Bodyshell and passive safety: Maximum stability and high-quality lightweight design

May 15, 2013
A high level of crash safety, lightweight design, outstanding rigidity for excellent handling with extremely low levels of noise and vibration. These were the aims when developing the bodyshell for the new S-Class. Application of the 3D Body Engineering approach has given it a third-generation aluminium hybrid bodyshell. The lightweight index – the torsional stiffness in relation to weight and vehicle size – has been improved by 50 percent compared to the predecessor model.
The bodyshell is the vehicle's functional backbone. All statutory, rating-relevant and internal crash load requirements are met, and even future requirements are already taken into account. To further improve suspension comfort, the shear forces acting at the force application points in the bodyshell had to be increased. These requirements were also met thanks to appropriate component design and special joining technology in the bodyshell. The lightweight index has been improved by 50 percent compared to the predecessor model. This index defines the torsional stiffness in relation to the vehicle's size (footprint) and weight. To this end, all the relevant node areas were optimised and systematically reinforced with respect to the force distribution.
The aim of the NVH measures for the bodyshell was to better the already very good preceding model with respect to vibration and noise comfort, but without dispensing with intelligent lightweight construction methods. The bodyshell structure of the new S-Class reaches new heights in dynamic rigidity thanks to new structural concepts and specific NVH measures in the bodyshell.
These include:
  • Integral NVH front section concept consisting of extruded aluminium side members, cast aluminium elements at the transition between the front section and passenger cell, also at the damper domes, and integral carrier as vibration-damping structural component
  • Increased use of bracing struts for specific increases in rigidity
  • Rear panel and cockpit cross-member of hybrid metal/plastic construction
  • Use of foam sections in the A/B/C-pillars to increase bodyshell rigidity
No weight increase in 20 years – despite stricter requirements
Since the 220 model series was developed in the 1990s, with an optimally coordinated materials mix the hybrid lightweight construction has been further developed into an aluminium hybrid bodyshell. During this period the share of aluminium has increased to more than 50 percent. It has therefore been possible to maintain practically the same body weight for 20 years and even slightly reduce it, despite far more stringent comfort and safety requirements and additional functions. In addition to this, structural foams are used at specific points in node areas in the new model series. The entire outer skin of the S-Class, including the roof and the front section of the body, consists of aluminium. The high percentage of aluminium is possible thanks to the use of the complete range of semi-finished products (cast, extruded and sheet aluminium). The safety passenger cell makes intensive use of high-strength steels.
This lightweight design by material and geometrical optimisation coupled with highly complex joining technology allows the new S-Class to further raise the bar in the demanding luxury saloon segment – without adding weight. With a torsional stiffness of 40.5 kN/degree (predecessor: 27.5 kN/degree), the S-Class achieves a new record in its segment.
Focus on driving dynamics: lightweight-construction measures
The front section is one of the preferred lightweight-construction zones for achieving balanced weight distribution of the entire vehicle. The decision to use aluminium as a lightweight material in this area was therefore made at an early stage. As well as being lighter than its steel counterpart, the aluminium front section also greatly enhances crash and NVH performance. Cast aluminium and extruded aluminium sections are used in addition to sheet aluminium. Die-cast aluminium was chosen for the shock absorber strut bracket because of its good integration properties. In this way it was possible to connect the front module without additional holders, for example. As well as reducing the number of components, die casting also enables the component to be designed to withstand specific loads and stresses. Morphological analyses were conducted to configure the wall thicknesses and geometry of the component so as to meet the strict functional requirements accurately.
In addition to local optimisation of the introduction rigidity at the shock absorber strut bracket and at the integral carrier, a cross-functional load path was conceived to further improve the system as a whole. The aluminium struts running in X-direction from the shock absorber strut bracket to the cowl provide support in the event of a crash. Supplemented by a multipiece framework, these struts also help to suppress the Y-movement of the side members. This design allowed the new load path to be incorporated into the limited package installation space in the front section. The forces are applied to the bodyshell structure in the three-piece cowl, which has been configured as a cast aluminium component in the centre section. In this case too, the casting allows a functionally perfect connection with distinct advantages in terms of weight and installation space.
The side members have been designed as combined aluminium extruded sections/castings to optimise crash performance, rigidity and component integration. The protrusions of the extruded aluminium sections of the side members required for package reasons have been designed to also have a positive effect on folding behaviour in the event of a crash. The cavity closed by a foam piece is also used as a resonance volume for the Frontbass system. The side members are connected to the steel passenger cell by means of cast aluminium components that allow a very rigid connection and integration of the integral carrier connection.
Clever combination: sophisticated steel and aluminium joining technology
The firewall area is a sheet steel construction. As well as meeting the functional requirements perfectly, this design also allowed integration of the complex hybrid joint between the aluminium front section and the steel cell. Here one of the major challenges was the mechanical joining technology used for the sophisticated high-strength steels.
In addition to the front section, the integral carriers are also made of aluminium and boast the lowest weight in their class. As well as acting as a component carrier for numerous components, the integral carrier is a central component of the front-end structure when it comes to performing crash and NVH functions. Compared to the previous model series, the complete cooling module is fastened to the integral carrier in addition to the engine, steering, torsion bar and front axle. The integral carrier's side members also form the third crash load path in the front section. In order to meet these multiple requirements yet still maintain a lightweight design, a complex aluminium mix comprising die castings/permanent mould castings, extruded sections and sheet metal parts was also required here.
The roof is another key area for lightweight design as reducing weight here has a positive effect on the vehicle's centre of gravity and on NVH characteristics. The new S-Class features an aluminium roof – a first at Mercedes-Benz. A major challenge here was integrating the roof into a steel structure. This was achieved by implementing an efficient and simple assembly solution in the bodyshop, which involves the roof being fixed to the bodyshell structure using shackles with defined spacing for the purpose of production in the factory. The resulting gap between the bodyshell structure and the roof ensures that the components are coated completely during the cathodic dip priming process.
One particular lightweight-construction measure involves the use of structural foam pieces in functionally critical node areas to ensure high NVH performance. Preliminary investigations have shown that comparable performance could only have been achieved by means of solid reinforcements, some of which would have required one-sided joining processes. Such reinforcements would have resulted in a double-figure increase in the bodyshell weight. Due to the way the foam pieces work, it is possible to position them precisely in the functionally critical node areas, since there is no need to take account of production-specific requirements such as joining technology limitations or component joining sequences. Furthermore, it is even possible to achieve frictional connections between cross-sections with several chambers.
Safety first: passenger cell made using high-strength steels
The safety passenger cell consists primarily of steel. Thanks to the use of higher steel grades, the weight has been kept the same as for the predecessor model although far more stringent crash requirements (angled mast) and stricter NVH requirements are met. All relevant components have been increased by one class in terms of material quality, one example being in the area of the B-pillar, roof frame and tunnel reinforcement, where thermoformed ultra-high-strength steels that have been weight-optimised as tailored products with different sheet thicknesses are used. In the lower side member area, an ultra-high-strength steel (CP 1000) is used as a roll-profiled component for the first time. This steel-based lightweight design by material optimisation is also backed by lightweight design by geometrical optimisation, including the formation of a frictional connection between the C-pillar and the rear centrepiece, which allows efficient support of the rear section's side members when the vehicle flexes.
In the case of detachable body parts such as the wings, bonnet and boot lid, the use of aluminium adopted for the previous model series was continued. The aim was to outperform internal and external competitors in terms of weight and to effectively implement the design in production whilst meeting strict requirements in terms of joins and seams. Proof that sophisticated design, lightweight construction and production-friendly product design can actually go hand in hand.
The aluminium hybrid design results in the use of additional mechanical joining technologies. The technical limits of the processes for these joining technologies were determined in collaboration with the supplier, the planning department and production. Consequently it was possible to combine high-strength sheet metal with aluminium for the first time and to reduce the sheet thickness of the components primarily from a functional standpoint. Only thanks to the interaction of lightweight-construction measures and lightweight-construction processes was it possible to make use of the 50 kg lightweight design potential. Conversely it was then possible to invest in a massive way in performance – in safety, comfort and customer benefit. Result: the bodyshell of the new S-Class is not only lighter, but also clearly tops in performance.
Restraint systems: meticulous design of every detail
Alongside the deformation potential of the body and major components, as well as the stability of the passenger cell, it is the quality of the restraint systems that largely determines the risk of injury to the occupants in the event of an accident.
The crash sensors, including appropriate software adaptation to detect the accident type and the collision severity, have been further refined compared to the predecessor model series. New components and features:
  • STAR2 electronic control unit
  • Pressure sensors for detecting collisions with pedestrians
  • Use of sitting position information for adaptive control of the restraint systems
As standard, the driver and front passenger each have a three-point seat belt with triple pyrotechnic retractor tensioning. Retractor tensioning and belt force limiting as well as an electrically reversible inertia reel tensioner are effective via the shoulder belt. The upper belt guide point is anchored to the B-pillar and height-adjustable. The PRE-SAFE® Impulse tensioning system is a world first in the new S-Class (see "Extended PRE-SAFE® protection" section). The rear passengers in the two outer seats have three-point seat belts with inertia-reel tensioner and self-adaptive force limiter, while the centre seat features a standard three-point seat belt system. As optional extras, the seat belt buckle extender, beltbag and cushionbag elevate rear safety to a new level (see "Extended rear protection" section).
Driver and front passenger airbags: adaptive filling
The driver's airbag (volume approx. 64 litres) is equipped with a two-stage gas generator. Two stages can be activated, depending on the detected vehicle deceleration values, with a delay between triggering of the two stages. In addition to the two-stage gas generator, the front passenger's airbag (volume approx. 112 litres) has a special feature in the form of a pyrotechnically activated adaptive stage. The cushioning of the occupant when plunging into the airbag is made harder or softer depending on the sitting position and gas filling, according to requirements.
The thorax/pelvis sidebags for the driver and front passenger with a volume of 17 litres are integrated in the front seat backrests whilst the rear sidebags with a volume of 12 litres are integrated in the rear side panelling where they are firmly attached to the bodyshell. In the event of a crash, the windowbag cushion (volume approx. 40 litres) is filled by a hybrid gas generator that is located in the roof area behind the B-pillar. The use of a new weaving technology (X-Tether technology) makes the windowbag cushion more stable, meaning that it is easier to ensure effective inflation over a prolonged period of time.
Triggering of the side protection systems is controlled by the Star2 electronic triggering device, which can detect and assess a side collision with the help of a central acceleration sensor, additional satellite sensors in vehicle lateral direction and pressure sensors in the doors. Furthermore, the belt tensioners are triggered together with sidebags and windowbags if a side collision is detected and together with the windowbags if roll-over is detected.
Pedestrian protection: active bonnet and more deformation space
To supplement the active safety measures which help to prevent accidents or reduce the severity of accidents, the measures aimed at mitigating the consequences of accidents in the event of collisions with pedestrians have been further developed for the new S-Class. In order to reduce the loads exerted on a pedestrian if their head hits the bonnet of the vehicle, the deformation space between the bonnet and the components beneath it has been optimised. This is achieved in part by appropriate positioning of components such as control units or fluid reservoirs in the engine compartment.
The S-Class also features an active bonnet. In the event of a collision with a pedestrian, sophisticated sensors combined with intelligent algorithms trigger pyrotechnic actuators in the area of the bonnet hinges. These raise the bonnet by around 80 millimetres. The deformation characteristics of the bonnet have been developed specifically to meet these requirements. Further reductions in the impact loads can be achieved by using aluminium and by homogenously reinforcing the inside face of the bonnet. Furthermore, the hardness of the foam in the front bumper has been optimised to reduce the load exerted on the pedestrian's legs in the event of an impact.
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