The new CLK-Class (W209): Body & Safety

Body and safety: extremely sound foundations

·       Bodyshell torsional stiffness increased by 40 percent

·       Percentage of high-strength steel alloys used almost doubled

·       Easy repairs thanks to modular concept for front and rear end

·       Large deformation zones on three levels for superb occupant protection

·       Optimum front airbag deployment depending on bodyweight

·       Large windowbags and sidebags as standard

·       World-record c_d figure in this vehicle class

Developing a coupé has long been one of the most exciting challenges for automotive engineers. What makes this particular species of car so appealing is its unique blend of sportiness and elegance, especially if it is a coupé displaying the Mercedes star and containing a unique combination of technical and stylistic features. In short, the new Mercedes-Benz CLK-Class

The body and safety experts at the Mercedes Technology Centre (MTC) in Sindelfingen played a major part in creating what is one of the most prominent and beautiful design features of the new two-door model: the elegant side lines and the continuous strip formed by the side windows and slender, filigree roof pillars are the result of a series of complex production processes. What is more, the Coupé still fulfils the most stringent of test requirements with respect to torsional stiffness and occupant protection, even in the absence of full-size B-pillars. In concrete terms:

The single-piece sidewall panelling on the new CLK-Class has individually welded inner shells which provide strong, stiff cross-sections for the roof pillars, the roof frame and the side longitudinal members.

The A-pillars contain high-strength steel tubing which is capable of withstanding even the extreme impacts experienced in the roof-drop test.

The (non-visible) B-pillar stumps also contain welded high-strength tubing. Together with numerous other reinforcements, this tubing provides effective protection in the event of a lateral collision. In addition, the area below the dashboard contains a solid square section which is bolted to both the A-pillars.

The doors consist of one inner and one outer shell, made of sheet steel with integrated reinforcement sections. Other integrated safety components include a flexible tubular member in the lower section, which reduces deformation in the event of a side impact. Mercedes-Benz used high-grade materials to develop separate panels for mounting the door hinges, to further enhance impact resistance in these areas.

40 percent of the bodyshell consists of high-strength steel alloys – as against 22 percent in the preceding series – designed to minimise weight and maximise stability and safety.

A three-section floor assembly forms the robust spine of the bodyshell. The floor panels were custom-made using the "tailored blanks" method, laser-welded together and then shaped according to requirements -- a sophisticated process that enables the use of tailored blanks of varying thickness and, therefore, also takes into account factors such as lightweight design and safety. The middle panel is the thickest of all and is used to shape the transmission tunnel which the Mercedes engineers further reinforce with additional panels. Crossmembers and diagonal members on both sides of the tunnel ensure renowned Mercedes rigidity; in the event of a frontal or side impact, these members absorb the impact and channel the force away from the passenger compartment so that the latter is stabilised.

Easily replaceable front and rear modules with integrated impact protection

The front and rear sections of the new CLK-Class also differ significantly from those used for the preceding series. As they did for the S-Class, SL-Class and E-Class, the Sindelfingen engineers have developed special modules which absorb crash energy at impact speeds of up to 15 km/h, thus ensuring that the supporting structures behind remain undamaged. Bolted connections make the front and rear modules cheap and easy to replace, eliminating the need for straightening, welding and repainting.

The front module consists of a series of components that are bolted together. At its core are two steel crash boxes and an extruded aluminium crossmember. In addition, there are two brackets for mounting the radiator and, on both sides, multi-section steel frames for mounting the headlamps in the front module.

The rear module also consists of two steel crash boxes and an extruded aluminium crossmember. The crash boxes are capable of absorbing crash energy at impact speeds of up to 15 km/h

For mounting the front axle, steering system and engine, the Mercedes engineers also employ a sophisticated modular concept that has already proven successful in other model series made by the Stuttgart manufacturer: in place of the welded-in front axle carrier, the new CLK is fitted with what is known as an integral support which is bolted to the front side members and absorbs some of the crash energy in the event of a frontal impact. The subframe is made of diecast aluminium and weighs around 20 percent less than an equivalent steel component.

Facts and figures: the main dimensions of the new CLK-Class

With an exterior length of 4638 mm, the new Coupé is also unique in terms of its body dimensions, positioned between the C-Class and the E-Class. The CLK-Class is 112 mm longer than the C-Class Saloon and 180 mm shorter than the new Mercedes-Benz E-Class. Compared to the predecessor model, the new CLK Coupé is 71 mm longer, 18 mm wider and 42 mm higher. The wheelbase has been extended by 25 mm and now stands at 2715 mm.

	New CLK-Class	Preceding series
Length	4638	4567
Width	1740	1722
Height	1413	1371
Wheelbase	2715	2690
Front overhang	840	860
Rear overhang	1083	1017

All measurements in millimetres

Further enhanced body rigidity and safety

The results of these and other innovative measures in the field of body technology are impressive to say the least. The new Mercedes-Benz CLK-Class

... has 40 percent higher torsional stiffness than the preceding series – an indication of the car's precise and safe handling.
• ... has a bodyshell that is practically immune (predecessor: 65 percent immune) to corrosion by virtue of its galvanised panels. Around half of the galvanised panels are treated with an organic paint that also contains zinc pigments. Extensive underfloor panelling made of fibreglass-reinforced plastic enables the Mercedes engineers to dispense with conventional, environmentally harmful underfloor protection made from PVC.
... is considerably quieter than the preceding series thanks to its highly-effective sound-deadening concept. Critical cavities in the bodyshell are packed with special, noise-insulating foam or fibre materials. There are 22 such "absorbers" in the sidewalls alone.
... passes the stringent European and American NCAP safety tests (New Car Assessment Programme) with flying colours, for example the frontal-impact test against a 40% offset, deformable obstacle, the ECE side-impact test or the pole test at 90° to driving direction, not to mention Mercedes in-house safety tests which are equally stringent.

Practical, multi-level occupant protection system

In addition, the new CLK-Class incorporates the pioneering, practical Mercedes safety concept. This concept also takes into account occupant protection in the frequent collisions that occur at mid-range speeds. For instance, in the event of a frontal impact, a three-stage protection system is activated, based on the severity of the collision:

At impact speeds of up to 4 km/h, the foam elements in the bumper absorb the crash energy, thus ensuring that the panels behind remain undamaged. The plastic bumpers even revert to their original shape after the accident.

At impact speeds of around 15 km/h, the energy is absorbed within the newly developed front module and its integrated crash boxes made of highly-resistant steel.

At impact speeds in excess of 15 km/h, the impact forces are channelled into four independent zones:
1. Via the aluminium crossmember in the front module, which channels the impact force to the side of the vehicle unaffected by the collision;
2. Via the substantially forwards-extended side members (behind the crash boxes);
3. Via robust sectional panels above the wheel housings, which provide an additional path for channelling the impact forces;
4. Via the front wheels which are supported by special crash elements in front of the side members (doorsills), enabling them to channel the impact forces into the body side structure.

Ellipsoid bulkhead for effective energy absorption during a frontal crash

The front end is connected to the passenger cell by means of an ellipsoid bulkhead comprising several panels of differing thickness. This set-up enables the material to be adapted according to the severity of the impact and represents an ideal compromise between lightweight design and crash safety. The thickness of the high-strength steel varies by up to 150 percent, meaning that the bulkhead is not only capable of withstanding extreme loads, it can also evenly disperse the forces generated by a frontal collision over a wide area. This prevents dangerous intrusions into the passenger cell. Consequently the previously installed front side member extensions are no longer required.

2-stage airbag deployment, based on impact severity and passenger weight

Adaptive front airbags – airbags that adapt according to the type of collision and deploy in two stages depending on the severity of the collision – prove the perfect complement to the practical protection concept in the new CLK-Class:

Stage 1: in the event of a minor frontal collision, the ECU only fires one chamber of the two-stage airbag gas generators. The airbags are therefore inflated to a lower pressure.

Stage 2: if the ECU detects a severe frontal collision, it also activates the second chamber of the gas generator after a delay of between 5 and 15 milliseconds. The airbags are thus inflated to a higher pressure, offering the occupants a level of protection in line with the severity of the accident.

Mercedes-Benz has further developed this adaptive airbag technology for the new CLK-Class: when calculating the required degree of airbag deployment, the airbag ECU does not just take the severity of the collision into account, it also considers the front passenger's bodyweight. This means that the protection systems are able to adapt with even more precision in the event of a frontal collision. By way of example, the microcomputer may even fire both generator chambers in the event of a minor collision, depending on the severity of the accident and the passenger's weight, so as to provide a heavy front passenger with the best possible level of protection. Conversely, if the front passenger is very light, the second chamber may only inflate in the case of more severe collisions. The ECU assigns the front passenger to one of four weight categories using data it receives from a special measuring membrane in the seat: 1 to 30 kg; 31 to 50 kg; 51 to 70 kg; over 71 kg. At the same time, the newly developed measuring membrane acts as a seat occupancy sensor and ensures that the passenger airbag, belt tensioner and sidebag are automatically deactivated if the seat is unoccupied.

The automatic child seat recognition system, which is fitted as standard, deactivates the passenger airbag when it detects that a Reboard child seat from the Mercedes range of accessories is fitted on the front passenger seat. This child seat uses a special transponder system that receives and responds to signals from two antennae inside the seat upholstery. This exchange of data tells the ECU whether a Reboard seat has been installed and, if this is the case, the ECU deactivates the airbag on the passenger side. The belt tensioner, sidebag and windowbag remain activated and offer the smaller passenger in the child seat additional protection in the event of an accident.

Windowbags and sidebags for optimum side-impact protection

Fitted as standard, the windowbags are a further example of the real-life safety concept at the heart of the new CLK-Class. They were also developed based on the findings of the latest accident research. During the course of this research, which took into account a wide range of impact angles, scientists established that an airbag in the side wall area needs to be as large as possible to ensure the best possible protection for all occupants, given the wide range of possible impact constellations. The windowbag meets the requirements stipulated by the accident researchers. It consists of several chambers which, in the event of a crash, are drawn like a curtain across the interior area between the front and rear roof pillars, all within the space of 25 milliseconds.

And whilst the windowbag prevents a person's head from colliding with the side window or roof frame, as well as catching glass splinters or other objects that can be propelled into the interior following a crash or subsequent rollover, the sidebags offer additional protection for the thorax area. Alongside adaptive front airbags, windowbags and sidebags, Mercedes-Benz equips the new CLK-Class with the best and most effective occupant protection systems available, including:

	Front seats	Rear seats
Automatic seat belts	x	x
High-performance belt tensioners	x	x
Belt force limiters	x	x
Automatic child seat and front passenger recognition with weight classification	x*	--
Automatic belt feeders	x	--
Head restraints,height and angle adjustable	x	x
Adaptive front airbags	x	--
Sidebags	x	optional**
Windowbags	x	x

x = standard; in front passenger seat; **standard in CLK 55 AMG

The automatic belt feeder is a further piece of practical technology used by the Mercedes engineers. It assists the driver and front passenger when putting on the automatic three-point seat belts. As soon as the occupants are seated, an electric motor extends the plastic rods holding the buckles on both sides of the vehicle so that the belts can be conveniently reached. The belt feeders retract again once the seat belts are fitted.

c_d figure of 0.28 a new aerodynamics record in this vehicle class

As well as providing class-leading body rigidity and occupant safety, the new CLK also points the way ahead when it comes to aerodynamics: a drag coefficient (c_d figure) of just 0.28 makes the two-door Mercedes the most aerodynamic coupé in its vehicle class. This important aerodynamic figure is almost seven percent lower than the figure for the preceding model.

This impressive performance is the result of numerous complex solutions jointly developed by engineers and designers in the wind tunnel. Features particularly worthy of mention include:

... the aerodynamic form of the front apron which channels the oncoming wind sideways along a pre-defined path so that it can flow over the relatively flat surface of the body without causing significant losses. Combined with the underfloor flow, this results in less lift acting on the front axle.

... the spoiler lip on the boot lid, designed to work in tandem with the rear edge of the roof and the C-pillars, to reduce the lift acting on the rear axle.

... the wheel spoilers in front of the front and rear wheels, which reduce the air backpressure in front of the tyres.

... the aerodynamic plastic shoulders in front of the front wheel housings, which allow the air to flow over the front suspension links without incurring significant losses.

... the large and flat-surfaced underfloor panelling which enables the oncoming wind to flow underneath the body without causing a significant vortex.

... the engine compartment encapsulation which channels the oncoming wind rearwards along a pre-defined path and prevents the air forming a vortex in the engine area.

... the sealing of the front section using rubber sections which expand due to the effects of backpressure and prevent the oncoming wind from entering the gap.

Facts and figures: the main aerodynamic data for the new CLK-Class

Wind resistance on the new CLK body is around seven percent less than on the body of the preceding model. Even the cd figure has been reduced by almost seven percent. And the lift acting on the front and rear axles is much lower than was the case in the previous CLK-Class.

	New CLK-Class	Preceding series
cd figure	0.28*	0.31**
Frontal area m²	2.06	1.99
Wind resistance cd x A m²	0.59*	0.62**
Front axle lift cLF	0.03	0.11
Rear axle lift cLF	0.07	0.11

*with automatic climate control as standard; **vehicle with air conditioning

However, detailed work in the field of aerodynamics did not just focus on improving wind resistance. Three further items, no less important, were also on the agenda: directional stability, dirt deflection and aero-acoustics.

Directional stability: it takes a great deal of experience and expertise to develop a coupé that is as aerodynamic and directionally stable as it is elegant. And the Mercedes engineers showed that they had both when developing the new CLK-Class: the two-door Coupé delivers exemplary performance figures in terms of the body lift acting on the front and rear axles. Front axle lift is up to 70 percent lower than for the predecessor model whilst rear axle lift has been reduced by around 36 percent.

Major factors behind these impressive advances include the new panelling concept for the underbody, the front and rear wheel spoilers, the discreet spoiler lip on the boot lid and the newly developed shoulders in front of the front wheel housings.

Dirt deflection: the aerodynamics engineers headed to the wind tunnel to develop a range of highly-effective solutions designed to keep the exterior mirrors, side windows and rear windscreen free of dirt, even after prolonged driving in the wet, and to ensure that the driver has a perfect all-round view at all times. For instance, the aluminium channels on the A-pillars act as water deflectors, collecting the rain that falls on the windscreen and deflecting it rearwards over the roof, assisted by the oncoming wind. The water then flows into a drainage channel between the rear edge of the roof and the rear window weatherstrip before draining down onto the ground via side channels. As a result, the rear window of the Coupé remains free of dirt, even at high speeds.

The narrow, inconspicuous channel in the exterior mirror casing also acts as a water deflector. It ensures that rainwater drips off at a very flat angle in relation to the body flank before being blown rearwards by the oncoming wind. The aerodynamic shape of the mirrors and the discreet spoilers on the lower section of the mirror casings enhance this effect.

Aero-acoustics: the low noise levels on board the new Mercedes Coupé are the result of a comprehensive raft of measures: hermetic sealing of the roof trim strips; main door seals with special edge trims; aero-acoustically shaped exterior mirror casings; special, complex-design seals for the 5-mm thick side windows.

New tilting/sliding roof with 34 percent larger opening area

Several of the new developments optionally available in the CLK-Class are sure to provide even greater driving pleasure and further enhanced comfort:

The electrically-operated glass (optional) can also be opened and closed by remote control. And, thanks to the larger roof area on the new CLK Coupé, the opening angle of the tilting/sliding roof is an impressive 34 percent larger than that of the preceding model.

To supplement the standard-fitted ELCODE access and drive authorisation system, which has an electronic key with integrated infrared radio remote control, the CLK-Class now has the option of KEYLESS-GO for the first time. Armed with this system, which has already proved successful in top-of-the-range Mercedes-Benz models, the driver does not need a conventional key to get into the car. Simply putting a hand into a shirt, trouser or jacket pocket and touching one of the door handles or the boot lid handle suffices. The rest of the work is performed by an induction-type antenna system which exchanges door-opening identification codes with the chip card at lightning speed. KEYLESS-GO also makes starting the engine easier: touching a button on the SEQUENTRONIC shift lever or the five-speed automatic selector lever is all it takes to start the vehicle, once the chip card and immobiliser have exchanged codes.


Twin headlamps with the option of halogen or bi-xenon technology

The new CLK-Class features a new reinterpretation of the twin-headlamp face, proving once again that this classic Mercedes design feature has lost none of its appeal. Displaying a sophisticated clear-glass look, the plastic lenses conceal some of the finest headlamp technology available: a two-chamber reflector that combines four lighting functions. The outer chamber contains dipped beam and, above this, the indicator, whilst the inner chamber is for main beam and the parking light. Switching on main beam in addition to dipped beam ensures wide-ranging illumination of the road ahead. The fog lamps, located in the front bumper covering, are positioned to provide optimum illumination in foggy conditions. Mercedes-Benz fits H 7 bulbs as standard for main and dipped beam.

The list of options includes even more powerful bi-xenon headlights, a technology lifted from the top-of-the-range S-Class, SL-Class and CL-Class models. Xenon headlamps are gas discharge lamps filled with the gas of the same name. Bi-xenon means that each xenon lamp is capable of providing both dipped beam and main beam. And whilst the entire luminous flux is available when main beam is on, switching to dipped beam causes a shutter to move between the lamp and the refractive optics, covering part of the pencil of light. One of the major benefits of xenon technology is its high light output: even at 35 watts, gas discharge lamps generate a luminous flux that is twice as intense as that provided by conventional H 7 headlamps.

Dynamic headlamp range adjusters, which adjust the inclination of the beam of light in line with the current position of the body and balance out brief periods of pitch during braking or acceleration, are also part of the bi-xenon package, as is a wash/wipe system with high-pressure water jet.

The exterior mirror casings with integrated LEDs have become a regular feature on many new Mercedes models. And the new CLK-Class also benefits from this technology. Turn signals are therefore clearly visible to other road-users, both from the front and from the side.

Elegant taillights and fast-acting LEDs in the boot lid

In a similar vein to the twin headlamps at the front, the taillights are an important styling feature at the rear of the Coupé. Sporting a design that is both dynamic and elegant, they are drawn deeply into the vehicle flanks, their powerfully sweeping lines enabling clear divisions between the individual lights: brake lamp (top), taillight (bottom), reflector (bottom), rear fog lamp (bottom, on the left-hand side of the vehicle) and indicator (middle).

The third brake lamp, which takes up a slightly more elevated position in the middle of the spoiler lip, is electrically connected to the brake lamps in the rear light clusters. The LEDs in the third brake lamp light up around 50 milliseconds faster than conventional bulbs. This means that the LEDs play an important part in preventing rear-end collisions.

Additional safety is provided by two reversing lights housed behind the clear-glass areas in the rear light clusters.

Windscreen-mounted light sensor fitted as standard

Electronic control units networked via CAN databus (Controller Area Network) enable additional safety and comfort-enhancing functions to be created. A prime example of this is Headlamp Assist: a windscreen-mounted light sensor detects when dusk falls or when the Coupé enters a tunnel and, in such an instance, turns on the car's exterior lights automatically. This convenient function is also activated when it rains or snows. Only if conditions are foggy do the lights have to be switched on manually in the conventional manner. Headlamp Assist is activated using a light switch on the dashboard ("Auto" position).

The auxiliary lighting circuit is automatically activated if a bulb relevant to safety fails. This system, which switches on different lights instead, is just one example of the extensive and sophisticated lighting technology at work in the new CLK-Class. Using the buttons integrated in the multifunction steering wheel, the driver can also program the lighting system so that dipped beam, parking lights, taillights and licence plate lights always come on automatically when the engine is started. Depending on the individual settings, the front headlamps can remain switched on for up to a minute after the occupants leave the vehicle, to assist them in the dark.

Two-arm windscreen wipers with highly-effective aero wiper blades

To replace the single-arm windscreen wipers that were a feature of the predecessor model, the Mercedes engineers have equipped the new CLK Coupé with an even more highly effective two-arm wiper system with special kinematics. Whilst the wiper on the driver's side has a fixed axis of rotation, its passenger-side counterpart performs an eccentric sweep so as to enable wiping of an even larger windscreen area. The wiper blades are a new feature and were developed in a wind tunnel, hence the term "aero wipers". This effectively means that, instead of the jointed clipping-in system used for conventional wiper blades – which involves inserting the wiper rubber into a series of claws on the blade – the aero wiper consists of a single-piece rubber section with integrated spoiler and externally positioned spring runners. These runners precisely follow the curvature of the windscreen, thus ensuring optimum contact between the wiper and screen at all times and improving the wiping quality, particularly at higher speeds.