Passive safety: Optimum protection before, during and after an accident

Mar 5, 2015
In an accident the high structural safety of the body plus the extensive equipment with up to nine airbags and further intelligent restraint systems offer optimum protection. The preventive occupant protection system PRE-SAFE® is available on request.
The bodyshell structure provides the basis for the high standard of passive safety, both in terms of material and with regard to its structural design. Key features of the front-end structure are the systematic implementation of an available crash length, load distribution over several planes, the new bulkhead and floor concept and the subframe as a deformation element.
A total of three longitudinal member planes – consisting of the straight front longitudinal members with inserted crash boxes made from extruded aluminium box sections, a second, upper plane as an extension of the load-bearing structure at the A-pillar node in the mirror triangle sector and a third plane at the bottom in front of the subframe – enable the controlled reduction of impact energy.
The subframe provides the torque support for the transverse engine/transmission block and serves to attach the components for the front axle and steering. It consists of several steel plates, some in ultra-high-strength steel, and a hydroformed tube. In order to optimise its deformation properties, the subframe is connected via two aluminium struts leading forward to the aluminium radiator mount located under the front end. In the event of a frontal impact, forces can be discharged at an early juncture into the subframe via this third load path, in order to ensure the best possible energy dissipation.
A crash wedge which is fitted at the rear of the front wheel arches helps to ensure that the wheels do not slide under the front doors in the event of a high-impact crash, irrespective of the turning angles. This means that it will be possible to open the doors, even after a serious accident.  The bulkhead also incorporates a special feature: so-called "skate runners" in front of the two middle longitudinal members discharge forces into the floor. The continuous floor structure consists of a total of four straight longitudinal members. The tunnel roof reinforcements at front and rear combine with the tunnel to provide a further closed profile supporting the front end.
Rigid side structures and defined deformation management help to safeguard the survival space for occupants in the event of a side impact. Elements here include a member fitted diagonally in the rear footwell between centre tunnel and floor side wall which is intended to prevent the floor from being torn open in the event of side impact with a tree. In the upper sector of the B-pillar high strength steels are used in order to ensure that as little intrusion as possible occurs, and that the passenger cell remains as intact as possible in the event of a side impact. In the lower sector, on the other hand, the B-pillar is designed to be softer, in order to enable energy reduction. In this manner, deformation is directed to a few critical areas beneath the seating surface and transferred to the other vehicle side via the tubular structures in the front seats, transversal members and massive transmission bridge in the main floor.
The CLA Shooting Brake completes the stringent Mercedes-Benz crash test schedule. This includes not only some 30 different impact configurations, which are laid down as requirements for safety ratings and international type approval, but also nine proprietary crash tests, such as the roof-drop test or the pole impact test, developed by the brand itself.
Extensive pedestrian protection measures including active bonnet
The protection of those road users who are most at risk has always been a top priority during the development of Mercedes passenger cars. The CLA Shooting Brake features numerous protective measures designed to help lessen the risk of injury to pedestrians. For example, in order to reduce the loads which arise in the event of a pedestrian impact on the bonnet of the vehicle, the deformation space between the bonnet and the components beneath it has been optimised. This is achieved in part through the appropriate positioning of components such as control units or fluid reservoirs in the engine compartment.
In addition to this an active bonnet is used in the CLA. In the event of a collision with a pedestrian, sophisticated sensors combined with intelligent algorithms trigger pyrotechnical actuators in the area of the bonnet hinges. These raise the bonnet by 60 millimetres. The additional space which this creates between the bonnet and the components in the engine compartment means that a pedestrian's head is subject to comparatively low internal acceleration forces on impact.
The deformation characteristics of the bonnet have been developed specifically to meet these requirements. Reductions in the impact loads can be achieved by using aluminium and reinforcing the inside face of the bonnet.
Intelligent restraint systems: belt force limiters in the rear, too
The CLA protects its occupants with up to nine airbags. The standard complement comprises driver and front-passenger airbag, a kneebag for the driver, large thorax-/pelvisbags incorporated in the seat to protect the chest, stomach and pelvis area, and windowbags. The windowbags extend over both rows of seats to the A-pillar triangle.
Here is an overview of all the airbags:
  • Airbags for the driver and front passenger: for the driver the volume is approx. 64 l and for the front passenger approx. 122 l. Both airbags are fitted with a two-stage gas generator. Depending on the severity of the impact, two performance stages can be activated, with a time delay between ignition of the first and second stages.
  • In a side impact, the thorax-/pelvisbags help to protect the upper body and pelvis area of the driver and front passenger. They also stabilise the entire body. This means that the risk of severe injuries in an accident can be reduced. The airbags are triggered if corresponding acceleration or deceleration processes acting on the car are detected by the central control unit and the lateral satellite sensors. The thorax-/pelvisbags are located in the seat backrests and are deployed at chest height between the side of the car and the occupant.
  • The windowbags between the A-, B- and C-pillars can protect the head area of the driver, front passenger and rear passengers sitting on the outside. In a side collision the windowbag covers the side windows and can thus cut the risk of a head impacting against the lateral face of the car or against intruding objects. The airbag is triggered at the same time as the thorax-/pelvisbags and always on the side facing the side impact. The large-surface windowbag is draped over the side windows like a curtain from the A- to the C-pillar.
  • The kneebag on the driver's side helps to protect the legs in a collision, and additionally stabilises the entire body. It can thus mitigate potentially severe injuries in an accident.
  • Sidebags in the rear are available on request. In a side impact they can offer additional protection in the chest area for the rear passengers sitting on the outside. The sidebags integrated into the rear side panelling inflate at chest height between the side of the car and the occupants.
As standard the CLA Shooting Brake has a seat occupancy sensor on the front-passenger side. If the pressure-sensitive sensor mat detects that the front passenger seat is unoccupied or occupied by a child seat, the airbag on the front-passenger side is deactivated. A transponder on the child seat is no longer necessary. A display in the centre console provides information on the "AIRBAG ON" or "AIRBAG OFF" status.
Not only the driver and front passenger, but also the rear passengers in the outer seats are protected as standard with belt force limiters and belt tensioners. A torsion bar in the roller mechanism twists when the load on it exceeds a defined level. In this way the belt force is limited and with it the load on the seat occupant.
The CLA's features also include the standard-fit ISOFIX child seat securing system on the outer seats. Child seats can additionally be fixed to the vehicle by means of special anchorage points with top tethers.
The head restraints are integrated into the backrests on the driver's and front passenger seat and help protect against injuries to the cervical spine in a rear impact.
In critical situations: preventive PRE-SAFE® protective measures
The PRE-SAFE® anticipatory occupant protection system is available in the CLA-Class as an option. The system offers the car occupants optimum protection through preventive measures in case an accident should occur. In order to do this PRE-SAFE® permanently evaluates the sensor data from other systems such as the Electronic Stability Program ESP® and the Brake Assist BAS and can therefore, for example, register unusually heavy under- or oversteering and emergency braking.
If a critical driving situation is detected, PRE-SAFE® can initiate various functions in fractions of a second. In the CLA these include the reversible belt tensioning for the driver's and front passenger seat plus the automatic closing of open side windows and the optional panoramic sliding sunroof.
After an accident: automatic emergency call and rapid rescue
After an accident various measures can lessen the consequences of an accident and facilitate rapid rescue. This includes unlocking the doors and lowering the front side windows by 50 millimetres, so that the interior is better ventilated following activation of the airbags. Depending on the type of accident and the severity of the impact the fuel pump is switched off. The hazard warning lamps are automatically switched on, as is the emergency lighting for better orientation in the interior.
In an accident, the Mercedes-Benz emergency call system, a "Mercedes connect me" service, automatically connects the occupants with the Mercedes-Benz Emergency Call Centre and sends the position and condition of the vehicle to the rescue operations centre. A rescue vehicle can then be dispatched immediately. The system works automatically, but can also be triggered manually.