BMW engineers took what they learned building performance cars and designed a top-notch sport-ute, the X5.
Not many cars can tear around the North ring of Germany's Nürburgring track a 14-mile, 176-turn course once used for Formula 1 races in just under 9.5 min. And the number of sport-utes capable of that feat can probably be counted on the fingers of one hand, with a few digits to spare. One SUV that is up to the task is the BMW X5, this year's Best Ride winner in the medium-sized SUV category.
Several judges commented that the X5 behaved like a sports car, not a sport-ute. Others noted its tight steering gave a "solid road feel," while still providing excellent maneuverability and the ability to avoid obstacles and potholes. And several raved about the suspension system, saying "it delivers a smooth ride and eradicates bumps and dips."
All this is evidence that BMW engineers hit their design goal: a vehicle that meets the company's high standards of performance, handling, and driving pleasure while still offering qualities many people really want in an SUV. The all-wheel-drive X5 might not be able to go toe-to-toe with the Hummer in off-road capability, but its all-wheel drive system and 4.4-liter, 32-valve V8 should be able to handle all but the roughest terrain. And besides, research shows that only 4% of luxury SUVs ever go off-road.
A SOLID FOUNDATION
A unibody welded to the chassis into a single entity creates a solid, strong, rigid structure which forms the base of the X5. The combined body and chassis reduces weight and helps distribute impact loads evenly. (The X5 is rated a "Best Pick" by the Insurance Institute for Highway Safety based on the vehicle's crash-test performance.)
The body/chassis design's biggest contribution to the X5's handling and ride is that it adds torsional rigidity, measuring 23,100 Newton-meters per degree of twist. Though the body/chassis approach is the rule in BMW autos, it is the rare exception in SUVs and trucks. But it helps the X5 counter the tendency to shudder, a quality most SUVs with heavy suspensions exhibit when going over bumps, dips, and ridges in the road.
The X5 uses a lot of high-strength steel to keep down weight. About 56% of the body/chassis shell weight is high-strength steel. Galvanized steel makes up 97% of the surface area of the body, an effort made to preserve body strength throughout the vehicle's life.
Weight distribution is almost exactly 50% on the front wheels, 50% on the rear, an optimum engineering detail. "This means no dramatic weight transfers despite acceleration, cornering, or braking," says John Buchko, an BMW spokesperson. "The SUV stays neutral and controlled even in extreme situations." The X5 also has a relatively low center of gravity compared to its high stance, an important factor to road feel and in preventing rollovers
The X5 allwheel-drive system employs a transfer case with a planetary center differential. It doles out 38% of the driving torque to the front wheels and 62% to the rear wheels. This reduces torque steer on the front wheels by sending them less than half of the overall torque and boosts directional stability by sending the lion's share of the torque to the rear wheels. It also gives the X5 the feel of a rear-wheel-drive vehicle, keeping with BMW's automotive heritage.
The X5 carries a full four-wheel independent suspension. This means that as one wheel goes up and over a bump, the wheel on the opposite side is unaffected. This is not the case with live axles, a common feature on other SUVs.
Front and rear suspensions use twin-tube shock absorbers. They have two sets of internal valves: one to provide smooth and easy compliance over small, sharp bumps; the other to control movement over larger bumps, an important consideration in SUVs going off-road.
The front wheels use a double-pivot system, featuring lower control arms for stability under all road conditions. The arm shape and positioning leaves room for large disc brakes (13.1 1.18 in.). Although similar to suspensions used on 5 and 7 Series Beemers, the X5 uses larger and stronger steel components. The subframe carrying the front suspension is also steel and uses tubular, stamped, and welded construction. It includes two hydraulic mounts (liquid-filled bushings) for damping road and powertrain vibrations.
A power-assist rack-and-pinion steering system eliminates most slack or freeplay at the steering's on-center position. Power assist varies with engine speed, giving drivers a break by reducing steering efforts at low speeds and during parking (when engine revs are below 1,500 rpm).
The rear four-link suspension is similar to that carried on the 5 and 7 Series. Three of the links are aluminum, the other steel. The multilink system controls rear-wheel angles precisely, thereby minimizing unwanted effects of dynamic load changes (as when lifting off the gas while cornering). To keep the X5 level when carrying heavy loads, the SUV is equipped with self-leveling air springs, along with an electric air compressor and ride-height sensors. The rear suspension, like the front, attaches to a tubular steel subframe. And the subframe incorporates two rubber mounts at the front and a hydraulic mount at the rear for noise and vibration isolation.
The $49,000 X5 carries several other high-tech systems designed to simplify handling and make it safer. A traction system limits wheelspin by reducing engine torque to the spinning wheel, applying brakes to individual wheels, or both. It gives the SUV sure-footed handling in rain and snow and on slippery roads. Dynamic Brake Control reduces driver pedal effort and stopping distances during emergencies. Dynamic Stability Control takes inputs from a variety of sensors to detect any abnormal under or oversteer, and gently applies individual wheel brakes to help keep the X5 on the intended path. Hitting the DSC switch on the dash raises the system slip threshold, which can be appropriate for deep snow, sand, or, as the manufacturer says, "very sporty driving."