Lightweight construction, stability and driving dynamics accompanied by typical Mercedes safety – these four factors were major focal areas in the specifications for the Mercedes-Benz SLS AMG. As the completely newly developed super sports car was based on no other Mercedes model series, its developers had more freedom than usual in the design and construction of the body.
The aluminium spaceframe weighing just 241 kilograms is a major component in the overall concept. For the first time not only the outer skin, but also the complete bodyshell structure is entirely of aluminium, leading to a low DIN kerb weight of 1620 kilograms. A new milestone for Mercedes-Benz and AMG.
In the new Gullwing model, the use of aluminium is more consistent than ever before. Maximum linear and lateral dynamics with first-class steering precision are only achievable with a very light but at the same time extremely rigid structure – which is where aluminium is the ideal material. It is not only high static and dynamic flexural and torsional rigidity that plays an important part here, but also the transfer and dissipation of extreme linear and transverse forces from the powertrain and suspension.
Combination of cast aluminium components and sections
The aluminium spaceframe of the SLS consists of cast aluminium components and aluminium sections. Cast components are used at the nodal points where forces come together or where functions are highly integrated, i.e. where large forces must be transferred or where large components such as the doors or dashboard are attached. Cast components have the advantage of specific redirection of forces, and make it possible to vary wall thicknesses locally according to the loads encountered. Areas of greater rigidity can therefore be incorporated where required, for example at the chassis connections. Moreover, only the necessary wall thickness is provided at any point of the structure, which saves weight in the areas subject to lower forces.
The cast components have been made weight-efficient with the help of so-called topology optimisation: ribbed structures precisely follow the directions of the forces encountered, while wall thicknesses are reduced in areas where the loads are lower. The roof side member is one example: this highly stressed but weight-optimised component carries the structural loads between the front and rear of the roof frame, and also bears the hinges for the gullwing doors. Especially in this component, topology optimisation also helps to lower the vehicle’s centre of gravity.
Torsionally rigid structure with an intelligent material mix
Lightweight aluminium sections connect the nodal points to a sturdy structure. The large, low-set cross-sections of these aluminium sections ensure high resistance torque, thus providing the required direct transfer of drive, braking and suspension forces. The structure prevents unwanted flexibility, which means that the vehicle rigidly, directly and with practically no torsion.
45 percent of the intelligently designed, weight-optimised aluminium spaceframe is made out of aluminium sections, 31 percent out of aluminium sheet, 20 percent out of cast aluminium and 4 percent out of steel. Maximum occupant safety requires the use of ultra-high-strength, heat-formed steel in the A-pillars. The bodyshell weighs a mere 241 kilograms – an absolute benchmark in the super sports car segment in relation to the peak output of 420 kW/571 hp.
Low centre of gravity and transverse reinforcing struts for superb dynamism
The entire vehicle concept has been adjusted for the lowest possible centre of gravity. This applies both to the low connection of the powertrain and axles, as well as to the arrangement of the rigidity-conducive bodyshell structure, which has been kept as low as possible. Examples include the rigid flexural and torsional connections between the front and rear section and the safety passenger cell, which have been realised consistently using force paths that are as low as possible. This results not only in a low centre of gravity but also in a harmonious and therefore efficient force path in the vehicle structure.
Another prominent feature of the lightweight construction are the transverse reinforcing struts at the front and rear axles, which are integrated into the bodyshell structure. The sections connect the side members precisely where the highest forces act upon the bodyshell under dynamic cornering. The advantages of this sophisticated solution include unrivalled transverse rigidity and the absence of heavy secondary reinforcement or supports.
Weight-optimised gullwing doors, body of aluminium and plastics
Lightweight construction is also the uncompromising guiding principle for the gullwing doors. During the production process for the outer door skin and interior, so-called superplastic forming – in this process, the aluminium panels and the forming tools are preheated to 500 degrees – makes it possible to dispense with multi-part components. This not only saves weight, but also simplifies the production process. Comfortable opening and closing of the gullwing doors is ensured by the specially reinforced window frames.
The aluminium spaceframe caries an equally lightweight outer skin: the bonnet, wings, gullwing doors and the side walls and roof are of aluminium, while the front and rear aprons, side skirts and boot lid are of plastic. The boot lid not only accommodates the automatically extending aerofoil, but — invisible from the outside — also the aerial systems for the radio, telephone and navigation.
Exemplary aerodynamics for an exceptionally high degree of handling stability
The best possible handling stability, low drag and low wind noise – the aero-dynamic requirements for the SLSAMGwere extremely demanding during its design and development. The specialists at Mercedes-Benz and AMGinvested a great deal of time to achieve the best possible aerodynamic balance – with the help of computer simulations, tests in the wind tunnel and test drives on various high-speed tracks. The result is a combination of slight lift at the front axle and some downforce at the rear axle. This is a desirable combination for a car with a front-mid-engine, as it ensures dampened responses to steering impulses at high speeds. Thanks to this aerodynamic configuration, critical driving situations can be prevented at source – for example during a sudden avoiding manoeuvre at high speed. The driver benefits from a constant feeling of safety and stability. All in all, the aerodynamic balance of the SLSAMGplaces it in the top echelon of the super sports car segment.
The downforce at the rear axle is determined by the automatically extending aerofoil. This feature attractively integrated into the boot lid extends at 120 km/h, and acts together with the rear diffusor to ensure the right downforce at the rear axle – with a particularly positive effect at high speeds. The aerofoil retracts again when the speed falls below 80 km/h. If required by the driver, the rear aerofoil can also be manually extended by pressing the relevant button in the AMGDRIVE UNIT. The aerodynamically efficient shape of the A-pillars, which have no drainage channels, the exterior mirrors fitted to the beltlines and the rounded B-pillars ensure that the airflow strikes the aerofoil at favourable angles in all speed ranges. The rigidity of the rear aerofoil is so designed that its angle of pitch is specifically modified by the air pressure at high speeds, which leads to improved air resistance with only a slight change to the rear axle downforce.
Good Cd value of 0.36
With a Cd value of 0.36 and a cross-sectional area (A) of 2.13 sq. m., the air resistance (Cd x A) amounts to 0.77 sq. m.. These figures are achieved by a favourable airflow into the front-end cooler modules and a precisely calculated airflow through the engine compartment. Spoilers in front of the front wheels improve the airflow around the tyres and reduce lift. The front wheel arch linings feature vertically installed louvres which conduct the airflow away from the radiator area with no effect on lift.
The engine compartment cladding, the almost completely smooth underbody and the rear diffusor play an important part in the aerodynamics of the SLSAMG. The favourable design of the front apron with a centrally integrated spoiler lip, plus the diffusors fitted at the sides of the engine compartment cladding, enable front axle lift to be effectively reduced. The rear diffusor is clearly visible between the exhaust tailpipes: this directs the airflow upwards, acting together with the aerofoil to create a downforce at the rear axle – in fact no less than 20 kilograms at the top speed of 317 km/h.
Painstaking attention to detail has also paid off where the boot lid is concerned: this is where air turbulences are normally created which can brake the airflow and unnecessarily increase fuel consumption. This is prevented by a discreet spoiler lip in the centre area of the boot lid edge, which effectively cuts off the airflow.
Low wind noise thanks to sophisticated aero-acoustics
The low wind noise of the SLSAMGis likewise a result of these extensive tests; this also contributes greatly to the long-distance comfort typical of any Mercedes. Not only the small, rounded surfaces of the doors and the flush side windows, but also the design of the A-pillars, door handles and exterior mirrors have a positive effect on this aspect. Effective sealing systems and the deliberate absence of drainage channels in the A-pillars further illustrate the sophisicated aero-acoustics of the SLSAMG.
Minimisation of soiling to the exterior mirrors, side windows and rear window makes a major contribution to active safety. The special shape of the exterior mirror housings redirects dripping rainwater to almost entirely prevent soiling of the mirror lenses and side windows. The side windows also have a water-repellent coating to optimise all-round visibility in poor weather conditions.
High standard of safety and the quality typical of a MercedesThe new Gullwing model also meets the high passive safety standards that are traditional at Mercedes-Benz. Right from the outset, the specified lightweight construction and outstanding crash characteristics were designed to be in line with the car’s low centre of gravity and the best possible distribution of load paths. The latter are specifically conducted around the occupants – this applies to front, rear-end and lateral collisions, as well as to roof impacts.
The entire bodyshell design is based on what actually happens in accidents.
During a frontal collision, for example, the continuous side member extends from the front cross-member to the side skirt, and directs the impact energy into the extremely rigid structure of the door sill. As a result the passenger compartment remains undistorted during the usual frontal impact tests. One typical characteristic of the SLS is the front-mid-engine layout of the drive unit. This positioning behind the front axle provides a large deformation zone in front of the engine. This in turn allows a firewall of reduced weight, as it is required to absorb far less energy during a frontal crash than in a vehicle with a conventionally positioned engine.
The torque tube connecting the engine to the double-declutch transmission located at the rear axle also helps during a crash: in the event of either a frontal or rear-end collision, the torque tube lowers the stresses on the bodyshell by specifically transferring and dissipating the impact forces. During a lateral impact, protection is provided by a side impact reinforcement of cold-formed, ultra-high-strength steel which is integrated into the gullwing doors. This is supported by the relavent A and B-pillars, and transfers the impact forces to the body structure via special elements. The load paths in the areas of the door waistlines are optimised by multilayered reinforcing sections.
Computer simulation of overall vehicle crashes
Sophisticated computer simulations helped to optimise all the structural components. To verify the results, a great number of overall vehicle crashes were simulated by computer. The aim was to achieve an outstanding crash performance combined with low weight. The relevant structural cross-sections were dimensioned according to the expected loads and load paths. The salient factors were the geometrical layout of the load paths and selection of the most suitable aluminium alloys for each component with respect to energy absorption, rigidity and strength. Choosing the best possible joining techniques and defining the wall thicknesses for all the components, taking into account the loads encountered during normal operation and during a crash, were also of decisive importance.
During the course of its development, the SLS AMG was subjected to a high number of crash tests, plus additional component tests to verify the results. The new super sports car complies with all country-specific impact configurations necessary for an operating licence. Plus all the current ratings and consumer tests, and also the particularly demanding, in-house impact tests of which some go well beyond the legal requirements. Passing these is a precondition for the highest accolade in automobile safety: the Mercedes star.
The very latest restraint systems, with up to eight airbags
The sophisticated body structure is perfectly complemented by the very latest restraint systems. Three-point seat belts with belt tensioners and belt force limiters are provided as standard for occupants of the SLS AMG. These are complemented with up to eight airbags: two adaptive airbags for the driver and passenger, a kneebag for each, two sidebags integrated into the sports seats and two separate windowbags deploying from the waistlines of the gullwing doors.
Exclusive, hand-built production
The aluminium spaceframe and body are hand-built with strict adherence to stringent quality standards. Highly qualified specialists join the aluminium components together using the very latest processes. The most suitable joining techniqe is used for the relevant requirement – riveting, bonding, welding and bolting. It goes without saying for every new vehicle from Mercedes-Benz that the greatest precision during production guarantees the highest possible product quality.
Source: Daimler AG