A robust basis is the ideal ingredient for a stiff bodyshell offering a high level of crash safety. It plays a decisive role in ensuring maximum noise-related comfort, outstanding vibration characteristics and hallmark Mercedes-Benz longevity.
In these terms, the E-Class Cabriolet had an ideal starting position, given that it is closely related to the E-Class Coupé – a two-door model which meets the very highest requirements when it comes to torsional stiffness and occupant protection. The same applies to the Cabriolet, which has one of the stiffest designs in its segment. The body’s static torsional stiffness can be seen as a reliable indicator of the excellent cumulative effect of the implemented measures. It has been increased by around 30 percent compared to the outgoing model.
To achieve this figure, the bodyshell of the E-Class Cabriolet received extra reinforcements compared to its Coupé sister model, the most important of which are as follows:
• The extremely robust A-pillar assembly consists of two high-strength steel tubes which are welded to the sheet-metal shells of the A-pillars. In the case of the E-Class Cabriolet, both tubes have an exceptionally robust Y-shape and stretch from the A-pillar intersection to the upper window frame.
• The plug-in B-pillars are extremely robustly connected as they engage in the side skirts, thus offering highly effective protection in the event of a crash. A shoe made from ultra-high-strength steel braces the inside of the B-pillar against the rear seat crossmember. The rear support against the rear panel runs in an arch-like shape to the rear seats.
• Reinforced side skirts and bulkhead platesin the doors protect the occupants in a lateral impact with a pole, for example. Likewise the reinforced shoulderline with its high-strength steel section enhances
the exceptionally robust nature of the design.
• The floor assembly has been reinforced solidly at several points.
• The robust rear panel behind the rear seats further enhances the body’s lateral rigidity.
Intelligent lightweight construction: state-of-the-art materials and innovative joining techniques
Despite the enhanced spaciousness, comfort and safety compared to the outgoing model, the weight of the bodyshell has been kept practically the same for the new E-Class Cabriolet thanks to intelligent lightweight construction. High-strength steel alloys account for around 60 percent of the weight. In the case of the side skirts alone, this high-strength, low-weight material reduces weight by some five kilograms.
The design and construction of the bodyshell are likewise weight-optimised. The energy-absorbing front crossmember, for instance, is inserted into the longitudinal members and no longer ends in flange plates. Tailored Blanks made from sheet steel are used in many places, such as on the underbody, where individual blanks of varying thickness and strength are joined by means of laser welding, or on the transmission tunnel, where the blanks optimise structural behaviour and crash performance.
In addition to these measures, the use of state-of-the-art structural adhesive plays a further important part in enhancing the body strength. The total length of the high-strength adhesive seams used in the bodyshell of the E-Class Cabriolet amounts to around 70 metres.
Mercedes-Benz employs innovative “RobScan” robot-guided laser welding for the E-Class Cabriolet. This new welding technology allows a very high welding speed with optimum weld quality. The technology is used in the rear centre section of the Cabriolet – with a total of 26 welds.
Quality right from the start: visual inspection and NVH optimisation
Alongside the usual quality assurance measures, all joining techniques are subject to process monitoring, which ensures quality in each individual stage of production. The MIG (metal inert gas) solder seams, for instance, are monitored by means of a 3D light-strip sensor. The position, width and completeness of the adhesive seams are checked by an integrated camera when the adhesive is applied. The spot welds likewise undergo a visual inspection: the photos taken by a thermography camera confirm whether the spot welds have the correct diameter.
A further important aspect of quality is a car’s NVH behaviour (noise, vibration, harshness). Here, too, the bodyshell plays a key role. Sturdy transverse sections in the floor assembly – known as transmission tunnel braces – along with optimised lower seat crossmember sections and reinforced engine mounts are just some of the NVH measures implemented to improve vibration characteristics.
Easy to repair: reversible bonnet
Development of the E-Class body focussed on customer benefit at all times, as shown by the Active Bonnet, which reduces the risk of injury to pedestrians and features a reversible design.
Further easy-to-repair and, therefore, money-saving solutions include the bolted-on crash boxes at the front and rear as well as the radiator fastening.
Source: Daimler AG