Safety | Bodyshell
E-ClAss
The Bodyshell as Lightweight Design Materials Mix Safety, comfort, a low fuel consumption, and reliability were the main characteristics on which development work for the new E-Class bodyshell structure had to concentrate. It was then a matter of employing lightweight construction methods to compensate for the weight increases due to demands for higher safety and comfort standards.
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The Authors 1 The Lightweight Design Materials Mix One objective in developing the new E-Class bodyshell was to find ways of compensating for the extra weight due to more stringent comfort and safety requirements, even though lightweight construction methods had already been employed in the preceding model. This was achieved by adopting the geometrical optimisation and conceptional lightweight design, combined with a materials mix based on the very latest, high-strength and ultra-high-strength steels and the use of aluminium attached parts. In order to optimise the axle load and consequently the vehicle dynamics, the drive towards lightweight design on the front-end structure by using aluminium and glass fibre reinforced plastic (GFRP) for the engine hood, fenders and front end assembly. Greater reliance was placed in other areas on lightweight steel design (high-strength and super-high-strength materials, tailored blanks and tailor rolled blanks, geometrical optimisation and conceptional lightweight design). In the front-end assembly it was possible to design longitudinal members with no need for necking to accommodate other major components and with
flared ends connecting to the firewall cross member. This allowed homogeneous load paths to be designed, obviating the need for any reinforcement of those parts of the structure subject to severe local stresses, which would have added to the weight. Similar design principles were applied to the structure of the rear and side members. Greater use of recently developed steel grades was made in the steel lightweight design of the bodyshell. There has been a significant increase in the proportion of the bodyshell’s weight made up of highstrength and ultra-high-strength sheet steel. Ultra-high-strength, heat-formed steels are used especially in areas with the most demanding crash requirements, such as the firewall and the sidewall roof area, Figure 1. Aluminium is mainly used in areas subject to rigidity stresses such as the parcel shelf and for attached parts such as fenders, the engine hood and trunk lid. The front-end support structure is an aluminium sheet and GFRP hybrid. As with the preceding model, the spare wheel well is made of glass mat reinforced thermoplastic (GMT). The methods described above have made it possible to reduce any weight increase in the new E-Class bodyshell structure to a minimum.
Heiko Kellermann is head of the department for bodyshell C-, E-Class.
Dr. Jörg Langner is head of the team for front end, exterior attached parts C-, E-Class.
Marcus von der Wehl is function group spokesperson for the bodyshell 1.
Bernd Artner is function group spokesperson for the bodyshell 2.
Hermann Tenfelde is function group spokesperson for the doors.
Figure 1: Bodyshell materials summary ATZextra I January 2009
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cross section, slightly narrower towards the vehicle centre, allowing the weight performance of an aluminium design to be achieved here using an inexpensive steel application. Generally speaking, increasing use is being made of flexibly rolled sheet metal in the floor structure of the new EClass so as to achieve the optimum thickness to withstand the relevant stresses. This technology, tested in the C-Class, represents an important element in the lightweight design of the new E-Class.
Figure 2: Attaching the front-end assembly
2.3 Sidewall, Roof 2 Structure, Body The bodyshell is essentially made up of an extremely rigid passenger compartment and includes the survival space in the event of a crash and the front -and rearend deformation areas. In order to ensure effective crash performance, even with the increased requirements in respect of high-speed rear-end collisions, side crashes and side pole crashes, the load on the bodyshell has been specifically distributed between several paths. Right from the start, because of the E-Class platform strategy, the bodyshell design took account of the planned derivatives in order to find the best design for both the platform and the derivatives.
DOI: 10.1365/s40111-009-0140-z
2.1 Front End A new structural feature, compared with the preceding model, is the lightweight steel frame-type integral support which, acting as an additional level of longitudinal support, can absorb the deformation energy in a head-on collision. The same design of integral support for all engine and body variants in the new E-Class (except for all-wheel models) can be used. An additional brace made of superhigh-strength steel between the damper dome and windshield cross member on the driver’s side distributes the energy loads at the upper longitudinal member level and reduces rearward displacement of the steering wheel and pedals. The front-end assembly, made of aluminium like its predecessor, and which can be removed for carrying out repairs, differs in that it follows the so-called principle of snap-in crash boxes. In addition to its ad94
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vantages for major component layout and for production line assembly processes, this construction method offers a weight reduction of 2 kg, Figure 2.
2.2 Floor Assembly The floor structure complies with the high Noise Vibration Harshness (NVH) and side crash requirements for today’s vehicle bodyshells. Together with the structural measures for high-speed rearend collisions, this is a further development of its predecessor’s design. Its main features can be summarised as follows: – profiled sections for reinforcement in the driver’s and front passenger’s footwells – continuous longitudinal member below the main floor – reinforced seat cross member (front and rear) with combined tunnel cross braces – reinforced longitudinal member structure at the rear. In order to satisfy the additional requirements of the ‘Oblique Pole’ crash test (FMVSS 214), the following measures were implemented: – an additional row of spot welds and adhesives to join the sidewall to the main floor over a large area – additional reinforcement of the seat cross members – a particularly strong plate connecting the main floor and cross member below the rear seat, sidewall. A special feature is the flexible rear bumper bracket which can be removed and exchanged when repairs are carried out. It is made of a flexibly rolled, ultrahigh-strength material, with a variable
Essentially, it is the very strict side crash performance which determine the bodyshell design. The main focus here is on the design of the B-pillar, the inside and outside of the side roof frame and the side member (rocker panel). The B-pillar is the component subject to the greatest stresses from side impact loads and, together with the existing sidewall body structures, possesses the optimum design to withstand other load cases, such as the roof drop test. The vehicle structure was designed to include an appropriate level of heatformed, ultra-high-strength steels and was then weight-optimised for all load cases. This design (tensile strength of over 1200 MPa) enables the vehicle structure to exploit the available installation space to the maximum for crash functions and packages. A reinforced, transverse roof frame helps to absorb the load on the roof structure in the event of a side crash. The sidewall structure of the A-pillar in the bodyshell profile was optimised in order to improve the visibility interference angle. Together with the multishelled, transverse roof frame, front and rear, both the A- and B-pillars help to satisfy the strict internal roof drop test requirements. The roof frame’s multi-shell design also enhances the body’s overall rigidity. The demanding NVH targets were also met by the selective use of structural adhesives in areas particularly subject to stresses. The new E-Class roof structure design applies to all roof variants; the roof frame, front and rear, is the same for solid roofs, tilting, sliding roofs and glass panoramic roofs.
3 Engine Hood The legal requirements for pedestrian protection were the most prominent considerations when designing the engine hood for the new E-Class. Here, it was essential to coordinate the layout of the major assemblies under the engine hood with the design of the inner section of the hood so that a pedestrian’s head would not impact against any “unyielding” components underneath the engine hood structure. A further element was to use an actuator which, in the event of a collision with a pedestrian, raises the engine hood in milliseconds, thus creating additional clearance.
4 Rigidity, NVH, Durability The strict comfort targets for the complete vehicle as regards ride characteristics and acoustics can only be achieved if the bodyshell structure forms a solid basis for the design of the major assemblies. Wide-ranging measures, including extra supports and reinforcements in the floor structure have been incorporated in the E-Class bodyshell to ensure this. In addition, a rigid design was emphasised for those points where the suspension and
drivetrain were linked to the floor structure. The complete package of measures is rounded off by the wider use of structural adhesive. The outcome is that the bodyshell’s structural resistance against distortion has been increased by 31 % compared with the preceding model. This figure is the basis for the improved dynamic behaviour of both the bodyshell and the vehicle as a whole and their outstanding durability.
5 Doors
path in the event of an offset crash. This energy path provides the passenger compartment with further protection against intrusion near the lower side impact reinforcement element by improved anchorage for the A-pillar. Protection against a side crash is provided by a side impact reinforcement element made of ultra-high-strength, cold-formed steel with a strength of >1200 MPa. This double-hat profile element has been installed diagonally in the driver’s and rear door and reduces door intrusion by being supported on the A- and B-pillar or in the transition area between the wheel arch and the side member. n
The basic structure of the doors has also played a significant part in improving the crash safety of the 212 series, Figure 3. By collaborating with those responsible for crash calculations and crash testing, an exemplary outcome has gradually been achieved in respect of rigidity, strength, vibration-proofing and crash behaviour while keeping the weight of the structure down. 99 % of the material in the doors is high-strength or ultrahigh-strength steel. One result of this design is demonstrated by the energy paths in the doors. The addition of a highstrength reinforcement profile has converted the inner beltline into an energy
Figure 3: Door structure ATZextra I January 2009
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