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Automotive Industry

Generation 5

Automotive Design & Production, Nov, 2003 by Christopher A. Sawyer

Despite the controversial styling, the fifth generation BMW 5 Series hasn't lost its sporting focus or dynamic prowess. It remains a benchmark for sport sedans and for the hard work needed to stay on top.

There is a lot of aluminum in the new 5 Series, but it doesn't fit the standard definition of an aluminum-intensive vehicle. That's because most of the unibody is made of steel. Only the structure forward of the A-pillars is aluminum. A full-contact adhesive segregates the materials so there is no chance of a galvanic reaction, while self-piercing rivets provide extra holding power in peel-prone areas. The shock towers are intricate aluminum castings. The hood inner and outer panels combine the expected rolled flange at the leading and trailing edges with a "tabbed" side flange. The inner and outer section side flanges extend vertically from the panel, and adhesive fills the gap. (It also fills the gap between the panels on the underside of the hood.) Small tabs located along this section provide enough area for self-piercing rivets to lock the pieces together.

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Although aluminum counts for just 15% of the unibody, for the first time in recent BMW history a new model is lighter than the model it replaces." This is despite an increase in both content and vehicle dimensions. The most common model, on a sales-weighted basis, is the 530i automatic which tips the scales at 3,494 lb, 45 lb less than before. Beyond the weight savings, another advantage of this construction was the fact that the aluminum front structure makes BMW's vaunted 50:50 weight distribution easier to achieve.

Both the front and rear suspensions are aluminum and sit on aluminum sub frames. The front is a strut design with double-pivot lower arms, coil springs and twin-tube gas pressurized dampers, while the rear is a multi-link design that shares componentry with the 7 Series. When the Sport Package is ordered, the conventional anti-roll bars are replaced by "Active Roll Stabilization," which adds hydraulic actuators to the front and rear anti-roll bars. These units decouple the bars during straight-ahead travel, and limit roll up to 0.80 g in corners. "When we limited the roll above that level," says Rich Brekus, manager, Product Planning and Strategy, BMW North America, "it didn't matter how experienced the driver was. They'd go flying off the road when they reached the limit of adhesion because there was no roll as a warning."

The Sport Package includes another bit of technical trickery: Active Steering. Supplied by ZF Lenksysteme, a joint venture between ZF and Bosch, this rack-and-pinion steering unit adds a planetary gearbox and electric motor to increase or decrease the steering angle of the front wheels. This makes the new 5 Series much more agile at parking speeds, where the steering wheel moves two turns lock-to-lock, and stable at high speeds, where more effort is necessary to divert the car from its path. It is combined with Servotronic power assist, which varies steering boost based on vehicle, not engine, speed. As expected, the springs and dampers are firmer, the ride height lower (by 0.6 in.), and the tires larger (17 in. on the 525i, 18 in. on the 530i and 545i).

Vented four-wheel disc brakes are used on all models, with the 545i receiving massive 13.7 x 1.18-in. front and 13.6 x 0.94-in. rear units. A two-part rotor is used on this model, and combines an aluminum "hat" that mounts the rotor to the hub with a high-carbon cast iron rotor. This reduces unsprung weight by 2.2 lb at the front and 1.5 lb at the rear, while lessening rotor deformation under severe use. The brake calipers also are made of aluminum.

No matter which 5 Series model is ordered, a six-speed transmission is part of the package. Standard on both the 525i and 530i models is ZF's Type H six-speed manual, while the V8-powered 545i gets the more robust Type G. The Type H is lighter than its five-speed predecessor, while the Type G holds a weight advantage over the six-speed it replaces. The ZF 6 HP 19 six-speed automatic is 10% lighter than the five-speed unit it replaces. A third option, SMG (for "Sequential Manual Gearbox") mates the six-speed manual with an electro-hydraulic shift mechanism, and does away with the clutch pedal. Each transmission is partnered with a friction-welded aluminum driveshaft that's more than 13 Lb lighter than the previous all-steel driveshaft. Steel, however, is still used in the yokes to provide shear strength.

The 2.5 and 3.0-liter versions of the M54 inline six-cylinder remain largely unchanged, though fine tuning of the intake and exhaust have improved part-throttle response. The 4.4-liter V8 in the 545i is the same N62 engine used in the 7 Series, and produces 325 hp @ 6,100 rpm and 330 Lb-ft of torque @ 3,600. Fitted with Valvetronic (variable lift), Double VANOS (variable timing of both the intake and exhaust cams), and a two-stage intake manifold the engine runs without a throttle under most conditions. A small auxiliary throttle is used as a fail-safe should the Valvetronic system fail, to control fuel tank ventilation, and to improve cold starts.

Automotive Industry

Generation 5

Automotive Design & Production, Nov, 2003 by Christopher A. Sawyer

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