by Adrian-Liviu Dorofte

The New Mercedes-Benz SLS AMG Roadster: Perfect open-top performance - IV

Bodyshell and safety: Lightweight construction and stability for outstanding driving dynamics and best-possible safety

Driving dynamics that are the hallmark of the brand, uncompromising lightweight construction, maximum stability and typical Mercedes safety – all these apply to the SLS AMG Roadster. These four factors take their origin from the aluminium spaceframe, which at 243 kilograms is just two kilograms heavier than that of the gullwing variant. As the Roadster variant was already taken into account during the conceptual phase for the SLS AMG, the lightweight yet extremely rigid aluminium spaceframe was defined correspondingly early. Dispensing with the fixed roof and gullwing doors meant that the side skirts needed to be of more robust construction. Research and driving trials showed that the driving dynamics can be improved even further by this means, therefore side skirts with larger wall thicknesses and more chambers were developed for the open-top SLS AMG.

A few specific modifications to the aluminium spaceframe

In order to achieve handling dynamics identical to those of the Coupé, despite the lack of a fixed roof, the Roadster has two features designed to increase the rigidity of the bodyshell: the cross-member carrying the dashboard has additional supporting struts at the windscreen frame and at the centre tunnel, and a strut mounting stay between the soft top and the tank rigidifies the rear axle. These features prevent unwanted vibrations from the start, and make the use of additional, weight-increasing vibration dampers such as those often employed by competitors unnecessary. As another important aspect, it is only if the bodyshell has the requisite rigidity that the soft top can be safely and reliably opened and closed electrohydraulically while on the move at up to 50 km/h.

Owing to the omission of the coupé roof and gull-wing doors, the open-top SLS AMG has a reinforcing cross-member behind the seats which supports the fixed rollover protection system. Not forgetting the 250-watt subwoofer of the Bang & Olufsen BeoSound AMG high-end sound system: this is accommodated in the cross-member, as the Coupé-specific location on the parcel shelf is not available owing to the Roadster's soft top. One particular challenge was acoustic insulation of the cross-member, which acts as a subwoofer housing for the two 165-millimetre speakers connected in series. Numerous improvements became necessary before the required listening pleasure was achieved. It was only with the help of a special bulkhead within the aluminium cross-member that all audiophile standards could be satisfied.

Lightweight yet rigid aluminium spaceframe

The aluminium spaceframe provides the ideal conditions for a very lightweight but extremely rigid structure. It is not only high static and dynamic flexural and torsional rigidity that plays an important part, but also the absorption and transfer of extreme linear and transverse forces from the powertrain and suspension. The aluminium spaceframe of the SLS AMG Roadster 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.

With the help of so-called topology optimisation the cast components of the aluminium spaceframe are specifically weight-optimised: ribbed structures precisely follow the force paths, while wall thicknesses are reduced to a minimum in less highly stressed areas. 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 responds rigidly, directly and almost without torsion.
50 percent of the intelligently designed, weight-optimised aluminium spaceframe is of aluminium sections, 26 percent of sheet aluminium, 18 percent of cast aluminium and 6 percent of steel. Maximum occupant safety is ensured by 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 designed to achieve 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 avoidance of heavy secondary reinforcements or supports.

Body of aluminium and plastics

The aluminium spaceframe carries an equally lightweight outer skin: the bonnet, wings, doors and side walls are of aluminium, while the front and rear aprons, side sill panels and boot lid are of plastic. The boot lid not only accommodates the automatically extending aerofoil, but also the third brake light and - invisible from the outside - also the aerial systems for the radio, telephone and navigation.

Lightweight soft top opens and closes in just eleven seconds

The three-layered fabric soft top of the SLS AMG Roadster, which is deposited behind the seats in a Z-formation to save space, is also an aspect highly relevant to handling dynamics. This weight-optimised, combined magnesium / steel / aluminium construction ensures a low centre of gravity and is designed for speeds up to the maximum of 317 km/h (electronically limited). Whether open or closed, even at top speed, the occupants hear no intrusive flapping, booming, hissing, clattering, whistling or howling. Likewise the push-on draught-stop and the panelling in the interior, on the soft top and along the beltline must be vibration-free. In short, nothing must be allowed to compromise the open-air enjoyment. The basis for verified, customer-compatible results in extreme conditions is provided by precisely defined test drives on the high-speed tracks in Papenburg, Nardo (Italy) and Idiada (Spain).

The excellent acoustics – which AMG engineers have naturally also verified with sophisticated measuring technology – also benefit from another special feature, namely the seamless, bonded-in rear window of single-layer safety glass. A special production process not only ensures a smooth transition between the outer skin of the soft top and the glass, as the sum of these design measures also leads to low wind noise when the roof is closed – at any speed.

125 years of experience with innovations in open-top vehicles

Ensuring that the soft top is wind and water-proof is a complex undertaking, and AMG and Mercedes-Benz have used the enormous experience gained during 125 years of innovation. There have always been open-top vehicles in the history of Mercedes-Benz – and unlike in the case of many competitors, in an uninterrupted sequence.

Every soft top is different, however, and even objectives already defined become more ambitious over time. The five challenges to which the AMG developers of the SLS AMG Roadster gave great attention and commitment were water, sand, dust, heat and cold. One special feature of the compact soft top is the continuous water pocket: this is attached below the soft top to catch rainwater and direct it down to the underbody via two apertures on each side.

Endurance test with 16 criteria: the "Sindelfingen rain test"

The rain test at the Mercedes Technology Center (MTC) in Sindelfingen is particularly demanding, and every new vehicle bearing the Mercedes star is required to pass it – whether it has a fixed roof, a soft top or a vario-roof. Extreme quantities of water are used to ensure that the result of the development work is 100% watertight – which is a particular challenge in the case of roadsters or cabriolets. 16 tests must be successfully absolved before approval is granted.
Whether during the hose test, when all soft top, door and flap seals are sprayed with a water-hose, the continuous, overnight rain test, the fording test, the icing, swirl and high-pressure tests, or the final session in an automatic car wash – the rain test simulates every conceivable situation that can occur on any continent.

Bench testing and practical trials in all climatic zones

In addition to various test facilities such as the water chamber and the climate/ wind tunnel, the AMG specialists have recourse to test drives in all climatic regions of the world, where problems can be identified and solutions sought. In Laredo, Texas, for example, there is a particularly fine dust which finds its way into practically any gap – and tests the seals to the absolute utmost.

The compact fabric soft top must also submit itself to various tortures. One of these is the standardised soft top endurance test used for all new Mercedes roadsters or Mercedes cabriolets: 20,000 closing cycles on a stationary test rig must present no problem for the hydraulic cylinders, electric motors and joints. There are also 2500 closing cycles while on the move, whether in great heat, icy cold, high humidity or dry desert winds. In this area too, nothing is left to chance – and for excellent reasons: the aim of this enormous effort is to give the customer limitless driving pleasure in their SLS AMG Roadster.

Final quality check on the complete vehicle

All the optimisation stages have been absolved, and the production tests at the Mercedes-Benz plant in Sindelfingen have been successful, but the developers have still not reached their goal. The quality of the overall vehicle is now the focus of the accompanying endurance trials. These simulate an entire vehicle life under the toughest conditions in accelerated test cycles. The aim is to verify the level of maturity before production of customer vehicles commences.

The endurance testing at a glance:

Long-term testing on a variety of different roads:
- all the components and systems are tested together in everyday operation. Loaded up to their permitted gross vehicle weight, the test cars are put through a precisely defined test programme on country roads, on motorways and in city traffic.

Endurance testing on heathland:
- in this case, the developers focus on the durability of the chassis and suspension components, the entire bodyshell and the integral subframe on which the front axle, steering and engine are mounted. The test cars are loaded up to their permitted gross vehicle weight.

"Accelerated" endurance testing:
- testing of the entire vehicle, focussing on the powertrain, chassis and suspension. Special features of the AMG programme include 10,000 kilometres on the Nürburgring's North Loop and 10,000 kilometres in city traffic.

Full-load endurance testing:
- extreme acceleration and braking manoeuvres with a high proportion of full-load operation, making extreme demands on the cooling, fuel-delivery and braking systems.

Long-term corrosion testing:
- corrosion testing of the entire vehicle simulates the toughest dynamic and climatic environmental influences.

Final board approval:
- all-inclusive verification of the degree of development and production maturity.

Exemplary aerodynamics for optimum handling stability

The best possible handling stability, low drag and low wind noise – the aerodynamic requirements for the SLS AMG Roadster were extremely demanding during its design and development. The specialists at Mercedes-Benz and AMG invested 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 downforce at the rear axle. This is a desirable combination for a super-sports 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 SLS AMG places 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 ensures the right aerodynamic balance in all speed ranges. 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 AMG DRIVE UNIT. The aerodynamically efficient shape of the A-pillars, which have no drainage channels, and the exterior mirrors fitted to the beltlines 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 2.11 sq. m., the air resistance (Cd x A) amounts to 0.76 sq. m. (Coupé in comparison: Cd x A = 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 diffuser also play an important part in the aerodynamics of the SLS AMG. 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 diffuser is clearly visible between the exhaust tailpipes: this directs the airflow upwards, acting together with the aerofoil to prevent lift at the rear axle.

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 SLS AMG is 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 sophisticated aero-acoustics of the SLS AMG Roadster.

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 Mercedes

The new SLS AMG Roadster 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 dual clutch 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 doors. This is supported by the relevant 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 beltlines are optimised by multi-layered reinforcing sections.

Computer simulation of crashes with the complete vehicle

Sophisticated computer simulations helped to optimise all the structural components. To verify the results, numerous crashes with the complete vehicle 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 numerous 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.

Eight airbags as standard, Blind Spot Assist on request

The sophisticated body structure with its fixed roll-over protection system is perfectly complemented by the very latest restraint systems. Three-point seat belts with reversible belt tensioners and belt force limiters, plus eight airbags, are provided as standard passive safety features for occupants of the SLS AMG Roadster. The airbag system includes 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 door beltlines.

The LEDs of the adaptive brake light flash 5.5 times per second during emergency braking. This significantly reduces the reaction time of traffic following behind. The new, optional Blind Spot Assist further improves active safety. This visual and acoustic warning system uses the short-range sensors of the standard PARKTRONIC system to detect vehicles in the driver's blind spot. If the system registers another vehicle in the blind spot, the driver is warned by a red triangle appearing in the relevant exterior mirror. An acoustic warning is given in addition if he nonetheless activates the indicators.

Blind Spot Assist can warn the driver at speeds of 30 km/h or more. The monitored area extends around 3.5 metres to each side of the vehicle, and three metres behind the vehicle rear. Registered vehicles which are overtaking are indicated immediately after entering the monitored zone if the relative speed difference between the vehicles is no more than 16 km/h. Vehicles which have just been overtaken are indicated with a delay of 1.5 seconds after entering the zone, which avoids unnecessary, distracting warnings.

Exclusive hand-production in three locations

The aluminium spaceframe and body are exclusively hand-built with strict adherence to stringent quality standards by Magna Steyr Fahrzeugtechnik GmbH in Graz/Austria. Highly qualified specialists join the aluminium components together using the very latest processes. The most suitable joining technique is used for the relevant requirement – riveting, bonding, welding and bolting.

Hand-assembly is also very much the order of the day at the AMG engine shop in Affalterbach, where the 420 kW (571 hp) AMG 6.3-litre V8 engine is produced according to the "One man, one engine" philosophy. This is visually confirmed by the AMG engine plate, which bears the signature of the technician responsible for its assembly. Final assembly of the SLS AMG Roadster is carried out by chosen personnel at the Mercedes-Benz plant in Sindelfingen. In summer 2008 the Sindelfingen plant was presented with the J.D. Power Award by the prestigious US market research institute J.D. Power and Associates. This award is given for the automotive production facility with the best delivered quality worldwide.

* Official photos and details courtesy of Mercedes-AMG GmbH *

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