The bodywork and vario-roof - Expertise and skill

Mar 15, 2004
Stuttgart
  • Intelligent use of materials for maximum strength and safety
  • Fully galvanised body and scratch-resistant paint
  • Substantially increased boot capacity when vario-roof open
  • Flawless aerodynamics enhance active safety and comfort
  • Bi-xenon headlamps in combination with Active Light System
Designing an open-top sports car with the structural stability and safety standards of a saloon calls for tremendous expertise and a great deal of experience. Mercedes-Benz has both of these attributes and the new SLK-Class is further testimony to this. The bodyshell delivers superlative results in terms of strength and rigidity, the essential key to delivering dynamic driving characteristics combined with optimum vibration damping and Mercedes standards of occupant safety. The body also provides a robust base for the vario-roof with its sophisticated deployment mechanism, thereby allowing it to operate to very high standards of precision.
The figures speak for themselves:
  • A measurement of the static bending strength on the body of the new SLK-Class reveals a 19 percent improvement with the roof down, when compared with its predecessor.
  • Static torsional strength, an important indicator for the vibration characteristics of the body, outperforms the exemplary figures delivered by the previous SLK-Class by a remarkable 46 percent with the roof down.
These impressive results are due primarily to the adoption of two measures:
An intelligent choice of materials and an elaborate body design process based on state-of-the-art calculation processes and a great deal of know-how.
Materials: 42 percent of all panels are made of high-tensile steel
When building the new SLK-Class, Mercedes engineers increased the proportion of high-tensile steel alloys to 42 percent of the total: these steels combine low weight with maximum strength, meaning that the great majority of components governing crash safety, strength and durability are now constructed from high-tensile sheet steel.
The specialists in the Sindelfingen plant also judiciously applied their tried and tested motto of using "the right material in the right place" to the use of other materials, wherever these delivered the greatest benefits and made the most significant contribution to the lightweight design concept. One such example is the curved rear panel, made of aluminium to deliver high strength combined with a substantial weight saving over a comparable component made of sheet steel. This aluminium rear panel is bolted to the body. Mercedes-Benz selected pressure die-cast magnesium for the partition separating the fuel tank and the boot area, weighing about 50 percent less than steel would have done.
Design: elaborate measures achieve impressive structural stability and safety
Mercedes engineers have also incorporated some clever ideas in the development of this new SLK-Class, some to achieve superlative bending and torsional strength figures and some to achieve further improvements in occupant safety. These have enabled the specialists to build on the already high standard of the previous model by incorporating improvements to virtually every aspect of the bodyshell:
  • Front-end structure
  • The front cross member features an additional reinforcement plate to create a twin-shell design.
  • The two straight longitudinal members are linked by means of cross members which reinforce the front firewall area, assisted by two pedal base cross members.
  • The curved firewall comprises several components. To counteract different stress levels, these sections feature different thicknesses of material.
  • The new design of the frame-shaped integral carrier for the steering unit, engine mountings and some of the wheel location components is intended to absorb energy in front-end collisions and is bolted to the front longitudinal members.
  • The second and upper longitudinal member plane is connected to a strut secured to the A-pillars. This absorbs collision impact energy in the event of an offset front impact accident.
  • Passenger cell
  • The A-pillars are reinforced using oval tubes made of high-tensile steel. These are securely anchored to the body structure. The material is shaped in the steel works while still in a semi-molten state, a process which makes the resultant component extremely strong. The oval shape of the A-pillars gives the SLK driver better all-round vision than in the previous model, an improvement quantified as twelve percent.
  • The side longitudinal members comprise an inner and an outer shell section reinforced by transverse bulkhead panels. The cross section of these longitudinal members flares towards the back of the vehicle where additional, surface-mounted profile sections are fitted, enabling these members to contribute substantially towards the superlative rigidity of the vehicle body.
  • The transmission tunnel, two straight connecting members on the underside which extend the front longitudinal members and two robust seat cross members constitute the reinforcing components of the floor pan assembly.
  • The B-pillars (not shown) have substantial supporting faces against the lateral longitudinal members and the cross member under the aluminium rear panel.
  • Rear-end structure
  • The rear longitudinal members on the SLK body are characterised by a multi-section design featuring different thicknesses of high-tensile steel. This approach has enabled engineers at Mercedes to define the strength and deformation properties of the longitudinal members very precisely, adapting each area to contend effectively with local stress loadings.
  • The spare wheel recess is made of steel and forms part of the rear floor pan assembly.
  • A robust cross member forms the rearmost rear component of the SLK bodyshell structure.
Four diagonal struts on the underside make a further important contribution towards the exemplary vibration damping characteristics of this roadster body. At the front end, they combine the integral carrier, which supports the engine mountings, the steering unit and some of the wheel location components, with the leading edge of the lateral longitudinal members. At the back end, these struts are connected at both sides to the longitudinal members and to an additional reinforcing member in the spare wheel recess.
The bonnet with its imposing front bulge around the Mercedes star, the characteristic identifying feature of the new SLK-Class, is also the result of innovative body and production techniques. Mercedes engineers developed a new process for manufacturing this good-looking one-piece bonnet section. A special technical feature of the boot lid is the two-section outer skin: the license plate recess – and this is a genuine first for Mercedes-Benz – is attached to the other panels on the boot lid using a laser brazing process. This laser process is so precise that the resultant brazed seam achieves a very high standard of quality.
Corrosion protection: fully galvanised body
To equip the SLK with long-term corrosion protection, the entire bodyshell is galvanised. In addition, some of the metal panels are treated to an additional organic coating on both sides which also contains rust-inhibiting zinc pigments. All the most vulnerable structural sections of the bodyshell are also protected with a cavity-fill preserving agent. A large area of the underbody is made from plastic so Mercedes engineers were able to dispense with conventional PVC underseal, a potential environmental hazard. The underbody trim panelling protects the bodyshell from stone chipping, wet and dirt. In keeping with all Mercedes passenger cars, the new SLK-Class is also protected by a 30-year MobiloLife warranty.
Paintwork: more scratch-resistant clearcoat based on nano technology
This new type of clearcoat is derived from the nano-technology sector, providing a new and enhanced level of scratch resistance and also enabling Mercedes-Benz to take another important step along the road towards improving the already excellent durability and value retention of the SLK-Class. This innovative paintwork system has been in service at Mercedes-Benz since the end of 2003, and the company was the first automotive manufacturer in the world to adopt it. This paint protection system is standard on the new Roadster and is used with all the metallic and non-metallic paint colours for this vehicle.
Thanks to the impressive progress made in the nano-technology sector, engineers have been able to incorporate ceramic particles less than a millionth of a millimetre in size into the molecular structure of the paint bonding agent. To begin with, these tiny particles float freely around in the clearcoat while it is still in its liquid state, then group together during the drying process in the paintshop. At this point, the particles bond together to form a very dense, regular network structure on the surface of the paint. The nano particles improve scratch resistance by a factor of three and ensure a long-lasting and visibly improved gloss sheen on the paintwork.
Vario-roof: new swivel mechanism creates more space in the boot
When it first appeared in 1996, the innovative vario-roof on the SLK-Class Roadster caused a sensation and set new trends in the construction of open-top cars. Mercedes-Benz has now developed this concept and has perfected the technology. The folding hardtop in the new SLK-Class now opens and closes even more rapidly than it did on the previous model. At the same time, a space-saving concept once again enables boot space to be increased when the roof is down.
Twenty-two seconds on a stopwatch clearly demonstrate that progress has been made. At the touch of a button, this is all the time it now takes to convert the new SLK-Class from a hardtop coupé to an open-top roadster, a whole three seconds faster than the previous model. Thanks to the ingenious roof design, the two-seater is able to change back just as quickly into a weatherproof, all-year-round car capable of meeting any of the everyday demands of road and weather. This means that the new SLK-Class is able to combine the driving pleasure of an open-top roadster with all the comfort of a Mercedes coupé.
Moreover, the new SLK-Class offers even more luggage space in the boot when the vario-roof is retracted. According to the VDA method, the boot of this car offers 208 litres of stowage volume, an impressive 63 litres more than the previous model – quite enough for the luggage of both driver and passenger.
This great step forward was achieved by a new and highly elaborate roof design, the crowning glory of which is a swivel-mounted rear window – not dissimilar to the one in the SL-Class of sports car. This rotary movement ensures that the window nests firmly in the curved surface of the roof shell, restricting the capacity of the boot much less than in the previous model.
Five hydraulic cylinders are responsible for ensuring that the vario-roof on the new SLK-Class opens and closes smoothly. Two of these are in charge of powering the roof, one of them locks the roof centrally with two catches on the windscreen frame and the remaining two power the movement of the tubular frame. The multi-piston hydraulic pump with integrated electromagnetic valve is housed in the cross member behind the seats. It develops a pressure of 160 bar to ensure that sufficient power is available for all movements in any given situation. Limit switches monitor every sequence while a dashboard display panel on the multifunction instrument cluster indicates when the vario-roof reaches its fully open or its fully closed position.
Aerodynamics: fine-tuning in the wind tunnel
Good design, high standards of road safety, superb aerodynamics – this combination is by no means always a given in the compact roadster market. However, it is a distinctive hallmark of the Mercedes-Benz SLK-Class. Right from the days of the 1996 model, this car has combined captivating lines with a streamlined body shape, delivering benefits to fuel economy, ride comfort and roadholding.
Starting from what was already a very high standard, Mercedes engineers have managed to achieve even more progress with the new SLK-Class. Although the wider and higher body shape and the new tyre sizes (205/55 R 16 replacing the previous 205/60 R 15) have increased the total frontal area of the car by four percent to 1.93 square metres, the Cd value or coefficient of drag used to express the aerodynamic properties of a body shape have actually been reduced, when compared to the previous model, by three percent, achieving a new level of 0.32. Here is a summary of the measurement results:
The aerodynamic data for the new SLK-Class
   New SLK-Class* Previous model*
Coefficient of drag Cd 0,32 0,33
Frontal Area A 1,93 1,85
Wind resistance Cd x A 0,62 0,61
*with vario-roof closed
Even with the vario-roof open, the new SLK Roadster demonstrates that the aerodynamics engineers at the Sindelfingen plant have achieved their challenging objective: the Cd value is now 0.37 compared with 0.38 on the previous model.
Road safety: lift forces reduced by as much as 30 percent
The engineers in Sindelfingen used aerodynamic details to achieve two things at once: they not only reduced the coefficient of drag, they also improved roadholding at high road speeds or when braking, thereby making an important contribution towards road safety.
The underfloor panelling, wheel spoilers and spoiler lip on the boot lid help to cut down the airflow forces acting perpendicularly on the car, which quite literally cause the body to start lifting off the ground at high speed. However, the new SLK-Class keeps its feet firmly on the ground and displays superb roadholding manners at all times. This fact is verified by the data relating to lift, which demonstrate substantial improvements over the previous model – about 30 percent on the front axle and 20 percent on the rear axle.
Ride comfort: effective measures to prevent draughts when driving with the roof down
The list of tasks presented to the aerodynamics engineers also included the heading of comfort. To be more precise, it specified the personal well-being and warmth of SLK occupants when driving with the vario-roof lowered. After all, given their many years of experience in the development of convertibles and roadsters, the specialists in Sindelfingen are perfectly aware of the fact that just being able to lower the roof of a car is by no means a reliable key to motoring pleasure. A cold wind which tousles your hair or sends cold draughts down the back of your neck can soon put a damper on the pleasures of open-top driving. To address this issue, the new SLK-Class has been tested and optimised in a wind tunnel. After an extensive series of in-depth studies, a new design of neck-level heating known as the AIRSCARF (an optional extra) is now celebrating its world premiere in this Mercedes Roadster.
However, AIRSCARF can only really come into its own if the other parameters are right as well – and one of the main issues here is that of draughts. This explains why the aerodynamic experts, when they began work on this product in the autumn of 1998, built 1:4 scale models to establish the basis for achieving perfect aerodynamics in the new SLK-Class. The angle of the windscreen and the seated height of the occupants were key aspects in this study right from the very outset because these are factors which play a key role in keeping the passenger compartment free of draughts. The result is a seat position carefully matched to the height of the wind-screen which ensures that the slipstream is directed above the heads of SLK occupants, so that sit out of the wind and therefore much more comfortably.
Even so, with the vario-roof down, it is not possible to completely prevent small pockets of turbulence from developing behind the occupants which can then prove to be a source of draughts. The answer to this problem is provided by the standard SLK draught-stop which is quick and easy to pull up over the two roll-over bars and which is then secured using press-stud fasteners. The closely-spaced mesh on this unit distributes air very evenly, thereby preventing the kind of fast airflow which is experienced as draughts by car occupants. When used in conjunction with AIRSCARF, the air pacified by the draught-stop is then heated as well.
The SLK-Class also delivers comfort benefits in the form of improved aero-acoustics. With the vario-roof closed, noise levels inside the car are as low as those of a coupé. Moreover, additional aerodynamic measures also help to reduce wind noise – one example being the streamlined shapes of the exterior mirror housings.
Windscreen wipers: better performance, lower noise levels
The windscreen wipers are another development from wind tunnel testing, which helps to explain why these are officially designated as "aero wipers": instead of the jointed bracket system of conventional wiper blades, to which rubber wipers are attached, the aero wiper comprises a one-part rubber profile with integrated spoiler and two curved spring rails which precisely match the convex curve of the wind-screen and which therefore always make contact at an optimum pressure. The shape and 50 percent reduction in height of these blades deliver tangible improvements in wiper action and operating noise – especially at motorway speeds.
Lights: projection-type headlamps employing halogen or bi-xenon technology
The headlamps on the new SLK-Class are not only very appealing in visual terms, they also satisfy the highest standards in terms of their technology. Mercedes-Benz has now replaced its reflection-type low-beam headlamps with a new development known as the projection system. In this system, light is distributed by means of an optical lens (70 millimetres in diameter). As standard equipment, clear-glass H7 halogen lamps mounted behind plastic lenses provide low-beam lighting.
Stylistic considerations were not the only ones which prompted the company to move away from its previous reflector system. The projection-type headlamps certainly did not occupy as much space at the front of the vehicle as the earlier reflector-type units, and this was something which gave the designers much greater latitude in their work. Equally important however was the very apparent progress made in terms of lighting power: the projection technique offers much broader and much more uniform lighting of the road ahead and also delivers substantially greater range for the low-beam headlamps. Main-beam lighting is provided by additional reflection-type headlamps which also employ H7 halogen bulbs.
With the optional bi-xenon headlamps, the lighting output is even greater. When these headlamps are on their main-beam setting, the H7 spotlights also come into play. To create a visual distinction between bi-xenon and halogen lights, the Mercedes designers have drawn a refined "eyelid" over the eyes of the new SLK. A transparent area above the bi-xenon module is backlit to create interesting light effects.
Cornering lights: fog lamps with an additional function
Fog lamps with halogen bulbs are also included in the standard equipment of the new SLK-Class. They are housed in the bumper trim and, in conjunction with the optional bi-xenon headlamps, also offer a new cornering light function, providing lighting for the area ahead and to one side of the vehicle. The advantage of this is that pedestrians and cyclists near the vehicle can be seen clearly, even at night. This cornering light function also enables drivers to orient themselves more effectively when cornering at low speed.
Whenever the bi-xenon headlamps are switched on, and up to a maximum speed of 40 km/h, the cornering light functions cuts in automatically in response to the driver using the turn signal indicator or turning the steering wheel by a corresponding angle. At this point, the cornering light illuminates the area to one side of the vehicle through an angle of up to 65 degrees and a distance of up to 30 metres, thereby lighting up areas of the road which would normally not be visible with conventional headlamp technology.
The cornering light design comprises a housing with a reflector and one H7 bulb per side, which also serves as a fog lamp.
Tail lights: the benefits of LED technology
Mercedes-Benz has also introduced leading-edge technology to the tail lights of the new SLK-Class: a grand total of 32 light-emitting diodes (LEDs) are responsible for brake lights and tail lights. Their field of light extends right across the full width of each tail light: when the brakes are applied, this red surface lights up to a substantially brighter intensity than it does for the tail light function.
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