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C-Class Sports Coupe

Technical Highlights of the C-Class Sports Coupe By Klaus P. Claar, Harald Colmsee, Martin Dannenberg

and Christoph Grevener In mid-March 2001 , a n e wcomer j oined the 203-series model family: the new C -C lass Sports Coupe.Thi s new mod el i s sportier and more compact in design than the 203 seri es Sa loon.The new exterior styling required a full-scal e redesign o f all visibl e body parts including the door s, b onnet , bumper trim, headlamps, r ear lights and w indows. The substructure o f the passenger cell and front end on the o ther hand was adopted unchanged from the Sa loon.

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Introduction

With a length of 4343 mm and a height of 1406 mm, the four-seater Sports Coupe is 183 mm shorter than the Saloon and 20 mm lower, while the width (1728 mm) and wheelbase (2715 mm) remain the same. The rear overhang has been reduced by 216 mm. The interior dimensions of the passen· ger compartment are virtually identical to those of the Saloon. New interior features which were specially developed for the Sports Cou pe include the front seats which feature a lower seat reference point, extended lateral supports and manual adjustment including an easy·en· try system for rear passengers, the two·seat rear bench with backrests and seat cushions which can be folded forward, a pivot· ing and removable rigid luggage cover be· tween the rear seat backrests and the boot lid, the multi-function steering wheel with three·spoke design as opposed to the fourspoke design of the Saloon. and the instrument cluster with curved frames superimposed on the glass cover which serve to separate the three arc-shaped instruments and the multi-function display.

Among technological innovations in the Sports Coupe which also serve as key styling features are the boot lid design and the optional modular glass roof (panoramic sliding sunroof).

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Boot lid

The structural concept and materials used in the boot lid represent a new departure for Mercedes·Benz vehicles, Figure 1. The basic structure of steel consists of a single· piece inner shell with various sheet metal reinforcements. The requisite strength is provided by an enclosing single-piece outer shell in the area ofthe roofframe and the Cpillar which also forms the spoiler below the rear windscreen. The joining technology dispenses with spot welding and instead uses a combination offolding. clinch· ing and bonding techniques. Underneath the license plate panel, an outer shell was not required.

2.1

Technology of the SMC license Plate Panel and Polycarbonate Strip

It would not have been possible to produce

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the license plate panel in this shape from sheet steeL To achieve Class-A surrace quality, SMC was used, amongst other things on account of its low thermal expansion prop· erties. The adapter plate and panel are formed in heated shear edge moulds. Cutting is automated. The moulding process is monitored throughout using pressure and temperature sensors which provide information about the flow, hardening and

shrinkage characteristics of the semi-fin ished product. The adapter plate and panel are bonded and then cut. The transparent strip continues theexpressive styling of the rear lights across the entire width of the vehicle. Because it would not have been possible to produce its com plex geometry using glass, injectionmoulded polycarbonate plastiC was selected instead. A Wide, large·size film gate is used. This ensures optimal mould flow and filling, with reduced stress. To enhance di· mensional accuracy, allowance for distor· tion has been incorporated in the design of the mOUld. This reqUired extensive prelimi. nary analysis (test series, simulations). The injection moulding parameters were chosen so as to reduce stress levels as far as possible in the processing of the polycarbonate granulate. The edge of the polycarbonate strip is coated with black paint. For this purpose, the transparent area is covered over with masking tape prior to automated painting. To improve scratch.proofing and chemical resistance, the polycarbonate strip under· goes all-over automated fiow-{'oating with a polysiloxane hard coat on a paced conveyor. Subsequent thermal IR treatment dries the paint to the point where the component can be transferred to the furnace line. where the actual cross-linking of the scratch-proof paint takes place. One of the major challenges during the production process is ensuring that the polycarbonate strip can cope with the extended exposure to a high thermal load. Appropriate design of the component carriers used on the furnace line ensures that the components remain within the specified dimensions.

2.2 Quality and Functionality Assurance The component must conform to rigorous quality standards. Since these are visible parts, particular attention has to be paid to the surrace quality of the offline-painted SMC panel and the transparent area of the polycarbonate strip. Functional features too. like dimensional accuracy and bonding and sealing properties, must be ensured.

Technische Highlights des-'c-Klasse SportcoupOs AU ",otfdrltide

C-Class Sports Coupe

Adherence to defined process parameters, for example during injection moulding and application of the scratch-proof coating, is critical to this. It is necessary to work to very close tolerances, since these parameters crucially affect the distortion occurring during the thermal treatment following scratch-proofing. To determine the influence of important process parameters (for example on dimensional accuracy), DOE (Design of Experiments) tests were carried out, in some cases using parameters from several different production stages. This statistical analysis pinpointed relevant process parameters and subjected these to defined variation in extensive tests. It was thereby possible to determine

the injection.moulding process window, - the parameters for thermal treatment following application of the black paint - the influence of upstream production stages on the dimensional accuracy of the components. The most important parameters in the individual production stages are recorded and documented by computer-assisted process data acquisition. Causes of irregularities in the production process which can negatively affect component quality can be pinpointed and problems can be eliminated as quickly as possible.

2.3 Assembly The basic structure of the steel boot lid is assembled and painted on·line together with the body. The SMC license plate panel with the polycarbonate strip is delivered to the final assembly line by the supplier as a painted module. The bonding of the panel to the steel basic structure take place online in an automated process (without separate pre-assembly) using a IK-PU bonder. An SMC adapter plate on the reverse side accommodates the license plate light and the handle. For ease of handling of the boot lid immediately after bonding, the adapter plate is also bolted to the steel structure. Using an assembly part for the license plate panel means that it is not necessary to produce additional bodyshell versions.

2.4 Aerodynamics The aerodynamic shape of the rear of the vehicle brings benefits in terms of safety and handling, Figure 2. The high spoiler edge underneath the rear windscreen ensures low rear axle lift. The rear spoiler has

ATZ Automobiltechn ische Zeitschrift 103 (2001) 5

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The aerodynamic shape of the rear

of tile velJicle brings beneflfS in terms of safely and handling

been harmoniously integrated into the design of the body. Also, since dirt is deflected away from the rear windscreen, a rear windscreen wiper can be dispensed with, which both enhances the appearance and keeps wind noise to a minimum. For the same reasons, all the aerial systems, as on the Saloon, were located invisibly on the inner side of the rear windscreen. The large opening angle of the boot lid means that tall people are not in danger of hitting their head when loading and unloading. To avoid damage to the boot lid in low garages, a gas-filled strut with extended damping travel is used. A steel pressure spring integrated in the gas-filled strut also makes the boot lid easier to open. As on the saloon, the boot lid lock is operated electrically. In the event of an electrical defect, the boot lid can still be opened using the vehicle key.

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Modular Roof

The Sports Coupe can optionally be supplied with an all-new panoramic sunroof. In this case, the top of the vehicle presents a continuous glass surface stretching right the way through from the front to the rear windscreen, with trim strips along the sides which form a fluent transition between the body and the glass. In addition to its elegant, flush-fitting design, and the excellent view through the roof for driver and passengers, the panoramic roof also creates a feeling of airiness and light in the interior. Added to this, when the new sunroof is opened it leaves an opening about a third larger than that of a conventional tilting/sliding sunroof, and when closed the transparent glass area is twice as large. The flush design with the rest of the body has been achieved by a

new opening mechanism with an externally located mechanism which takes over from the internally located mechanism partway through the opening operation. The external mechanism is located under the trim strip and is sealed very effectively against dirt.

3.1

Technical Design and Operation

The outer skin of the modular roof consists of three separate glass sections, Figure 3: - A pop-up wind deflector at the front A moving glass panel in the centre - Afixed glass panel at the rear. Supplied as a complete pre·assembled module, the roof is based on a steel frame work acting as a component carrier which is bonded to the roof frame of the bodyshell from above, after painting. At the touch of a switch, the moving glass section can be electrically raised (tilting position), lifted and then slid backwards (slid· ing position), Figure 4. During closing, these operations are performed in reverse order. In the fully open position, the moving glass section of the roof is positioned over the fixed rear glass section. Even in this position, the passengers' view through the rear glass section is unimpeded. The wind deflector pops up at a pre·defined angle as the roof slides backwards, Figure 5. Two black-painted aluminium strips incorporating a seal cover the joints at the left and right-hand sides between the bodywork and the glass panels, Figure 6. A new water management scheme specially designed for the panoramic roof means

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C-Class Sports Coupe

that it is possible to dispense with the water drain hoses used on a conventional tilting/sliding sunroof. Switch operation (press upwards for tilting and backwards or forwards for sliding) follows the same logic as for the conventional glass tilting/sliding sunroof of the Saloon. The moving glass section can be stopped in any intermediate position by releasing the switch. If the switch is pressed beyond a resistance point in the "open" direction and then released, the moving glass section automatically moves to the fully open position. Should the electric drive fail, mechanical emergency operation is possible using a socket wrench. As on the Saloon, the modular glass roof can also be opened ("summer opening" function) and closed (extended locking function) from the outside using the electronic ignition key.

3.2 Protection against Solar Radiation To protect against solar radiation, the glass is tinted as on the conventional glass tilting/sliding sunroof. This filters out almost all the ultraviolet light. Furthermore, two electrically powered roller blinds housed in the centre dividing strip of the modular roof can be extended to coverthe glass panels on the inside. The electric motor used to operate the blinds is located in the roof frame under· neath the control panel. along with the drive for the roof itself. Intelligent use of the linkage cable ensures that only one drive system is required for both blinds, the front and rear blinds being controlled by the two ends of the linkage cable. Since the two blinds extend to different lengths, a decoupling of the rear blind has also been provided for. Thus, when the shorter rear blind reaches the end of its travel, the front blind can continue moving. The blinds and the moving glass panel are both controlled by the same switch. The blinds can only be deployed when the roof is closed. They can be stopped in any intermediate position by releasing the switch. If the switch is pressed beyond a resistance point in "open" direction, the blinds are automatically fully retracted.

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3.3 Safety

short rear section is formed by the side members without adapter pieces.

Polyurethane plastic is injection-moulded around the single·layer safety glass. As on the conventional glass tilting/sliding sunroof, if the glass breaks no sharp edges are formed which could injure the passengers. Compliance with the stringent safety standards was confirmed by successful testing (crash and roll-overtests).

The component-sharing concept is so designed that all body versions fulfil the same high standards in all areas such as crashworthiness and fatigue resistance. A standardised rear module flexible bumper bracket from the Saloon and the Estate is used.

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Thanks to the component-sharing concept, the numberoftxxlyconfigurations has been limited to four versions: left-hand drive and right-hand drive and with or without panoramic roof.

Bodyshell

With the introduction of the Sports Coupe, a further body version has now joined the CClass model family. During the development of the C-Class, an important focus was not only to make further advances in the fields of safety, strength and lightweight design but also to make provision in the bodyshell concept for a number of different versions. The bodyshell of the Sports Coupe, like that of the Saloon, consists of various standard modules which are combined with modelspecific components. With a standardised front section and floor, the modular concept helps to reduce bodyshell diversity. It also ensures uniformly high standards of accident safety, strength and corrosion resistance for all body versions.

4.1

Component-Sharing within the C-Class Bodyshell Family

The Sports Coupe bodyshell is based on the C·Class component-sharing concept. Shared components include the main floor unit, rear floor unit, the firewall with windscreen cross· member, the front side members, the wheel arches and thedie-cast aluminium integral subframe. All parts like weld studs, brackets etc. required for the mounting of optional equipment items are incorporated in the component-sharing concept. The weld stud layout is the same for all bodyshell versions. The bracket functions have been located in the components. The Sports Coupe body version has a short overall length. At the front, this is achieved by a model·specific front-end module in which the length of the crash box differs from that of the other models. At the next bodyshell assembly stage, additional modelspecific components are accommodated in the rear. Depending on the version, adapter pieces are welded to the rear of the side members which have the same profile as the side members and thus form the transition between the substructure and the rear end centre assembly. On the Sports Coupe, the

4.2 Production Concept In order to meet the requirements for higher process reliability and minimised production tolerances in bodyshell production, a bodyshell assembly concept was developed based on the principle of "stresHelieved joining". The connecting flanges of the components are designed so that any tolerances are compensated for as soon as the stampings are inserted in the bodyshell assembly systems. As a result, bodyshell components can be welded with reduced stress. Areas where this joining technique is used include the cross-members under the rear seats, the firewall and, especially, the side walls. The firewall region comprises six different se<:tions. This reduces the complexity of the stamping and gives greater design flexibili ty than a single-piece solution. Although the normal preference is for large stampings, this has been compensated for in the case of the centre firewall and lower firewall by dual part production. The possibility for tolerance compensation between the different components results in a high standard of production quality. The use of different sheet metal thicknesses and different strengths of steel allows locally differentiated matching of the bodyshell structure to the forces occurring in the event ofa crash. In bodyshell production, precision design of components, innovative tolerance compensation and state-of-the·art spot welding all playa part in corrosion protection. Soldered connections and metal electrode active gas (MAG) welding at the connecting points between sheet metal parts can largely be dispensed with. Such connections have always been viewed as particularly susceptible to corrosion. The total length of MAG welding seams has been reduced to just 60 millimetres. •

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