Double Feature

The SMC ‘bustle’ decklid on the coupe does double duty as a styling cue and a rear spoiler.

You may not know his face, but I’m sure you’d recognize his tail end. Adrian van Hooydonk is president of DesignworksUSA, BMW’s U.S. design studio that has done product design and brand identification for clients as diverse as Nokia and John Deere.

His personal portfolio includes the exterior designs for the MINI ACV30 concept and BMW Z9 Grand Tourismo. He can also take the credit (or blame) for the bustle-tail of the BMW 5 and 7 Series and can now add the new 6 Series coupe and convertible to his list of achievements.

Hooydonk incorporated all of the necessary BMW styling cues in the 6 Series, (not forgetting the famous Hoffmeister kink in the C-Pillar). It sits on a 109.4-in. wheelbase, with a long nose, power dome hood and low profile. He points out the distance between the front axle and dash.

“It’s the tell-tale sign whether or not a coupe is a re-badged sedan,” he says. While the new 6 Series coupe and convertible share manufacturing facilities at the Dingolfing, Germany, plant (the 6 is more closely related to the 5 Series than the 7 Series), it was important that the 6 Series didn’t just look like a two-door 5 Series. All of the class-A surfaces on the 6 Series are unique, in more ways than one.

Body-in-white (above) features aluminum doors and hood, SMC decklid and injection-molded plastic fenders that allow for intricate designs like the fender-mounted side marker light (below).

Van Hooydonk says that once the design was signed off on, a variety of materials were chosen to make sure the finished product kept the integrity of the original design and also make for a lighter, better handling vehicle. The aluminum hood is a first for BMW. Both inner and outer panels are joined by a two-component adhesive that not only makes for a stronger finished piece but reduces the weight by 6.2 lb. The aluminum door panels save about 22 lb. over steel.

The front fenders are made from injection- molded plastic, to reduce weight, but primarily to retain the crisp body-side line that flows into a deep-set indent that carries the side marker, something that would have been impossible to stamp in either aluminum or steel.

The decklid was designed as an integral part of the vehicles aerodynamics. To achieve the proper balance, a small spoiler lip, just a few millimeters tall, was added. That and the deep draw of the decklid made it impossible to stamp this part from steel or aluminum. SMC (sheet-molded compound) not only made it possible to recreate the fine detail but cut the decklid’s weight by 25 percent.

Hooydonk points out that the 7 Series decklid has a taillight band running through the middle of the decklid’s rear panel. Because of that dividing line BMW was able to stamp the upper portion of the 7 Series decklid and use an SMC panel for the lower portion.

The engine compartment is made completely of aluminum with the remainder of the body structure made of steel tailor-rolled blanks. Aluminum body components are joined with rivets and a special isolating adhesive is used to join the steel and aluminum parts, preventing direct contact of the metals which would cause corrosion. In addition to the adhesive 4,000 welds and highpressure rivets and some 70 bolts hold the body together.

The body shop at Dingolfing is almost 100 percent automated. 500 robots at 150 stations build the body shells. A coupe body/chassis unit including doors, hood and decklid weighs 728 lb.

Dropping in a Drop Top

The convertible gets some added rigidity courtesy of additional diagonal bracing front and rear, a specific front subframe, front thrust plate of 3 mm aluminum, vs. 2.5 mm on coupe, specific reinforcements to the side sill, strengthened attachment of B-pillar to lower body, reinforced attachments for diagonal braces at rear subframe and lateral lower-body reinforcement.

The Edscha-supplied soft top mimics the shape of the coupe when up. A unique, nearly verticle glass backlight can be raised or lowered with the top up or down.

The windshield frame is manufactured using an inside high-pressure forming process. The frame is formed under high heat with oil inside it, ensuring an even distribution of forming pressure that makes for consistent wall thickness.

The convertible measures 15,000 Nm per degree of torsional rigidity without the front glass in place.

The convertible top modules are installed on the same trim line with the coupes. The unique design of the top creates extending rearward on either side of the vertical glass backlight that mimic the flow of the coupe in profile. The interior of the top is manufactured by Edscha, with a layer of Butyl- Rubber sandwiched in between a top layer of Polyacrylnitryl (PAC) fibers and an inner layer of Polyethylen (PET) fibers.

Workers install the top module on the same trim line that the coupes go down.

An insulation layer or polyurethane foam is adhered to the inside surface of the canvas top. The inside of the top is coated in polyurethane to reduce vibration and wind noise. The rear window can be lowered while the top is up or raised when the top is down to serve as a windscreen.

One button operation raises and lowers the top in 25 seconds for the full cycle.