The bodyshell of the new CL-Class is another excellent example of how very different aspects can be successfully integrated. As a robust backbone it stabilises the running gear, ensures precise handling and prevents uncomfortable vibrations. It also meets the most stringent Mercedes safety standards with its intelligent lightweight construction, has exemplary aerodynamic properties and has made audible progress in the field of vehicle acoustics.
These results are based on the careful selection of materials on the well-tried Mercedes principle of "the right material in the right place". The material mix consists of steel, aluminium and plastics, with steel continuing to play the major role. Here too, a technological change is taking place: conventional deep-drawn steels are becoming less important, and only account for around 26 percent in the new Mercedes-Benz CL-Class. Instead the specialists in Sindelfingen have opted for high or higher-strength steel alloys, which achieve maximum strength with minimum weight. Accordingly the proportion of these steel types has increased from eleven to around 44 percent compared to the preceding model.
In addition, ultra high-strength, high-tech steel alloys are used in body areas which are important for occupant protection. These are already formed in a warm state, thereby achieving an extremely high tensile strength which exceeds that of conventional steels by a factor of three. This means that half the bodyshell of the new CL-Class consists of high-tech steel alloys.
Mercedes-Benz uses the lightweight material aluminium where it has most advantages compared to steel. In the new CL-Class these areas are:
- Front wings
- Exterior door panels
- Front-end module member
- Rear-end module member
- Bulkhead behind the rear seat backrest
- Rear end centre assembly
The boot lid and spare wheel recess are likewise of lightweight material, namely plastic.
High-tech adhesive in addition to welded connections
In addition to these high-tech materials, a combination of modern joining techniques contributes to the exemplary torsional and flexural rigidity of the bodyshell. A new, high-strength structural adhesive supplements conventional processes such as spot/laser welding and low-stress clinching. The adhesive creates a firm bond between the steel flanges, significantly increasing the load resistance and transfer of forces in safety-related areas. The total length of high-strength bonded seams in the new CL-Class has more than doubled versus the preceding series.
The precise dimensions of the body components, low-stress joining techniques and the latest spot or laser welding processes make additional brazed connections and MAG welding seams between the steel panels almost completely unnecessary – a major contribution to the durability of the bodyshell.
Easily repaired modules at the front and rear ends
The engineers in Sindelfingen adopted the well-proven modular concept for the front and rear ends of the new CL-Class, though with certain detailed design improvements. The flexible cross-members of these modules are now of high-strength extruded aluminium sections. These are connected to the longitudinal members via steel crash boxes, and designed so that the impact energy of a low-speed collision is first absorbed by the aluminium sections and crash boxes. This means that the downstream body structure remains undamaged. During an offset frontal collision, the robust cross-members also have the task of transferring impact energy to the unaffected side of the body structure.
The front and rear-end modules are bolted to the bodyshell, and can be replaced without troublesome welding work if repairs are required. The individual components within the modules are also connected with bolts.
Hybrid construction of two lightweight materials for the doors
The doors are of frameless, lightweight aluminium/magnesium construction: the interior parts are of magnesium, while the outer skin is of aluminium alloy. Side impact protection is provided by internal reinforcing sections of high-tensile aluminium. Mercedes engineers also devoted particular attention to the door hinges, for which they developed special, high-strength mounting plates. This creates a robust, integrated structure which provides effective protection to the occupants in the event of a lateral collision.
In addition to a high level of safety, the doors of the new CL-Class also have functions which make access and egress easier. A newly developed hydraulic system ensures that the open doors remain in position above an opening angle of 15 degrees – even on gradients or in strong wind. An electric servo mechanism pulls both doors (and the boot lid) closed.
Long-term protection: fully galvanised bodyshell and paint with nano-particles
Mercedes-Benz has developed an effective package of measures for long-term anti-corrosion protection. This is based on fully galvanised body panels, some of which have an additional organic coating on both sides depending on their location, e.g. on the doors or on the front, side and rear longitudinal members. This coating also contains rust-inhibiting zinc pigments. Mercedes-Benz also protects the most vulnerable structural areas of the bodywork with a cavity-fill preserving agent, for example on the front side members, the upper side member plane, the side members and the rear wheel arches.
The Mercedes-Benz experts also weather-seal the welding seams in order to prevent the onset of corrosion. This seam sealing benefits not only the bonnet, doors, tailgate, rear wheel arches and fuel filler flap, but also a large proportion of the welded joins in the floor structure of the new CL-Class. Using plastic for a large area of the underbody panelling has allowed Mercedes engineers to dispense with conventional PVC underseal. This underbody panelling protects the body from stone chippings, water and soiling.
In keeping with all Mercedes passenger cars, the new CL-Class is also protected by the 30-year MobiloLife warranty.
Mercedes-Benz also makes a major contribution to exemplary long-term quality and value retention with a scratch-resistant clearcoat based on nano-technology. This innovative paint system, which celebrated its world debut at Mercedes-Benz, is a standard feature of the new CL-Class and is used for both metallic and non-metallic finishes.
Thanks to advances in the field of nano-technology, it was possible to integrate the tiny ceramic particles measuring less than one millionth of a millimetre into the molecular structure of the paint binder. These particles effect a three-fold improvement in the scratch-resistance of the paint finish and ensure a visibly brighter, long-lasting sheen.
Aerodynamics: exemplary wind resistance and wind noise
The aerodynamics engineers at Mercedes-Benz have achieved another triumph where the new CL-Class is concerned. Despite larger wheel arches and greater demands in terms of engine cooling and interior climate control, the luxury Coupé has an even better coefficient of drag than its predecessor, with a Cd value of 0.27. This means that the new Mercedes Coupé also sets the standard for its class where aerodynamic efficiency is concerned.
The key aerodynamic figures for the new CL-Class at a glance:
of drag (Cd value)
drag area (Cd x A)
axle lift (cFL)
axle lift (cRL)
The fact that the good aerodynamics of the CL-Class not only produce an exemplary Cd value, but also have a positive effect on handling safety, is demonstrated by the favourable values for front and rear axle lift. These are up to eleven percent lower than those of the preceding model, and are reliable indicators of high braking stability, good roadholding and a low susceptibility to side winds.
Comfort as an audible experience
The good aerodynamics of the CL-Class are also noticeable in acoustic terms, for if the airstream passes across the outer skin with minimal turbulence in the interests of low drag, this also leads to less noise. As early as the conceptual phase, Mercedes engineers took pains to ensure that the bodyshell was optimally streamlined so that wind noise was reduced.
It was with this in mind that they designed the airflow-efficient A-pillars, developed a more rigid bodyshell with reinforced outer surfaces and achieved a new sealing concept for the doors, which is acoustically even more efficient than in the preceding model. This exemplary noise insulation is also helped by the side windows, which have a thickness of six millimetres in the new CL-Class.
Many other components were also developed with a view to aero-acoustic efficiency: for example the windscreen wipers, which are positioned very low down, out of the airstream, when parked. A narrow stem between the exterior mirror housings and the doors also prevents wind noise, and a specially designed rubber seal between the rear window and the boot lid also makes for aero-acoustic efficiency. The tilting/sliding sunroof features serrated wind deflectors whose ridges create designed-in longitudinal air turbulence, thereby reducing thrumming noises when the roof is open.
Noise analyses with a three-metre parabolic mirror
To carry out their aero-acoustic measurements, Mercedes engineers developed a new procedure involving a three-metre parabolic mirror fitted with numerous microphones, which acts as a sound reflector. From a distance of five metres, the microphones at the focal point of the mirror record the noises caused by the airstream as it passes around the vehicle body.
While numerous individual measurements were formerly necessary for such analytical procedures, the new parabolic mirror process requires only a single measurement to examine complete areas of the vehicle body, e.g. the A-pillar, exterior mirror, side window and roof.
A video camera makes the area under examination visible on the computer screen. At the same time the system superimposes the wind noises onto the video image as different-coloured areas, depending on the sound volume and frequency. This enables sources of irritating noise and its distribution to be identified and examined.
During the development of the new CL-Class this innovative aero-acoustic measuring procedure was supplemented with well-proven artificial head technology: a dummy with an artificial head, whose anatomically accurate auditory passages were equipped with capacitor microphones, was seated behind the steering wheel. These made both spatial sound recordings and precise measurements of sound pressure and frequency possible.
Subsequent evaluation of the test results showed how effective the noise reduction methods were. The acoustically sensitive area at the driver’s head level appears as a blue, particularly quiet zone on the computer screen of the parabolic mirror microphone system. At a wind speed of 140 km/h, the interior noise level in this area is below 66 decibels.
Background noise subjectively perceived as pleasant
In addition to taking measurements, Mercedes engineers also consider it important to establish how male and female drivers perceive the level of acoustic comfort. This is because some noises which the equipment registers as quiet can in fact be irritating, and compromise comfort on long journeys. They therefore use analytical methods which better reflect human audio-perception and provide key psycho-acoustic information about subjective perception of background noise.
One of these parameters is loudness, for example. This is calculated on the basis of different frequency groups, and is better able to describe how humans perceive loudness than the noise level in decibels. At a wind speed of 140 km/h, an outstanding result of just 19 sone was measured in the new CL-Class. Other criteria for noise comfort include harshness, which predominantly evaluates high frequency noises such as hissing or whistling, and the articulation index. This indicates how easily the car occupants are able to converse during a journey. For example, it is irritating if the airstream creates a continuous, high-frequency rushing sound which makes conversation more difficult.
Mercedes engineers arrived at an articulation index of 78 percent for the new CL-Class. This is more than ten percentage points above that of the preceding model, and significantly exceeds the value for other cars in the luxury class. The ideal conditions are therefore in place for easy communication between the driver, front passenger and rear seat occupants.
Hearing tests with 60 male and female drivers confirmed the effectiveness of these aero-acoustic developments in the new Mercedes Coupé. They were asked to evaluate certain noises, and thereby helped the engineers to achieve favourable aero-acoustic values.
Windscreen wipers: aero-technology with two motors
The windscreen wipers of the new CL-Class were also developed in the wind tunnel: they are known as aero-wipers. Instead of the articulated retention system used for conventional wiper blades, in which the rubber blades are claw-mounted, the aero wiper consists of a one-piece rubber section with an integral spoiler and externally mounted spring rails; these precisely follow the curve of the windscreen. The spring rails ensure an even distribution of wiper pressure along the entire length of the wiper blade, so that it always operates with the greatest possible contact pressure. The result is significantly better wiping quality, while wiper noise is considerably reduced.
The wipers on the driver and front passenger sides are powered by two motors, and have different kinematics: while the driver's-side wiper arm moves around a fixed axis, its counterpart on the passenger side executes an additional lifting movement to wipe an even larger area of the windscreen. The rain sensor which controls the windscreen wipers according to rain intensity is standard equipment, and can be adjusted in two sensitivity stages using the multifunction stalk on the steering column.
Windows: silver foil as an infrared reflector
The new CL-Class leaves the production line with infrared-reflecting, laminated glass which also has an invisible coating of pure silver. This not only filters out UV radiation, but also light in the infra-red spectrum which is mainly responsible for heating up the interior. Compared to green-tinted glass, this reduces the energy radiation into the interior from 60 to around 47 percent at the windscreen.