are available yet, but after trying the near-identical combination in the BMW 7 Series, Road & Track's June '02 issue reported that it "responds instantaneously to the driver's right foot as it delivers dose after dose of pure unadulterated acceleration." The '04 X5 4.4i's 0-60 time has improved from 7.4 to 6.8 seconds.

The X5 3.0i keeps the same 6-cylinder engine as before - the unit Car and Driver noted in a January '03 comparison test: "To be sure, the X5 was a delight to wail through the endless serpentines of the Hocking Hills while its 225-hp, 3.0-liter inline six sang yet another haunting Bimmer exhaust tune." Here, the power train upgrade for '04 is a new 6-speed manual transmission, which conveys benefits similar to the 4.4i's automatic. With the 6-speed, the X5 3.0i accelerates to 60 mph in 7.8 sec. (0.3 sec. quicker than before).

Even if many consider the X5 the most handsome vehicle of its type, BMW has given it a discreet updating for the '04 freshening

From the windshield forward, all sheet metal and the bumper / air-intake unit are new.

Hood contours are more pronounced, the traditional BMW "kidneys" grilles are widened and set higher, and the headlight clusters are newly shaped; inside these, the luminous rings recently introduced on other BMW Series make their first appearance on the X5, and Xenon lighting (optional 3.0i, standard 4.4i) powers the low and high beams, vs. low-only previously.

BMW's Adaptive feature, which "steers" the headlights to help the driver see around curves and corners, will be added later in the '04 model year.

The front bumper continues with its upper portions in body color and lower section in matte black; new here are the overall contours, differently configured fog light nacelles, and an upper section with a split air intake.

As before both the upper and lower bumper grilles are screened; with the optional Sport Package, these screens are in a Titanium finish.

Rain-sensing windshield wipers, formerly included in the optional Premium Package, are now standard; so is automatic headlight control, which switches on the headlights and related lighting at dusk and in low-light conditions.

Four new wheel designs also contribute to the visual freshening

The 3.0i's standard 17 x 7.5 and Sport Package 18 x 8.5 wheel designs are both new; so is that of the 8.5 x 18 standard wheels and the beefy 19 x 9.0 front / 19 x 10.0 rear wheels available on the Sport Package-equipped 4.4i. Sizes and speed ratings are unchanged for '04.

The taillights are in a new "brilliant" look; Adaptive brake lights, which illuminate an additional lamp brightly under hard braking, continue. There are seven new exterior colors.

New standard and optional equipment inside the '04 X5

Previously, the 3.0i had an 8-way power driver's seat and a 6-way manual front passenger's seat; an 8-way power passenger's seat was part of the 3.0i Premium Package and standard in the 4.4i. For '04, both front occupants get standard 8-way power seats with fore-aft, height, cushion-angle and backrest adjustments.

Other interior changes affect colors and trim materials. Truffle Brown is added to the leather color choices, bringing the selection to five; and in addition to the two colors of wood trim (3.0i Premium Package, 4.4i standard), Maple Anthracite - a handsome gray-tone wood first seen in the new 5 Series - is also now available.

X5 4.4i is more than just more power

As the model that launched the X5 Series, the 4.4i occupies the top position until the upcoming even higher-performance model is introduced. Compared with the 3.0i model, the 4.4i embodies the following increments of engineering and equipment:

• The new 4.4-liter V-8 engine, vs. the 3.0i's 3.0-liter 6-cylinder unit
• Automatic transmission as standard equipment (3.0i: optional); 6-speed automatic vs. 5-speed
• Standard self-leveling rear air suspension (3.0i: optional)
• Standard 18-in. wheels and tires (3.0i: 17-in. standard, 18-in. in Sport Package); 4.4i Sport Package wheels are V-rated vs. 3.0i's H-rated
• 19-in. wheels and tires available; not available on 3.0i
• Various distinctions in exterior trim
• More extensive Check Control monitoring functions in instrument cluster (3.0i: Premium Package)
• Automatic climate control with separate left / right temperature settings (3.0i: Premium Package)
• Rear-compartment climate control (3.0i: Rear Climate Package)
• Leather upholstery and wood interior trim
• Differently composed option Packages.

X5 3.0i: sporty 6-cylinder power in the most accessible X5

Base-priced $11,200 below the V-8 4.4i, the 6-cylinder X5 3.0i is positioned in a market segment with such upscale 6-cylinder SUV competitors as Mercedes-Benz ML 350, Infiniti FX35 and Lexus RX 330.

The 3.0i's most obvious distinguishing feature is its inline 6-cylinder engine, an engine type for which BMW is justly famous - and not just in recent times, but all the way back to 1933. Largely because V-6s are easier to package with front-wheel drive, inline 6-cylinder engines are a rarity today; BMW nurtures the inline-6 tradition because of this format's superior sound, natural smoothness and linear power delivery

The M54 6-cylinder engine develops 225 hp and 214 lb-ft. of torque. An aluminum block and cylinder head help keep weight moderate; BMW's VANOS 1 system varies intake- and exhaust-valve timing steplessly to optimize torque, power and emission control through the engine's broad rpm band.

Uniquely in the X5 Series, the 3.0i is available with a manual transmission; the new 6-speed described earlier.

From the outside, the 6-cylinder 3.0i model is distinguished most noticeably by its different wheels. The 6-cylinder model comes standard with 17 x 7.5-in. wheels, carrying 235 / 65R-17 all-season tires; on the 4.4i model, 18 x 8.5 wheels with 255/55R-18 tires are standard. With the available Sport Package, the 3.0i moves to 18 x 8.5 wheels with 255/55R-18 H-rated all-season tires. The 4.4i Sport Package includes 18 x 8.5 wheels and V-rated all-season tires; or the customer can specify 19-in. wheels and V-rated performance tires of differentiated sizes: 19 x 9.0 front with 255/50R-19 tires, 19 x 10.0 rear with 285/45R-19.

Packages and stand-alone options available for individual tastes

All X5 models are extensively equipped in base form; yet BMW recognizes that buyers have varying tastes and priorities. Thus an appealing range of options, both Packages and stand-alone, is available. All Packages are available for the 3.0i and 4.4i, and all can be combined.

Packages:

• Premium Package. Given the 3.0i's lighter standard equipment, its Package has the greater content: automatic climate control, Multi Information Display with 8-function On-board Computer, power moonroof, leather upholstery, Light Poplar trim and power-adjustable rear-seat backrests. As the leather and wood are standard in the 4.4i, its Premium Package omits these features but adds others: auto-dimming interior and exterior mirrors and 4-way power lumbar support. BMW Assist, described later, is newly included in these Packages.
• Sport Package. Both Packages (3.0i and 4.4i) include sport suspension, sportier wheels and tires, titanium-finish front bumper grille, Shadowline (matte-black; high gloss for 4.4i) trim around the side windows, Black Chrome exhaust tips, sport steering wheel, sport seats and Anthracite-color headliner. With the 4.4i Package, the electronic maximum-speed limiter is deleted for a 149-mph top speed vs. the limited 130. Wheel designs are specific to each model; the 3.0i tires are H-rated all-season, the 4.4i V-rated all-season.
• Cold Weather Package. identical for 3.0i and 4.4i; includes a high-intensity liquid headlight cleaning system, heated front seats and an interior ski bag.
• Rear Climate Package. This group includes pull-up sunshades on the rear door windows and privacy glass aft of the B-pillar. The 3.0i Package adds rear-compartment climate controls, which are standard in the 4.4i.

Stand-alone Options:

• STEPTRONIC automatic transmission. Optional only on the 3.0i, where it is a 5-speed unit; a 6-speed version is standard on the 4.4i.
• Self-leveling rear air suspension. Optional 3.0i, standard 4.4i. Air springs replace coil springs in the rear suspension. To keep the X5 level even when heavy loads are carried, ride-height sensors recognize changes in the vehicle's attitude and signal an electric air compressor to increase or decrease the air pressure and keep the vehicle on an even keel. Normal ride height is 0.3 in. lower with this system.
• Adjustable ride height. To the self-leveling rear air springs, this system adds front air springs and adjustable height for all four air springs. Via a switch to the left of the steering column, vehicle height can be set to -
  • Normal, with 8.1-in. ground clearance.
  • Offroad - 20 mm/0.8 in. higher than normal.
  • Access - 40 mm/1.5 in. lower than normal, for ease of entry and exit.
• The offroad setting can be used up to approximately 31 mph, at which the vehicle returns to the normal setting even if Offroad is selected. The access setting can be held until the vehicle reaches approximately 22 mph, or engaged if the vehicle is traveling 16 mph or less. Available on both models; requires Sport Package.
• 19-in. wheels and tires. Available on 4.4i in combination with Sport Package for sportier handling. New Star Spoke wheel design; sizes and tires described earlier.
  Xenon headlights - optional 3.0i, newly standard 4.4i. Include dynamic auto-leveling to adjust the lights not only for vehicle loads but also for more transitory changes in vehicle attitude, such as acceleration or braking.
• Park Distance Control - Via ultrasonic sensors in the front and rear bumpers, PDC senses when the vehicle is approaching obstructions that may not be visible to the driver. Beepers at the front and rear of the cabin alert the driver.
• Auto-dimming interior / exterior mirrors, available as stand-alone option on 3.0i.
  Power 2-way moonroof - optional 3.0i and 4.4i. Also part of both models' Premium Package.
• BMW On-board Navigation System, available in both models. Includes the 8-function On-board Computer.
• Premium audio system - optional both models. In place of the standard 10-speaker system, includes 12 speakers (including two subwoofers) plus Digital Sound Processing and upgraded components throughout.
• 6-disc CD changer, available in all models for BMW Center installation.
• 16-way Comfort front seats with articulated, power-adjustable upper backrests and 4-way power lumbar support. Available in 4.4i only.
• 4-way power front-seat lumbar support - optional in 3.0i, part of 4.4i Premium Package.
• Leather upholstery. High-quality Dakota leather, standard in the 4.4i, is offered as a stand-alone option (or part of the Premium Package) in the 3.0i.
• Heated steering wheel - available in 3.0i and 4.4i without Sport Package. Requires the Cold Weather Package.
• Heated rear seats - available in both models in combination with Cold Weather Package; in 3.0i, requires leather upholstery.
• Interior trim options. In the 3.0i with Premium Package and any 4.4i, Dark Poplar or new Maple Anthracite wood can be substituted for Light Poplar at no extra cost.
  Rear-seat side-impact airbags are available in both models; front and rear Head Protection System is standard.
• Retractable load floor, available in both models. An attractive and useful feature for convenience in loading objects into the cargo area. After being released via its handle, the carpeted floor panel rolls out on ball-bearing rails.

In the recent past, BMW Assist - a comprehensive program of safety- and convenience-oriented customer services - has been a part of the available BMW On-board Navigation System. Now these services, in enhanced form, are offered as a stand-alone option as well as being included in the X5 Premium Packages.

The in-vehicle equipment for BMW Assist includes GPS technology and an SOS button that in the X5 Series is in a panel above the windshield. Vehicle occupants may request emergency or other services simply by pressing this button; the BMW Assist system transmits the location and vehicle information to the BMW Assist Response Center. A response specialist will then speak with the occupants to coordinate dispatch, notify emergency contacts on file, and link BMW Roadside Assistance or emergency services as needed and requested. Deployment of an airbag automatically activates BMW Assist as well.

A year's subscription to BMW Assist is included with the option; after the initial year, the annual fee is $240. In addition to the safety-related benefits, subscribers can call a toll-free 800 number to avail themselves of many BMW Assist Concierge services, from travel planning to dining reservations, shopping assistance and event tickets.

Like all current models, each X5 is covered by BMW's 4-year/50,000-mile limited warranty and comes with a 4-year/50,000-mile Full Maintenance Program for customer convenience, peace of mind and economic benefit.

The unique character of BMW's Sports Activity Vehicle continues unabated

The X5 continues as a unique alternative to conventional SUVs. Its special character, as only BMW could have created it, springs from (but is not limited to) features and qualities such as -

• Unit body/chassis structure for exceptional structural rigidity without unduly high weight, excellent riding comfort and responsive handling (still relatively rare in SUVs, particularly large ones).
• 4-wheel independent suspension for best on road riding and handling. Publications have reported cornering capability as high as 0.82g for the X5 4.4i with 19-in. wheels and tires (Motor Trend, April '03). Used to be rare in SUVs; now on the increase.
• Optimum weight distribution, in the BMW tradition, also for optimum handling. No '04 X5 model departs from the ideal 50% front / 50% rear weight distribution by more than 1.9%.
• Fulltime all-wheel drive, with the new xDrive system delivering amazing traction and a new standard of agility for this type of vehicle.
• Dynamic Stability Control (DSC) for enhanced stability in cornering or accident-avoidance situations.
• Hill Descent Control (HDC), which intervenes at low speeds to help the driver maintain speed down steep grades.
• Variable-assist power steering with excellent road feel, as in all BMWs.
• State-of-the-art brakes, with 4-wheel discs, electronic proportioning of braking force at all times, and Dynamic Brake Control to reinforce the driver's pedal effort in emergency braking.
• BMW design. Tasteful, smooth, sophisticated and timeless, inside and out.
  BMW ergonomics. Motor Trend's April '03 report noted, "We like the X5's familiar interior layout, with the hefty gauge hump, clean center stack, rich materials, and impeccable fit and finish."

Safety and Security

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.

Performance with a Conscience

BMW strives to produce its motor vehicles and other products with the utmost attention to environmental compatibility and protection. Integrated into the design and development of BMW automobiles are such criteria as resource efficiency and emission control in production; environmentally responsible selection of materials; recyclability during production and within the vehicle; elimination of CFCs and hazardous materials in production; and continuing research into environmentally friendly automotive power sources. Tangible results of these efforts include the recycling of bumper cladding into other vehicle components; water-based paint color coats and powder clear coats; near-future availability of hydrogen-powered models; and various design and engineering elements that help make BMWs easier to dismantle at the end of their service life.

source: BMW USA