Safety - Safety takes a front seat

Mar 15, 2004
  • Two-stage front airbags and belt force limiters
  • Newly developed head/thorax side airbags in the seat backrests
  • Sophisticated all-round protection for the passenger compartment
Real-life safety – this tried-and-tested practical approach to safety as applied by Mercedes-Benz has been reinforced and thereby consistently honed over the course of recent years by a series of groundbreaking new developments from the Stuttgart-based vehicle brand’s engineers. The new SLK-Class reaps the benefits of the progress that has been made: the safety technology on board the sporty roadster is in line with the very latest findings from the field of accident research, and sets new benchmark standards for this vehicle segment.
The Mercedes concept centres around restraint systems capable of adapting to different types of collision. The result is a further decrease in the loads placed on occupants in the event of a severe collision, as well as optimisation of the protective functions triggered following more moderate collisions. It is in this key aspect that the new SLK Roadster differs noticeably from its predecessor, which was itself deemed to be one of the safest sports cars on the road. On the new model, both the driver and passenger airbags feature two-stage operation, as do the new two-seater's belt force limiters. This means that the systems are deployed in such a way that they are able to offer the greatest possible protection for the situation at hand.
The driver's airbag is fitted with a two-stage gas generator: in the event of a minor impact at the front, the control unit only ignites the first stage and the airbag inflates more gently. Should the control unit detect a severe frontal collision, however, it deploys the second stage of the gas generator after a delay of some 5 to 15 milliseconds. As a result, the airbag inflates at a higher pressure to provide the driver with the level of protection required in a collision of this severity.
The airbag on the passenger side operates on precisely the same principle, as well as incorporating brand new technology which further improves the way in which the airbag inflates evenly to cover a wide area. In addition to this, the initial pressure increase of the two-stage generator is reduced, meaning that the airbag inflates even more gently. This shows just how much consideration Mercedes engineers have given to the fact that children are often seated in the front passenger seat, where the newly developed airbag now offers them even more protection.
The belt force limiters, which make up part of the standard specification of the new SLK-Class, are also able to adapt to the severity of the collision. Should the control module detect a high impact load, it will reduce the restraining force acting on the belt strap after just a short time; as a result, more of the seat belt strap will be fed out and the airbag will be able to cushion the seat occupant more effectively.
Head/thorax airbag: new air cushion integrated into the seat backrest
Mercedes-Benz is also equipping the new SLK-Class as standard with the newly developed head/thorax sidebags which have already debuted in the top-of-the-range SL-Class sports cars. They are accommodated in the backrests of the seats where they are positioned between the sturdy magnesium frame and the cushion holder. Following a side impact, the head/thorax airbag transforms into an asymmetrically shaped cushion in a matter of just 30 milliseconds, with the top edge reaching high above the seat when the airbag is fully inflated. This reduces the risk of the occupant's head colliding with either the side window or any objects that penetrate the interior of the vehicle, regardless of the occupant's seating position.
The middle of the head/thorax airbag slides between occupant and door in the event of an impact from the side or the vehicle rolling over, so that the chest area is also cushioned.
Sensor system: upfront sensors permit quick detection of accident severity
The swift, adaptive response of the restraint system is made possible by a highly advanced sensor system. In the event of a head-on collision, fast response is taken care of by two extra upfront sensors. Their remote positioning on the radiator cross member in the front end of the SLK body enables them to detect the severity of a collision even earlier and with greater accuracy than the central crash sensor on the transmission tunnel. The information from the upfront sensors is used by the electronic control module to shorten the time that elapses between the moment of impact and the deployment of the belt tensioners, to adapt the operation of the belt force limiters to the severity of the collision and to trigger the airbags both earlier and in two stages, depending on the situation.
Rapid activation of the head/thorax airbags in the seat backrests is the task of two additional satellite sensors positioned on the outer edges of the bodyshell's seat cross members which work together with the central crash sensor.
Quite apart from this, the new SLK-Class is also equipped as standard by Mercedes-Benz with a roll-over sensor, which reliably recognises this type of accident and relays its data to the restraint systems' central control module. In response to this signal, the micro-computer activates the belt tensioners and the head/thorax sidebags in the SLK seats, regardless of whether the vario-roof is open or closed.
Still to be found on the list of standard equipment for the SLK-Class are a sensor pad in the passenger seat which detects whether the seat is occupied, as well as the inhouse-developed automatic child seat recognition facility. The child seat recognition facility comprises special aerials in the seat cushion which exchange data with the transponders that are fitted in the base of special child seats. Using this data, the central control module is capable of detecting when a child seat has been fitted, causing it to deactivate the airbag on the passenger side, as deployment of the airbag could otherwise increase the risk of injury.
Bodyshell structure: larger crumple zones at the front end
The bodyshell structure of the new SLK-Class also offers occupants greater safety reserves than the predecessor model. The crumple zones in the front end, which absorb energy in the event of a frontal collision, have now been enlarged and the flow of forces optimised in order to offer even better protection for the passenger cell. This fact is clearly demonstrated by the results of numerous crash tests which the new Roadster successfully completed as part of a development process that lasted several years. As well as the statutory test specifications of various countries and the requirements laid down by the Euro and US NCAP (New Car Assessment Program), the SLK-Class was also subject to Mercedes' own, extra-strict test criteria. These test criteria generally reflect the findings of the Mercedes accident research department and allow vehicles to be designed with real-life accidents in mind.
Virtually every safety-related body component has been checked and redeveloped by the Sindelfingen engineers with the aim of optimising geometry, material thickness, joining technology and material quality. The proportion of high-strength steel alloys, which ensure maximum strength combined with a low weight, has been increased to around 42 percent, an important prerequisite for achieving exemplary standards of safety. The majority of the components which determine the bodyshell's crash characteristics are manufactured using high-strength sheet steel.
Front end: split-level side members and sophisticated firewall design
Key features of the structure at the front end include straight front side members and the twin-wall front cross member. These elements form a sturdy composite assembly, so that in the event of a frontal collision where a load is exerted on one side only at the front end (offset impact), the opposing side is also able to absorb some of the energy. Mercedes-Benz engineers have also developed a second side member level which is positioned above the wheel arches and front wheels and has been specifically designed to enhance occupant protection in the event of an offset crash at the front. This construction also takes account of the fact that a new McPherson front suspension is now being used which permits longer crumple zones compared to the dual-wishbone suspension used on the previous model.
The forwards-arching firewall is reinforced by an elaborately designed cross member, which is in turn welded to the front side members. This allows the impact forces which are released following a frontal collision to be distributed over a large area in the vicinity of the firewall. Plus, the front end of the new SLK-Class is equipped with pedal-floor cross members which also link up with the front side members and form a protective barrier in front of the footwell area.
The pendulum support on the master brake cylinder, which was already used successfully on the predecessor model, has now been redesigned; the pendulum support prevents the brake pedal from moving further backwards into the driver's footwell. A generously sized impact absorber made from polypropylene which is positioned below the footrest is also capable of cushioning some of the impact energy.
The frame-style assembly carrier, which holds the steering gear, the engine mountings and some of the wheel location components, is also incorporated into the crash concept of the new SLK-Class and is capable of effectively dispersing energy in the event of a frontal impact. The carrier is bolted to the front side members. In the event of a crash, the wheels are braced by special impact elements positioned in front of the lateral side members.
The passenger cell of the new SLK-Class is built on a sturdy floor assembly with an inserted transmission tunnel made from thick-walled steel, plus an additional locking plate. Straight support sections running underneath the body on both sides link the front end's side members with the rear-end structure. Lateral reinforcement for the floor is provided in the form of two solid supports underneath each seat.
The lateral side members consist of an inner and an outer wall, which are strengthened by bulkhead plates. The cross-sectional area of the side skirts increases towards the rear to produce a large connection with the (concealed) B-pillars. The bolted-in aluminium rear wall, a cross member running underneath it and the vario-roof's solid support together form a robust composite crossways structure at the rear which protects occupants in a lateral collision.
Roll-over protection: sturdy tubular reinforcements in the A-pillars
Engineers at Mercedes also devoted a great deal of attention to roll-over protection in the new SLK-Class. In addition to the characteristic roll-over bars behind the seats, roll-over protection also comes in the form of a high-strength, A-pillar composite structure. At the core of this assembly are two oval pipes made from heat-formed, high-strength steel, which are welded together with the A-pillar's metal walls and extend up to the top of the windscreen frame. These complement the protective effect of the roll-over bars and ensure that the passengers' survival space remains as intact as possible should the vehicle roll over.
To enable them to withstand high loads, the oval tubes are integrated by means of high-strength steel plates into the structure of the A-pillars, which are in turn braced against the lateral side members by sturdy gusset plates. In order to give it extra strength, the material is formed at the steel works while it is still glowing hot.
Just how rigid this new construction is can be plainly seen from the results of the Mercedes-Benz roof-drop test, an internal rigidity test in which the body is dropped onto its roof at a slight angle from a height of 50 centimetres. The vehicle lands with its full weight on one of the two A-pillars, which may only deform slightly if the vehicle is to pass the test. And the new SLK-Class came through this roof-drop test with flying colours.
Mercedes engineers opted for an oval shape for the tubular reinforcement so that they would be able to keep the A-pillars as slender as possible, ensuring minimal restriction of the driver's angle of vision. Thanks to this technique, the all-round view in the new SLK has been improved by around twelve percent compared to the model it replaces.
In the event of a collision from the rear, passengers aboard the SLK will be protected by the box-shaped rear side members made from high-strength steel of varying thicknesses. The Mercedes engineers have therefore been able to precisely define the strength and deformation characteristics of the side members, and to align these criteria with the levels of stress placed on the different body sections.
The fuel tank and filler neck are located above the rear axle. This creates a sufficiently large crumple zone for collisions from the rear. The SLK bodyshell is completed at the rear by a rigid cross member, while the steel spare wheel well forms part of the floor assembly at the rear.