Body and passive safety: Safety and quiet running at the highest level

Feb 2, 2018
Stuttgart/Amsterdam

A rigid and intelligently designed body provides the basis for both crash safety and pleasant acoustics. Safety is also enhanced by specifically deformable crumple zones, and quiet running by numerous other measures. Nonethelesss the weight of the bodyshell has been kept at the level of the preceding model.

The new A-Class is also the first Mercedes-Benz model to have been developed at the new Technology Centre for Vehicle Safety (TFS). The design of the vehicle structures incorporates findings from research into real accidents. Every single bodyshell component was developed according to the loads and stresses encountered, with respect to geometry, material thickness, joining technology and material quality (increased proportion of high-strength and ultra high-strength sheet steel).

The centrepiece of the body's safety concept is the highly rigid passenger cell. Its great rigidity when subjected to accident-induced stress e.g. in frontal, lateral or rear-end collisions and rollovers is above all due to the greater use of high-strength, ultra high-strength and press-hardened sheet steel.

The strength of the entire side wall is increased by a floor assembly with high transverse rigidity. In the front footwells this is brought about by two inner and one outer reinforcing section between the inner shell of the front side members and the central tunnel. There is a continuous seat cross-member in the rear. These are augmented by seats with high transverse rigidity featuring tubular sections in the seat frame.

The upper area of the B-pillar is extremely rigid, with hot-formed ultra high-strength steel, while the lower area is slightly less rigid with deformable, ductile steel. This gives the B-pillars very good deformation kinematics. The steel doors have specific reinforcements, and additional protection is provided by impact elements in the side seat and B-pillar claddings.

The so-called Inertia door handles are the result of in-house accident research. These even more reliably prevent a door from opening during an accident, as the door is locked in place at the handle when subjected to impact-induced forces.

Special measures for the structural safety of the vehicle's front section include the following:

  • the robust firewall cross-member of hot-formed, press-hardened steel between the two A-pillars
  • several parallel load paths for improved load distribution in a partial frontal collision (offset crash)
  • side members at the upper impact level connected to the A-pillars
  • effective load distribution from the front side members by means of specially reinforced A-pillars
  • a middle impact level with side members having specific crash kinematics
  • multi-layered floor structures for optimum energy absorption
  • an additional pedal floor cross-member to protect the footwell
  • continuous floor side members to improve energy transfer into the underbody structure, with a larger cross-section and material thickness than in the previous model
  • compatibility with other vehicles in the design of the front structure in the event of a frontal collision (protection of other road users).

The more compact drive units also play an important role. During a frontal collision, and acting together with the body structure, they allow more homogeneous deceleration and help to ensure the occupant protection for which Mercedes-Benz is well-known. Depending on impact severity, the engine and transmission are specifically displaced and disengaged from the new integral carrier.

Vehicles with a tailgate require a particularly well-conceived rear body structure to compensate for the large aperture. For many years, e.g. in the E-Class Estate models, the structural concept of two peripheral sections has proved successful for Mercedes-Benz: the so-called C-ring at the level of the C-pillar and the D-ring around the tailgate aperture. Thanks to the geometry of the D-ring, it was possible to position the lower section of the C-ring completely below the rear floor while retaining the excellent rigidity of the previous model. This enlarged the load capacity and made a level load area possible.

Restraint systems: partnership with the safety bodyshell

The most important restraint system is the seat belt. The driver and front passenger each have a three-point seat belt with belt force limiter and belt tensioner. In conjunction with the PRE-SAFE® system (optional equipment), the front seats are equipped with reversible belt reel tensioners. Both of the outer rear seats are fitted with a seat belt with reel tensioner and belt force limiter. These excel with a relatively low level of force for small and lightweight occupants. The centre belt is a standard three-point belt.

The new A-Class is equipped with driver and front passenger airbags, a driver's kneebag and windowbags as standard. Unlike in many competing models, the windowbag also covers the A-pillar for more complete passenger protection. Thorax-pelvis sidebags are likewise standard in the front, and available as optional equipment for the rear.

Child safety: automatic deactivation of the front passenger airbag

A pressure sensor in the front passenger's seat cushion can detect whether the seat is unoccupied or whether an infant or other child seat has been placed on the front passenger seat, and in the latter case deactivate the front passenger airbag automatically. Contrary to other systems that require the airbag to be deactivated with a key, this system reduces the risk of incorrect operation. As no special transponder is required, the system can be used for all conventional, rear-facing child restraint systems. The system is standard or optional equipment depending on the market or region.

i-Size child seat attachments are used for i-Size child seats. This internationally standardised attachment system, the successor to Isofix, is provided for the outer rear seats as standard. It improves protection with a fixed connection between the child seat and the vehicle.

Pedestrian protection: active bonnet

Reducing the severity of an impact is particularly important in the case of a collision with vulnerable road users such as pedestrians, as these have no "crumple zone" of their own. This is where the active safety systems of Intelligent Drive come into their own. If an impact is unavoidable, the further developed measures to mitigate the consequences of accidents involving pedestrians can help.

The active bonnet is an important factor in this. The technical basis is a comprehensive sensor system in conjunction with intelligent algorithms in the airbag control unit, which decides when to trigger. After activation of the pyrotechnical actuators, the bonnet is raised on its hinge by a significant 80 mm or so within fractions of a second. This creates additional space between the bonnet and the components in the engine compartment. The protective effect is increased by a comparatively flexible cross-member below the windscreen. This is designed in the form of a shackle, enabling it to give way better in the event of a head impact.

Noise and vibrations: there is strength in serenity

Quiet, vibration-free driving characteristics are a major factor in the grown-up, premium driving impression made by the new A‑C lass. Alongside effective insulation of the suspension from the vehicle body (see section on Suspension) and aeroacoustic measures, the bodyshell itself plays a decisive role.

During the development of the A-Class, particular attention was given to high overall structural rigidity and the connecting points between the body, suspension and powertrain. Significant increases in introduction rigidities were especially achieved at the suspension connecting points that are so important to road roar, and the interior noise level was considerably reduced.

Following examination of the previous model, the guide bearing of the front axle and the rear connection of the subframe were identified as major introduction points. The introductory rigidity at the front axle guide bearing was significantly increased with a compact, rigid integral carrier.

The subframe of the multi-link rear axle is elastically isolated by rubber bushings. The front subframe connection is integrated into the C-ring structure of the bodyshell, and therefore has the rigidity to isolate it. A cross-member is integrated into the multifunction recess to increase the introductory rigidity of the rear subframe connection. This not only gives transverse support to the subframe carrier, but also stiffens the membrane surface of the recess. This reduces sound radiation as a good basis for efficient noise insulation.

To reduce the noise impact to the passenger compartment, the firewall is a high-quality injection-moulded component. In contrast to deep-drawn components, this has the advantage that constant wall thicknesses are possible: large changes in contours do not thin out the material – a classic acoustic weak point. In addition a modular major assembly compartment partition made of plastic is used by Mercedes-Benz for the first time for a platform with transversely installed engines. This makes models with the top engine variants particularly quiet-running.

The comprehensive noise insulation concept of the new A-Class includes components such as the air and coolant management systems or trim parts in the interior being configured so as to improve noise insulation as well. Sound insulation measures also include the specific use of absorbers e.g. in the area of the C-pillars, in body cavities and with spring-mass systems on the wheel arches. The tool used for all sound insulation measures was statistical energy analysis (SEA). This makes it possible to assess the effect of sound insulation concepts at a very early stage with the help of computer models.

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