Blitzing a Cabrio into production - Mercedes-Benz's CLK Cabriolet

Automotive Industries, Sept, 1998 by Norman Martin

Nichemeister Karmann took Mercedes' new CLK convertible from concept to production in two years.

When Mercedes-Benz needed turn-key production of its new CLK Cabriolet, it handed the job over to an old pro. Wilhelm Karmann GmbH, Germany's veteran manufacturer of low-volume convertibles and niche vehicles, handles BIW-production and final assembly of the $47,200 drop-top. Based on the gorgeous CLK coupe launched last year, the 1999 4-seat convertible is remarkable, both in how its clever roof retracts into the trunk, and how it retains the sensuous lines of the coupe with the top up.

Mercedes called upon Karmann for two reasons. The automaker's production facilities were at capacity, and Karmann's cost structure is scaled for the relatively low-volume (11,000 units slated for this year, rising to 30,000 in '99) CLK Cabrio--even though the convertibles have to be hauled back and forth 74 miles by rail for painting.

Manfred Remmel, executive vice president for production at Daimler-Benz passenger cars, says Karmann already proved to be an able partner, also building the smaller SLK roadster. "It was only natural for us to draw on a previous good experience," he says.

While Karmann is probably still best-known in the U.S. for its production of Volkswagen's Karmann-Ghia models in the 1960s, the company has existed for nearly a century as a coach-builder in Osnabruck, Germany. Today, it's the major employer in the German states of Niedersachsen and Nordrhein-Westfalen, with 7,500 employees. Specializing in convertible production, Karmann has produced more than 3 million vehicles at its German plants, with some 900,000 (most of them VW models) exported to North America over the years. It develops and/or assembles various specialty models for Audi, BMW, Daimler-Benz, Ford, Jaguar, Renault and VW.

The Program

Following the CLK's concept intro at the Frankfurt Auto Show in September 1995, Karmann and Daimler-Benz immediately began discussing development of a convertible version off the coupe. By the end of month Karmann was given program approval, and a full-scale prototype of the car.

"We cut the (prototype's) roof off, and started to rework the body into a convertible," says Siegfried Licher, director of technology development at Karmann in Osnabruck. The new design was presented to the Daimler-Benz board just eight weeks later. "It was a very short process for such a complex styling rework," Licher says.

A two-year development timetable was set. After the design 'was finalized in CATIA (the CAD/CAM platform used by both companies), Karmann engineers produced their first prototype in nine months. A total of 22 prototype iterations were built for safety and structural stiffness testing. Following testing, dies and production fixtures were built and installed by September 1997. Karmann spent some $70.5 million in production facilities, equipment and tools.

Actual production of the convertible is a complex process. Mercedes' plant in Bremen stamps various body and structural components for the Cabrio and ships them to Karmann. Powertrain and suspension assemblies go to Karmann from Mercedes' factory in Stuttgart.

Much of the body above the floorpan, from the A-pillars back, is stamped by Karmann. The parts include revised A-pillars and windshield header, sidesills, rear fenders, rear bulkhead, trunklid and soft top cover. The doors are also slightly modified from those used on the coupe. The body-in-white is assembled at Osnabruck, and transported by truck 74 miles north, back to Bremen. There they are painted and returned to Osnabruck for assembly.

The vehicle enters Karmann's assembly line, where wiring and acoustic insulation are installed, followed by fitting the doors and instrument panel. The vehicle then moves to a turntable station, where axles, exhaust systems and lower parts of the car are added. The turntables can be turned up to 42 degrees. Next the interior trim section, where seats, carpets and interior trim are fitted. The car then undergoes a rain test, before moving to a test track.

Overall, the process is similar to that which Karmann employs for VW Golf Cabrio production, Licher says. What is different in the CLK's case is Karmann has full responsibility for testing. This testing phase, which was run in conjunction with a small Daimler-Benz engineering team, is extremely important in such turn-key projects, Licher says.

And while it is unusual to move vehicles miles back and forth from the paint shop, Licher says, documentation and run rates are clearly defined. "Initially, the challenge was handling ail the parts we needed to build this car, compared to products we already have in production." The CLK, with its high number of special options, has double the number of parts Karmann normally handles.

While the Cabriolet uses the same basic structure as the coupe, the body is unique from the A-pillars back. One problem that appeared from open prototype testing was the wind flow circulation entering from the back side of the vehicle. A wind deflector had to be developed for inside the cockpit. Licher recalls that 10 different iterations were tried; in the end, both the back seat and rear quarter panel were modified. The three-piece deflector itself is anchored by a crossbar that attaches to the side panels. A mesh pulls out vertically from underneath the rear seats and attaches to the crossbar.