The new Mercedes-Benz CLS: Refined sportiness - PART VI


Stuttgart, Germany, Oct 04, 2010

The bodywork: A first: frameless, all-aluminium doors

- Made to be light: aluminium and high-strength steel alloys
- Made to be safe: stable bodywork structure and a high degree of rigidity

Intelligent lightweight construction plays a decisive role in bridging the classic conflict of objectives between low weight and high strength in the new CLS. The CLS is thus the first vehicle from Mercedes-Benz to have frameless, all-aluminium doors. These are made from deep-drawn aluminium panels with extruded sections, and in comparison with conventional steel doors, are some 24 kilograms lighter. This is not only beneficial to the environment, operating convenience is increased too: the Coupé doors can be opened and closed more easily, particularly on an incline. The new construction has called for new joining techniques: instead of welding, a combination of gluing and riveting has been used.

In order to assure quality, in the production process the doors are positioned in the frames by extremely precise robots; and the side windows are fitted with the greatest precision using laser measurement technology. As a result, this door design meets the same seal requirements as for the door frames on a saloon, which has to be demonstrated in a Mercedes internal test (one of several) in which it is subjected to high-pressure testing with water pressure at 80 bar. To put this in context: this pressure is about five times higher than the pressure needed by an espresso machine to produce a good crema.

Apart from the doors, the bonnet, front wings, boot lid, parcel shelf, various support profiles and substantial parts of the suspension and engines are all made of aluminium. The front end is a hybrid construction made of aluminium panels and plastic strengthened with fibre-glass. The one-piece aluminium crash boxes in the front area are fitted in the side members and are screwed into the side with them.

The large front bumper with the integral grille with the centred star is made of polypropylene. The materials experts at Mercedes-Benz have worked together with the experts from the supplier of this part to develop the plastics formulation, which has a specific talcum content of 15 percent, in order to achieve an optimum compromise between weight and rigidity, as well as excellent thermal properties. As supporting structure for the bumper there is a front end which is a hybrid construction of aluminium sheet and plastic strengthened with fibre glass. For every CLS, special adjustable fixtures are used to ensure the front fits exactly and the gap dimensions are the same all round.

About 72 percent of all panels used for the bodyshell of the new CLS are made from rigid and ultra-rigid steel alloys. The ultra-rigid high-tech alloys, which have three to four times more tensile strength than conventional rigid types of steel, account for around eight percent of total weight. They are deployed in areas in which there could be extreme material stresses in the event of an accident - for example, in the event of a side-on collision, in the B-pillars and side frame of the roof; as well as in the rear, to create a stable crossmember.

Front-end structure: crumple zone on four levels

Compared to the previous model series, the Mercedes engineers have further enlarged the deformation zones substantially in the front and rear sections of the new CLS, as well as improving the energy flows. The front crumple zone has four independently acting impact levels, meaning that the forces can be distributed over a wide area while bypassing the passenger cell.

- Sectional panels above the wheel arches form the upper side-member level. From here, the impact forces are channelled into the A-pillars and, subsequently, into the roof frame.

- An aluminium crossmember connects the forward-extended side members and ensures that the forces are transferred to the side facing away from the impact. The crossmember and the forward-extended side members form the central impact zone.

- The subframe to which the engine, steering and front axle are attached also serves as an impact level in the event of a frontal collision. It is made of high-strength steel and, depending on the engine variant, can be connected to the newly developed floor side members by means of special supporting tubes. As a consequence, the subframe can deform in a predetermined manner and absorb energy in the event of a crash on the one hand and channel high impact forces straight into the vehicle floor on the other.

- The side skirts have been extended forwards to support the wheel and prevent it from entering the footwell in the event of an offset frontal collision. In order to provide specifically targeted front-wheel support and location, Mercedes-Benz has also developed special struts and additional energy-absorbing elements for the wheel arches. The struts are arranged diagonally and prevent the passenger cell from sinking in the event of an impact.

The firewall is a four-part construction. This design enables Mercedes engineers to vary the material thickness according to the level of vulnerability in an accident. As the load acting on the firewall during a frontal crash is greatest in the lower section, the sheet steel used here is almost 50 percent thicker.

As well as being a major reason behind the high level of impact resistance, this intelligently designed bodyshell not only enhances ride comfort, it also reduces noise and vibration. The Sindelfingen engineers paid particular attention to the connecting points between the suspension and the bodyshell, which are required to withstand very high forces. These were specifically reinforced to ensure that road-induced vibrations are not transferred to the body at the expense of driving enjoyment. An indicator of the excellent overall result is the static flexural strength of the bodyshell, which shows a 28 percent improvement over its predecessor. Torsional strength increased by six percent.

Passenger cell: custom-designed floor panels and robust load-bearing sections

The passenger cell of the new CLS has been shown to be a structure which is virtually immune to deformation and which keeps the occupant space intact, even at high impact speeds, regardless of whether the collision is head-on, from the rear or from the side, or whether the vehicle rolls over. The use of high-strength steel and thicker panels plays as important a role here as the installation of additional load-bearing members.

The main floor assembly thus consists of different sheet-metal plates that either undergo flexible rolling or are welded together by laser beam and subsequently shaped. Flexible means that the high-tensile steel can be processed in such a way that areas with different steel thicknesses can be produced within a single component. The middle blank forms the tunnel – the actual backbone of the passenger cell. Here the thickness of the custom-designed panels varies between 0.7 and 1.1 millimetres, and between 1.55 and 2.0 millimetres for the tunnel reinforcements, depending on the stresses and loads to which they are subjected.

The continuous floor side members, the insides of which are further reinforced with extra sections, are very important both for occupant protection and the rigidity of the bodyshell. Their front faces connect to the side members, thereby lengthening the load-bearing paths along which forces can be distributed in the event of an impact. At the rear, the floor side members extend as far as the crossmember beneath the rear seat unit to stabilise the entire floor structure.

The Mercedes engineers have also incorporated sturdy aluminium transverse sections – known as transmission tunnel braces – into the floor assembly. One is located beneath the transmission, and is designed to direct forces to the side of the vehicle facing away from the impact in the event of a side-on collision. The second forms a connection between the two side members. It likewise braces the floor assembly and is able to channel impact forces into the floor structure at an early stage following a side-on collision.

Rear-end structure: side members with specifically graduated material thicknesses

Multi-piece side members and a robust, flexible crossmember made from ultra-high-strength steel form the key components of the rear-end structure. The rear side members are continuous, closed box sections with carefully graduated material thicknesses. These are able to absorb high forces, thereby making a decisive contribution to occupant safety in the event of a rear impact. The bolt-on flexible crossmember is manufactured using a flexible rolling process which likewise allows the material thickness to be varied as required. Accordingly, the material thickness on the outside of the crossmember – where impact loads are highest – is greater than on the inside.

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