The once and future king: BMW's 3 Series; Just when the competition thought they had it in their sights, BMW raises the sports sedan bar with its 5th Generation 3 Series

Automotive Design & Production, June, 2005 by Christopher A. Sawyer

Dr. Wolfgang Epple could have come from central casting at a movie studio. Put the call out for a German car engineer, and you'd expect to find a dozen trim, balding German men in early to mid-50s milling around the casting office. However, it's doubtful any would have Epple's quiet confidence or could carry the same weight on his shoulders. The 3 Series--which comprises about 50% of BMW's total production and most of its profits--is the literal heart and soul of BMW. Without it, the company ceases to exist as a high-margin, privately held entity.

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"Like anyone else in my position, I have a strict budget to keep to, and a board that makes certain I do," he says with a wry smile. That budget included fewer prototypes than he was hoping for. Dr. Epple complied by building the same number as had been used on the previous program. "I did more with them," he says simply. For example, 50 of the prototypes were used to test the complete electrical system under real-world conditions while carrying out their normal tasks because--despite nearly 80% commonality with the 5 Series' electrical system--the 3 Series has more control modules that are integrated into common units. "Making sure the parts work isn't enough," Epple cautions. "You have to make sure that each one of those components that checks out fine in bench testing works with the rest of the system under conditions the customer is likely to encounter."

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STRUCTURE

Unlike the larger 5 Series, the 5th generation 3 Series does not mix aluminum and steel in its structure. Instead, high-strength and multi-phase steel make up 38% of the total structure by weight, and is used to add strength without adding weight. The body structure is 25% more rigid, but 10 lb. lighter than its smaller predecessor. It's also used to transfer crash forces from one side of the vehicle to the other. "This interaction involves more of the structure in major accidents," says Epple, "making it possible to control how the forces are dissipated in an optimal manner."

Most of the high-tech steels are found in the A- and B-pillars, the roof bow between the B-pillars, the lower B-pillar, door reinforcements, and in the diagonal beams within each door. Tailor-welded blanks are used in the doors, but the trunk is laser welded and built with two sets of tools. "One set was what we engineers wanted," says Epple, "but the Design Dept. wanted a more stylish look around the upper lid and license plate. They won the argument."

The new 3 Series meets FMVSS 301, a 50 mph offset rear-impact standard that contacts 70% of the structure and requires no fuel leakage. It also meets a side-impact test promulgated by the Insurance Institute for Highway Safety (IIHS) that replicates a SUV impacting the vehicle's side at 50 km/h. "It's a tough test," says Epple, "because the entire impact is above the side sills." The new 3 performs so well in side-impact testing that BMW was able to remove the side airbags in the rear doors. Up front, the side-impact bags are now in the seats, not the doors, meaning they can be smaller yet offer better protection. Plus, the previous tubular head air bag has been replaced by a side curtain bag that offers greater injury protection. The driver's feet and knees are protected via a two-stage system that lessens crash severity. First, the footrest is designed to crush under the driver's left foot so the impact isn't transmitted to his foot or lower leg. Second, the brake and master cylinder rotate as the front end deforms, moving the clutch and brake pedal forward.

POWERTRAIN

Two new engines are available, both are inline sixes and part of the N52 family. They continue BMW's recent fascination with bedplate construction, first seen in the M5's V10, and use a composite magnesium/aluminum construction. A ridged high silicon content aluminum alloy insert forms the cylinders and coolant passages, and interlocks with the ridged magnesium upper crankcase that shrinks onto the aluminum insert while cooling. Magnesium also is used for the bedplate that forms the lower block. The upper case is lowered onto the bedplate, and a liquid sealing compound is injected into a groove in the contact surface.

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The head is cast via the lost foam process, but the real technology lurks within. Hollow camshafts built from hydroformed steel tubes save 2.6 lb. compared to conventional cams, and sit under a magnesium cam cover. The cam drive is integrally cast into the block structure--this eliminates the need to seal separate components, saving a step in the production process--and the exhaust header flanges are formed from 2.0-mm steel instead of the 12-mm pieces used on the previous engine.

The coolant pump is electrically driven, consuming a maximum of 200 Watts--versus 2.0 kW for a conventional design--and runs only when needed. It allows faster engine warm-up, circulates coolant to provide heat to the cabin when the engine is off, and eliminates the need for an external drive belt, making the engine shorter. A variable-volume oil pump also is used.