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 kilograms 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 cover behind the tank. 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.
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 kilograms and is thus 110 kilograms 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. As the SL 500 it weighs 125 kilograms less than its predecessor, while the SL 350 weighs 140 kilograms less than its previous version. And its preceding model already had a lightweight aluminium bonnet and doors.
“The effect is rather as if a heavyweight-class passenger had got out of the car and taken their 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 decreases.”
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 an 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 what are known as the 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 to aluminium on account of its low melting point.
Particular highlights of the bodyshell:
– The firewall 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 consists of 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 the 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 the natural environment. 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 model 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 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 a 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 collision.
A front end deformation zone acting on several levels and in which the aluminium front axle subframe 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 road roar characteristics is a very rigid connection between the front section and rear floor, allowing relaxing travel even over long distances, and at the same time offering impressive driving dynamics.
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 wheel arch 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 precision workmanship on the acoustics goes so far as to ensure that the hydraulic pump for the electrohydraulic vario-roof is encapsulated and decoupled from the bodyshell. In addition to this, a newly developed insert nozzle sees to it that the pressure pulsation in the pump is reduced to a minimum right from the outset. This means that pump noises are only barely audible for the driver and passenger.
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 does not need to carry a bothersome aerial stump on its aerodynamically refined body.
The bonnet of the SL is made from aluminium, as was that of its predecessor. It was optimised in terms of form and materials, contributing towards the 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 are infinitely adjustable to any desired angle when opened, 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 garage or car park.
Unique item of equipment: MAGIC SKY CONTROL
The proven electrohydraulic vario-roof – which turns the roadster into a closed coupé with a fixed roof in just a few seconds at the push of a button – has been intelligently enhanced, with Mercedes-Benz continuing to play the pioneering role here. For the new SL Mercedes-Benz is for the very first time offering drivers the choice between three variants of the lightweight-construction vario-roof:
– The base version is a roof painted in the vehicle colour
– As an alternative, a transparent variant with a roller blind can be selected
– The third variant from which drivers can choose is the panoramic vario-roof with MAGIC SKY CONTROL. This glass roof switches to light or dark as required at the press of a button. When light it is virtually transparent, offering an open-air experience even in cold weather. In its dark state the roof provides welcome shade and prevents the interior from heating up when the sun’s rays are very intense. In other words: a feel-good atmosphere at the touch of a button
The operating principle is based on the physics of a plate condenser: if the glass structure is subjected to an electrical voltage, particles in the structure position themselves so that light is able to pass through the glass. If the voltage remains switched off, however, the particles position themselves randomly. This partially blocks the light, and the glass remains dark. The control unit and converter are integrated into the front section of the roof, and the switch is in the overhead control console.
In addition to the unique panoramic effect and transparency at the touch of a button, MAGIC SKY CONTROL also provides an atmosphere of wellbeing. UV and infrared light are also effectively blocked in transparent mode, while insulation increases even more significantly in the darkened mode, thereby helping to keep the temperature on interior parts such as armrests cooler by up to ten degrees Celsius compared with conventional green glass.
Since the innovative glass also shields against the sun’s rays far more effectively than conventional thermal insulation glass or sunblinds, the occupants of the new SL will also enjoy the highest level of climatic comfort at the same time. When driving with the roof open would be unpleasant due to intense heat, the driver and passenger are able to remain composed under the MAGIC SKY CONTROL roof – making an important contribution towards driver-fitness safety, a concept which Mercedes is well known for promoting. Even when the vehicle is at a standstill the interior does not overheat, since the roof is switched to darkened mode when powered off.
But MAGIC SKY CONTROL not only benefits the occupants. Some of the strain is also taken off the air conditioning, thereby saving on CO2 emissions.
The frame of the three roof variants is now made of magnesium. This makes each roof around six kilograms lighter than in the previous model, giving the car a lower centre of gravity, and therefore better agility.
The engineers have refined the operating mechanism for the roof and boot lid. It now takes less than 20 seconds to open or close the roof completely.
Sophisticated corrosion protection concept
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 simultaneously protecting 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.
Intelligent Light System and LED tail lights as standard
The headlamps on the new SL have been enhanced. As part of its standard specification, the new roadster is equipped with the Intelligent Light System (ILS) featuring bi-xenon headlamps. A driving direction indicator, sidelights and the daytime running lamps with LED technology complete the Intelligent Light System.
The tail lights on the new SL benefit from the advantages offered by the LED technology. LEDs light up approximately 150 milliseconds faster than conventional bulbs, plus they have a considerably longer life and conserve current. In the SL’s tail lights, the LEDs also light up with differing intensity in a targeted manner in certain sections of the tail lights, depending on the light function. This technology ensures an unmistakable night design.
Rear reversing lights and rear fog lamps are integrated into the rear bumper. The third brake light featuring LED technology is integrated in the boot lid above the Mercedes star.
Source: Daimler AG