The new Mercedes-Benz SL: well thought-out right down to the smallest detail (II)

Aluminium bodyshell in the new Mercedes-Benz SL: Under the bottom line: 140 kilos lighter

Cars are becoming increasingly safer, more comfortable, but often heavier as well. The new Mercedes-Benz SL not only stops this trend, it turns it around. In spite of increased comfort, performance and safety, it is 140 kilos lighter than its predecessor.

The greater part of the weight-reduction "diet" is hidden from the eyes of the observer of the SL. Under the aluminium outer skin there is a bodyshell made almost entirely from aluminium, only very few components being made from other materials. The even lighter magnesium is used in part for the rear panel. The A-pillars and the roof frame are of steel sheet metal incorporating high-strength steel tubing. For these elements steel is the best solution to provide survival space for occupants in the event of the vehicle overturning. Everything else is made of aluminium.

The bodyshell of the SL is the first all-aluminium bodyshell to be produced in large series at Mercedes-Benz. This entirely new development weighs 254 kilos and is thus 110 kilos lighter than a comparable steel bodyshell. Further extensive lightweight design features compensate the additional weight unavoidably caused by the increased comfort, the new assistance systems and other technical features. Under the bottom line an enormous weight advantage remains for the new SL. The new SL 500 weighs 125 kilograms less than its predecessor, while the SL 350 weighs the above-mentioned 140 kilos less than its previous version. And its preceding model already had lightweight aluminium bonnet and doors.

"The effect is rather as if a heavyweight-class passenger had got out of the car and taken his heavy flight luggage, too" says Dr Thomas Rudlaff, responsible for the aluminium bodyshell at Mercedes-Benz. "The result is perceptible and measurable. Less weight means more dynamism and less consumption. In other words: the motoring enjoyment increases and the environmental burden sinks."

For the roadster the aluminium bodyshell is superior to a steel construction
The developers at Mercedes-Benz did not rest content achieving weight benefits alone. The aluminium structure had to be superior to a steel construction in terms of rigidity and comfort as well. In order to attain this high objective, developers consistently went for intelligent lightweight construction, and explored many new paths to do this. Every single component of the aluminium bodyshell was specifically optimised for its particular function and expected loads. Thus, diverse processes are used to make different kinds of aluminium depending on the use the component is to be given: the parts are made by chill casting or vacuum die casting, worked into extruded aluminium sections or into aluminium plates of thicknesses that vary within one and the same component; these are the so-called tailored welded blanks. Expressed in numbers, the bodyshell weight is made up of: 44 percent cast aluminium, 17 percent aluminium sections, 28 percent aluminium sheet metal, 8 percent steel and 3 percent of other materials.

At the Bremen production facilities the parts are assembled using diverse load-adequate joining methods, some of which are innovative processes. Secure joints are ensured for example, by MIG welding, hemming, bonding, self-piercing rivets, flow hole bolting, or friction stir welding – a joining method by which a highly resistant weld seam is produced by means of friction heat; a method particularly well-suited for aluminium on account of its low melting point.

Particular highlights of the bodyshell:

- The Front Wall is at present the largest aluminium cast component made in large series for vehicle bodywork
- Many sheet metal parts are designed in such a way that for the first time they can be made from 100 percent recycled aluminium, saving 80 percent of the energy used in their production.
- The main floorpan is a 3-layer shaped panel made from thin, extrusion-moulded hollow sections, welded together by friction stir welding.
- The longitudinal members in the vehicle front end are made using high-pressure hydroforming (IHU) technology, which enables the creation of highly complex and robust components, permitting optimum use of reduced installation spaces.
- The door sills (longitudinal members) consist of 1.7-metre long, 7-chamber extrusion-moulded aluminium sections; these provide rigidity in the lateral sectors and safety in the event of a collision. Flexible chamber distribution makes possible a minimum component weight coupled with optimum characteristics.
- The tunnel is made of aluminium sheet metal with a reinforcement of varying thickness (3 different thicknesses depending on sector, a so-called tailored welded blank TWB).
- The rear sector floor is a MIG welded frame with a hollow chilled cast longitudinal member as its central element. This technique is employed in the SL for the very first time in automotive bodyshell construction.
- The rear sector floor frame structure is closed by floor sheet metal panels and the boot tub made by vacuum die-casting.
- The boot recess is made from recycled sheet metal.
- The central member connects the front end with the rear sector floor. The mounting points for the drive shaft, the transmission cross beam, the transmission tunnel braces and the seat bolting points on the tunnel side are all integrated into a single element. The wall thicknesses and rib distribution are oriented bionically towards the requirements and loads.
- Many other components were optimised bionically, i.e. based on examples from nature. These structures reduce the vehicle weight compared to a classic design even further.

The sum total of all the design measures leads to a lightweight, torsionally and flexurally rigid bodyshell with an optimum rigidity/weight ratio. It was possible to increase the bodywork's torsional rigidity by more than 20 percent over the already highly rigid preceding series. This is confirmed by measurements of the new SL's torsional strength – at 19400 Nm per degree the roadster achieves an absolute top value (its predecessor already reached the astounding figure of 16400 Nm per degree).

Meets the highest safety standards

At the same time the high-strength structural elements of the aluminium bodyshell make the new SL even safer than the preceding model in the event of a collision. Extrusion-moulded sections, connecting cast nodes and double-thickness plate floor form a passenger compartment that is just as lightweight as it is sturdy. Two aluminium sections in each door together with the side sills (very rigid thanks to their internal chamber structure) and crash-resistant seats provide the greatest possible survival space in the event of a side-on collision.

A front end deformation zone acting on several levels and in which the aluminium front axle integral carrier is integrated as an additional third longitudinal member, distributes collision forces to a large area, conducting them past the passenger compartment. And in the rear, too, there is sufficient energy-absorbing deformation potential. A structural cage made from chilled cast longitudinal members, transversal sections and a cast magnesium tank separation bulkhead also contribute to this. This is also where the fuel tank is nested in a crash-protected manner above the rear axle. This way all the statutory safety requirements are met as well as the even stricter ones from Mercedes-Benz.

Best marks for NVH comfort

Thanks to its innovative aluminium bodyshell concept, the new SL is also unique among the roadsters in terms of NVH comfort (noise, vibration, harshness). One contributing factor towards its optimum vibration- and top-level roll characteristics is, among other things, the extremely rigid connection between front end and vehicle rear section. This makes for relaxed travel, even on long journeys, while delivering thrilling driving dynamism at the same time.

Although aluminium features a higher sound emission and radiation level than steel, the new SL is the quietest vehicle in its class. Mercedes-Benz compensates for the acoustic disadvantages of aluminium by means of a consistent sound insulation concept with targeted adaptation of the sound damping materials to each problem zone, and through the use of innovative sound insulation materials:

- A special firewall damping with variable mass distribution and a significant heavy layer proportion attenuates engine noise.

- In conjunction with the plate floor, single-piece, foam-backed carpeting with virtually no seams reduces the roll noise of the vehicle in the interior.

- A spray-on lining attenuates the structure-borne sound in the vehicle interior and exterior.

- Textile wheelhouse linings, acoustically absorbing shield plates and bodyshell foam elements for critical sectors also contribute towards a smooth ride.

- Damping in the rear wall transversal cross member and boot lateral elements as well as in the forced ventilation openings prevent disruptive tyre and wind noises from making themselves unpleasantly noticeable in the vehicle interior.

A further contribution to the good interior acoustics is the laminated glass windscreen with acoustic film. The transparent, highly resilient film absorbs the vibrations of the windscreen and reduces the perceptible sound level in many frequency ranges.

The doors and the bonnet, too, are lightweight

The intelligent material mix is completed by the boot lid, of SMC-hybrid design (SMC = sheet moulding compound). It consists of a single-shell synthetic material panel mounted on a steel reinforcement. Both materials have virtually identical thermal expansion coefficients and complement each other very well. The interior steel construction permits maximum rigidity with minimum use of space, while the plastic panelling allows the full integration of the aerials for navigation, digital radio and mobile telephony out of sight in the rear area so that the SL, unlike many other vehicles, does not need to carry an aerial stump on its aerodynamically refined body.

The bonnet of the SL is made from aluminium, as that of its predecessor. It was optimised in terms of form and materials, contributing towards outstanding pedestrian protection.

The doors, too, are of a lightweight design and made from aluminium. They are fashioned from a combination of sheet metal, extruded sections and cast metal parts, joined by diverse methods: riveting, bonding and hemming. Their aluminium and steel hinges are friction-based and can be opened infinitely adjustable to any desired angle, so that when getting into or out of the car they can remain securely open at any angle permitted by the space available at the side. This is particularly desirable in cramped parking conditions such as in a parking garage or on a parking lot.

Sophisticated corrosion protection concept

"The corrosion prevention measures used on the new SL are the best you can find on the market" says Dr Paul Dick, responsible for corrosion prevention at Mercedes.
The aluminium bodyshell of the new SL offers no point of attack for corrosion. A sophisticated surface protection concept ensures the preservation of the brilliant look both of the outside skin and of the underside of the sheet metal panels, thus protecting at the same time the renowned reliability and value stability of Mercedes-Benz vehicles. The protection concept was developed and tested on the basis of the environmental burden in different climate zones of the world and the specific loads the vehicle is subjected to. The foundation for maximum corrosion resistance is formed by corrosion-resistant aluminium alloys and design features, while the few steel vehicle components are all fully galvanised. High-quality zinc-nickel coatings or special electrochemical insulation measures prevent contact corrosion with aluminium. All the seams are meticulously sealed, the surfaces protected through cathodic dip priming and multiple coats of paint. Sectors particularly exposed to corrosion are additionally protected with wax, to ensure that the pristine aspect of the new car is preserved for a long time.

Source: Daimler AG

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