Body shell, passive safety and vibration comfort (NVH): Robust shell with lightweight design

Jun 24, 2019
Stuttgart/Salt Lake City

The body shell of the GLS has a particularly rigid design – because this addresses many requirements. Especially a long vehicle body with large liftgate is subject to strong torsion moments during fast driving manoeuvres in terrain and must be able to withstand these. The right rigidity is also an important characteristic of a high level of vibration and noise comfort, which suppresses resonances and reduces the transmission of bothersome noises and vibrations. And finally, the stability of the passenger cell is the foundation for effective protection in the event of an accident. Given all these requirements, it is obvious that an SUV body shell cannot be a flyweight. And yet it is to be just as heavy as needed – that is to say as light as possible. This requires a great deal of effort, a number of compromises – and a lot of good ideas.

The high stability of the body shell of the new GLS is the result of a combination of high-strength sheet steel and lightweight materials for assemblies where this achieves the desired attributes, and of the optimal sizing and geometry of all components.

The bonnet and front wings of the new GLS are of sheet aluminium, the strut towers at the front and rear axles are of die-cast aluminium and the side members at the rear are die-cast aluminium partially reinforced with sheet steel. The front-end member is of innovative Organo panelling, fibre-reinforced plastic panels, which after being heated up are formed into three-dimensional components in a press.

Precision-fitted panels with variable material thickness

For the first time, the floor pan of the passenger cell uses so-called tailored rolled blanks. These panels are rolled to different thicknesses, so that the finished, pressed component has the ideal wall thickness in every area. This means that high wall thicknesses only occur where they are really needed - on the new GLS, this is in the area of the centre tunnel that forms the backbone of the floor panel and greatly influences the rigidity of the body shell in a crash.

For the greatest possible rigidity, the body shell components are for the most part bonded and spot-welded, and the flanges connecting the parts are designed to allow joining with minimal tension so that tolerances between the panels are compensated during assembly. The resulting body shell is so torsionally rigid that despite the large roof aperture to allow for the optional panoramic sliding sunroof, it offers more resistance to torsional forces than that of the preceding model.

At the same time, with the same equipment level, the body shell of the new GLS is no heavier than that of the previous generation although it has a longer wheelbase and a substantially greater overall length, and although it is designed to meet the substantially more stringent requirements of the US/NCAP and Euro NCAP safety tests.

Crash requirements take real-life accident data into account

The foundation of the occupant protection is the body shell structure with a particularly rigid passenger cell. For crash safety, it is important that its strength is tailored to the deformation resistances of the front-end and rear-end structures, which on the new GLS were likewise modified compared with the predecessor. All Mercedes-Benz models must comply with in-house safety standards, which in many cases go far beyond the legal requirements. Especially the crash-related requirements are aligned to the so-called Real Life Safety philosophy. These take findings from in-house accident research into account in the development specifications, e.g. the roof drop test.

In addition to the passenger cell, the area of the body structure that accommodates the fuel tank is of particularly rigid design to limit the consequences of a serious collision. To this end, the new GLS uses aluminium castings with steel inserts, which limit the deformation of this area ahead of the rear axle even in very severe accidents. In addition, the strength of this area is also important to protect the rearmost seats in a rear-end impact.

PRE-SAFE® with new functions

For many years PRE-SAFE®, the preventive occupant protection system, has supplemented the classic design measures. The result is comprehensive protection that starts well before an accident and is still effective after the accident, as in the new GLS. The extensive driving assistance systems of the new GLS, as well as the sophisticated crash sensor system, enable PRE-SAFE® to recognise a likely impact in even more situations than before. The protective effect of the systems is improved with a precisely coordinated response of the restraint systems and a number of other measures. As a result, PRE-SAFE® is now able to

  • incorporate an impending side-on collision, for example, in the event of accidents at junctions by means of the close-range radar sensors (PRE-SAFE® Impulse Side, part of the Driving Assistance Package Plus),
  • recognise an impending impact of a vehicle following behind by means of the radar sensors in the rear bumper and warn its driver with rapidly flashing hazard lights. At the same time, the PRE-SAFE® measures of the occupant protection systems are initiated. If the vehicle is stopped, the brakes are locked to keep it in place and reduce the forward jolt and thus the risk of whiplash and a secondary collision (PRE-SAFE® PLUS).

In the new GLS, PRE-SAFE® also protects the passengers in an area whose endangerment has been discussed rather infrequently: the hearing. If a probable impact is detected, the standard-fit PRE-SAFE® Sound system transmits a noise signal through the sound system of the vehicle, which can trigger a reflex. It causes the stapedius muscle in the inner ear to contract and muffles the noise level of a major collision.

Restraint systems: state-of-the-art belts and airbags

The GLS has 3-point inertia-reel seat belts with belt tensioners and belt force limiters on all outer seats, including those in the optional third seat row. The centre belt in the second row is a standard three-point seat belt. The belts of the front seats are equipped with reversible PRE-SAFE® reel tensioners. For child seats, all three seats of the second row and the two outer rear seats of the third row are fitted with i‑Size or ISOFIX child seat attachment points, depending on the country.

For severe frontal collisions, the airbags in the new GLS include a driver knee airbag, a driver airbag and a front passenger airbag. The latter is automatically deactivated by sensors when a rear-facing child seat or an unoccupied seat is detected. The window airbags can also be activated in a frontal collision involving a lateral component.

A severe side-on collision usually triggers the airbags only on the side facing the impact: Window airbags cover the side windows all the way to the D-pillar from the roof and thereby protect all the passengers on outer seats. Thorax-pelvis side airbags for driver and front passenger are standard, side airbags for the outer seats of the second seat row are optionally available. The airbag control unit also recognises a roll-over and triggers, e.g. belt tensioners and window airbags, if needed.

High level of vibration and noise comfort

The rigidity of the body shell structure of the new GLS is key for the noise and vibration comfort. Excitations by the suspension, the vehicle's mechanical components themselves and the airstream can be dampened and muffled by many measures, but the decisive factor for the perception is how the body shell structure responds to the excitation. That of the GLS has natural frequencies that are far from the resonances caused by the typical excitation frequencies of wheels and powertrain. The areas carrying the elastomer bearings of the powertrain and the subframes of both axles are particularly rigid. This means that these assemblies and their vibrations are very effectively isolated from the body shell.

The geometry of the engine and suspension mounts was redesigned to transmit less vibrational energy to the body shell structure. The housing of the electro-mechanical steering was also made more rigid, so that steering and road noises transmitted into the interior are reduced.

The insulation of transmitted sound from the powertrain to the passenger cell has also been optimised. The firewall insulation is injection-moulded rather than deep-drawn, and therefore does not have the uneven wall thicknesses resulting from the stretching of deep-drawn sheet metal. So despite its complex shape extending to the side area of the A-pillars and the windscreen cross-member, there are no acoustic weak spots. The weight per unit area of this noise insulation is locally configured for the actual noise input – heavy where needed for effective insulation, but with a weight-saving overall design. Insulation is augmented by the powertrain partition of sound-absorbing plastic and the engine compartment insulation. In terms of thermal and acoustic insulation, these are configured to suit the engine variant.

The insulation measures for the vehicle floor and wheel arches are also specific to each engine variant. Computer simulations were used to configure the torsional dampers and isolation elements, and incorporate reinforcements into the floor assembly. All these measures were rounded off with insulating membranes, foam-lined cavities and floor carpeting that performs far more functions than just a floor covering: it consists of a four-layered acoustic structure with foam, a heavy layer, matting and finally carpet.