Mercedes' grand convertible switches to aluminium lightweight construction
22 November 2011

Mercedes will break new ground with the sixth generation of its inimitable Mercedes SL roadster next year by making it the very first Mercedes in large-scale series production to use an almost exclusively aluminium lightweight construction.

At a preview event held close to last week’s Los Angeles motor show, Mercedes showed journalists a completed example of the new two-seater grand convertible’s ‘body-in-white’ superstructure.

See the official pics of the new Mercedes SL's tech and in testing

Made from a mix of chill cast-, vacuum die cast-, stamped- and extruded aluminium, it allows the finished 2012 SL to be at once 140kg lighter than the outgoing steel car, and 20 per cent more torsionally rigid – to the enhancement of handling precision, rolling refinement, performance and fuel-efficiency.

Mercedes’ CAD-optimised design for the SL’s monocoque is even stiffer than other aluminium-constructed rivals – or so the firm claims. “Our engineers have measured a Jaguar XK’s body stiffness at around 16,000Nm per degree of torsional deflection,” said SL Product Manager Bernd Stegmann. “The new SL has just under 20,000Nm per degree.”

The various metal castings and tailored blanks of the SL’s body-in-white are connected via different methods, depending on how much load they transmit. While some parts of the underbody are MIG welded, hemmed, bonded and bolted, others are joined via state-of-the-art friction stir welding – a technique that allows for particularly stiff, precise joins.

With mainly aluminium body panels, the only significant parts of the new Mercedes that aren’t aluminium are its pedestrian-protection-oriented plastic ‘soft nose’ grille, its super-lightweight magnesium rear bulkhead, and its tubular steel A-pillars and header rail. Steel is chosen here for its extremely high strength in the event of a rollover.

Mercedes’ engineers wouldn’t be drawn to confirm very many other details about the new convertible last week, which will be unveiled for real at the Detroit motor show in January. But Stegmann did tell Autocar that diesel or petrol-electric hybrid verions of the SL had been ruled out.

“SL customers are extremely loyal, and they know what they want from the car,” he said. “Quality, refinement, safety, design appeal, and a certain amount of dynamic ability. But most of these cars are the third or fourth in someone’s collection: so fuel economy really isn’t a priority.”

Matt Saunders

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