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Bodyshell and safety: Exemplary occupant protection and aerodynamics

• Self-supporting body instead of structural frame

• PRE-SAFE® and innovative NECK-PRO head restraints as options

• Nano-technology paint offering improved scratch resistance

• Lowest Cd value in this vehicle class

The assignment to develop a modern off-roader with a future-oriented concept was taken very seriously by the bodyshell developers at the Mercedes-Benz Technology Center. Nothing remained sacred, and all the body parts and components were re-examined. Safety, lightweight construction, operating reliability, handling stability, aerodynamics and long-term durability were just some of the many aspects which had to be considered and reconciled, even though some requirements seem contradictory at first sight. However, resolving apparently conflicting aims made the M-Class project all the more interesting for the engineers in Sindelfingen, and encouraged them to redouble their efforts.

A self-supporting, rigid bodyshell creates the conditions for safe handling and a high level of ride comfort, and forms the basis for even better occupant protection and modern lightweight construction methods. This is where the new M-Class differs from the preceding series with its structural frame. In order to cope with the high loads occurring in off-road operations, the highly stressed areas between the vehicle suspension and the bodyshell were redesigned and – above all - redimensioned.

One indication of the bodyshell’s qualities is the considerably higher torsional rigidity, which is now 2.18 millimetres per metre (static) and therefore significantly betters that of the preceding series. Accordingly the bodyshell makes an important contribution to the noticeably improved vibration comfort and excellent handling stability of the new M-Class. This outstanding torsional rigidity is in part due to a continuous frame structure in the roof area, which is used to attach the tailgate and also forms a robust connection between the side walls, floor and rear roof frame. This "D-ring", as it is known, literally has a firm grip on the entire bodyshell and increases its torsional rigidity.

The body of the M-Class has been generally redimensioned: the new Off-Roader is 150 millimetres longer, 71 millimetres wider and five millimetres lower than its predecessor. The increased length is mainly due to the 95-millimetre longer wheelbase and the 56-millimetre longer rear overhang. Here is a look at the new dimensions compared to the preceding series:

	New M-Class	Preceding series
Length	4780 mm	4630 mm
Width	1911 mm	1840 mm
Height*	1815 mm	1820 mm
Overhang front rear	865 mm 1000 mm	866 mm 944 mm
Wheelbase	2915 mm	2820 mm

*with roof rails

Two thirds of all body panels made from high-strength steel alloys

The latest production processes and the intelligent use of materials have made progress possible with respect to lightweight construction, tolerances and ease of repair. While conventional steel continues to predominate in the material mix for the new M-Class, Mercedes-Benz has made a five-fold increase in the proportion of high-strength alloys, which offer maximum strength for the minimum panel thickness and weight. In terms of total body weight, 62 percent of all the body panels are now of high-strength steel alloys. Some of these, e.g. those of dual-phase steel, are even in the "very high strength" category. The special, two-phase microstructure of this alloy is capable of withstanding very high loads and therefore contributes significantly to the exemplary stability of the front end and passenger cell.

Numerous sheet metal components in the bodyshell of the new M-Class are assembled according to the "stress-relieved joining" principle, which allows a high level of precision. For example, the flanges at the edges of steel panels are designed in such a way that any tolerances are already compensated when positioning the sheets together, which permits body components to be welded together without significant stresses. This joining technique and the precise design of the body components also makes a major contribution to corrosion protection, as it substantially dispenses with additional brazed connections and MAG-welding seams which are generally seen as areas particularly vulnerable to corrosion.

Fully galvanised body panels and nano-technology paint with improved scratch resistance

As in all the latest Mercedes passenger cars, the long-term corrosion protection of the new M-Class is based on fully galvanised sheet metal which is also organically coated on both sides, depending on the area of application. This coating also contains rust-inhibiting zinc pigments. Highly stressed structural areas of the bodyshell are also protected by cavity preservation, and the welding seams are carefully sealed. The six-section plastic underbody cladding, which gives protection against stones, water and soiling, makes it possible to dispense with a conventional PVC underbody coating. The wheel arch liners of three-millimetre thick plastic likewise give protection against stone impact.

Mercedes-Benz also makes a major contribution to exemplary long-term quality and value retention with an innovative, more scratch-resistant clear paint coat based on nano-technology. This innovative paint system, which had its world debut with Mercedes-Benz, is standard equipment for the new M-Class; it is used for both metallic and non-metallic paint finishes.

Thanks to remarkable advances in the science of nano-technology it has been possible to integrate ceramic particles measuring less than one millionth of a millimetre into the molecular structure of the paint binder. These nano-particles achieve a three-fold increase in the scratch-resistance of the paint finish, ensuring a visibly improved and lasting sheen.

Front-end structure with crash boxes and robust structural members

Dispensing with the previous structural frame has made it possible to design the front-end structure of the new M-Class to passenger car standards. This particularly benefits occupant safety during a frontal collision.

Two straight side members of high-strength steel reinforced with two interior sheet steel liners, a front element and two outside members above the wheel arches are the most important energy-absorbing features of the front end. Energy is absorbed in stages, depending on impact severity. The front end is equipped with two crash boxes of high-strength steel which absorb the impact energy in the event of a frontal collision at low speeds (up to 15 km/h).

The crash boxes are bolted to the side members and can therefore be inexpensively replaced without welding in the event of damage. All the other components in the front end are also bolted together for ease of repair.

Only at higher impact speeds (above 15 km/h) are the robust side members in the front end activated to form a deformation zone. If the body structure is subjected to one-sided loads, i.e. during an offset frontal impact, an aluminium section and other transverse connections in the front end ensure that the impact forces are also dissipated to the unaffected side of the vehicle. During an offset crash the second level of side members above the wheel arches also serves to dissipate impact energy.

The high-strength steel subframe which carries the front axle, steering gear, engine and transmission is also specifically deformed during an impact, and thereby makes a major contribution to energy absorption. The front wheels are also integrated into the safety concept; during a collision they are supported by the robust side walls and direct impact forces away from the passenger cell.

Effective all-round occupant protection

While the front-end structure is designed and constructed to deform and absorb energy during an accident, the passenger cell of the new M-Class forms a protective area with a high deformation resistance. The floor assembly, roof pillars, side members and side walls are the most important components of this rigid safety zone:

• The main floor consists of three assemblies. The centre section - the transmission tunnel – has a higher steel thickness and therefore acts as the backbone of the floor structure. The areas on both sides of the tunnel are reinforced with cross-members which provide a solid mounting base for the seats and ensure a high level of lateral stability, which particularly benefits occupant protection in the event of a lateral collision. In addition a diagonal member extending from the firewall to the B-pillars strengthens the floor assembly.

• The external side panels and multi-piece interior side wall components form the multi-layered cross-sections of the roof pillars, roof frame and side members.

• The reinforced outer shells of the side walls are one-piece units; the inner shells consist of several steel sheets which are particularly robust at the nodal points where the roof pillars join the roof frame and the side members.

In addition to aero-acoustics, sound-absorbing foam components contribute to the excellent noise comfort on board the new M-Class. These are concealed at various points of the bodyshell – 34 such foam elements are integrated into the two side walls alone.

Easy-repair rear-end design

The rear side members of the new M-Class are designed as continuous, closed box sections with gradually reducing material thickness. These make a major contribution to occupant safety during a rear-end collision. The fuel tank is located in an impact-protected position in front of the rear axle.

In the interests of repair-friendly design the rear end also features steel crash boxes and a deformable aluminium cross-member. These absorb impact forces during minor collisions (up to a maximum of 15 km/h) so that the main body structure remains undamaged. Bolted connections allow damaged crash boxes and the cross-member to be replaced inexpensively.

The world’s first off-roader with PRE-SAFE® anticipatory occupant protection

With adaptive front airbags and belt force limiters, belt tensioners, sidebags and windowbags, as well as newly developed crash-responsive head restraints (optional), the M-Class is well-prepared for all types of accident where occupant protection is concerned. But the developers of the Off-Roader were not satisfied to leave it at that. They are offering customers an even more intelligent system which can already become active in the few seconds preceding an impending accident: PRE-SAFE®.

With this anticipatory occupant protection system, which Mercedes-Benz first realised in the S-Class in 2002, the M-Class underlines its role as a trendsetter and example. No other off-roader offers this unique, trailblazing technology.

PRE-SAFE® creates the link between active and passive safety. It is networked with Brake Assist and the Electronic Stability Program (ESP?) and recognises critical handling situations which might lead to an accident. PRE-SAFE® intervenes in such dangerous situations within milliseconds, preparing both the occupants and the vehicle for the possible collision:

• The seat belts of the driver and front passenger are pulled taut as a precaution.

• If the electrically adjustable front passenger seat with memory function (optional) is adjusted to an unfavourable fore-aft setting or cushion/backrest angle, it is moved to a better position.

• The sunroof is closed if there is a risk of the vehicle rolling over.

Thanks to these precautionary PRE-SAFE measures the vehicle occupants are in a better seating position even before the accident occurs, enabling the seat belts and airbags to work more efficiently. If the accident is avoided, the preventive tensioning of the seat belt is automatically relaxed and the passengers are able to return the seat and sunroof to their previous positions.

Adaptive airbags and belt force limiters

The restraint systems also respond according to need on the basis of an efficient sensor system. In addition to the central crash sensor in the centre console, Mercedes-Benz employs up-front sensors for this purpose; these are located on the radiator cross-member in the front end structure, and their remote positioning in the front end of the body enables them to detect the severity of a collision even earlier and with greater accuracy. This makes it possible to shorten the time that elapses between the moment of impact and the deployment of the airbags and belt tensioners even further. In other words, the seat belts are tensioned at a very early stage so that the occupants are optimally linked to the passenger cell and take part in the vehicle deceleration process during an impact.

The electronic control unit also uses the information from the up-front sensors to deploy the driver and front passenger airbags sooner and as the situation demands. This takes place in two stages, depending on the severity of the impact: during a minor frontal impact the electronic control unit only ignites one stage of the two-stage airbag gas generators, 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 slight delay. As a result, the airbags inflate at a higher pressure to provide the occupants with the level of protection required in a collision of this severity. By virtue of a newly developed, energy-absorbent and telescopic steering column which compresses by up to 100 millimetres during a collision, the driver is given far more space for forward motion.

The belt force limiters are also adaptive, i.e. they operate according to the accident situation: if the sensors send information that a frontal collision is severe, the seat belts are tensioned and reach their maximum retaining power. Briefly afterwards the force limiters switch to a lower level of force, which slackens the belts slightly and allows the front occupants to sink more deeply into the airbags. This reduces the loads acting on the chest area.

Newly developed crash-responsive NECK-PRO head restraints

Mercedes-Benz has perfected occupant protection during a rear-end collision with newly developed, crash-responsive NECK-PRO head restraints for the driver and front passenger (optional), which celebrate their premiere in the M-Class. These are likewise linked to the control unit: if the sensor system detects a rear-end collision with a defined impact severity, it releases pre-tensioned springs inside the head restraints which immediately cause these to move forward by about 40 millimetres and upwards by 30 millimetres. This means that the heads of the front occupants are supported at an early stage, reducing the risk of whiplash injuries. After NECK-PRO activation the head restraints can be unlocked and returned to their original position using a tool supplied with the car.

Sidebags and windowbags as standard

With standard-fitted sidebags for the driver and front passenger, as well as windowbags, the restraint system of the new M-Class provides two further, highly-effective systems whose protective effect is complementary: while the sidebags primarily reduce the loads acting on the chest area, the windowbags provide head protection over a wide area for both front and rear occupants. During a lateral impact the windowbag inflates and extends from the front to the rear roof pillar like a large curtain. Accordingly it also gives protection against objects which might penetrate into the vehicle interior during a crash. Sidebags are also optionally available for the rear.

A special sensor system is responsible for deployment of the sidebags within milliseconds: in addition to the central sensor in the interior, satellite sensors located in the side members provide information about the severity of a lateral collision.

The sophisticated crash sensor system in the new M-Class is rounded off by a roll-over sensor which activates the belt tensioners and windowbags during certain types of lateral impact. A special, laminated coating on the weave ensures that the windowbags remain inflated for some time after a roll-over.

The occupant restraint system of the new M-Class at a glance

	Front seats	Rear seats
Inertia-reel seat belts	o height-adjustable	o
Belt tensioners	o	o
Belt force limiters	o adaptive	o also for centre seat
Head restraints	o adjustable for height and angle, optional crash-responsive NECK-PRO head restraints	o
PRE-SAFE®	optional	--
Front airbags	o with adaptive control according to accident severity	--
Sidebags	o	Optional
Windowbags	o
Automatic child seat recognition	optional in front passenger seat	--
ISOFIX child seat anchorage	--	o
Front passenger recognition	o	--

o = Standard

Cd value reduced by 15 percent

A low fuel consumption, a high level of handling safety, good noise comfort – the excellent results the new M-Class can boast in these disciplines is also the result of painstaking, detailed work in the wind tunnel. Designers, aerodynamics specialists and engineers from other areas worked together closely to ensure that the Off-Roader was at its best in every respect, and with convincing results:

• The coefficient of drag (Cd) is 0.34, which is 15 percent below the figure for the preceding series. This means that the new M-Class has the lowest wind resistance of any off-roader.

• Mercedes engineers have reduced the lift at the rear axle (cRA) to 0.033 – 70 percent less than for the previous M-Class – making an important contribution to exemplary handling stability, safe braking and good directional stability.

	New M-Class*	Preceding model
Cd-value	0.34–0.36	0.40–0.42
Frontal area (A)	2.81	2.78
Air resistance cW x A	0.96	1.11
Rear axle lift cAH	0.033	0.110

*Measured with road simulation

These results are the culmination of various aerodynamic measures in almost every area of the bodywork, but these advances are also based on the latest test procedures, which allow a particularly realistic analysis of the airflow characteristics. The wind tunnel tests on the new M-Class were carried out at the University of Stuttgart, with the wheels turning: two steel belts powered the wheels to simulate the aerodynamic interaction between the road surface, the tyres and the bodywork, for example to examine the influence of turning wheels on the wind resistance and lift forces – particularly at high speeds.

Major details which contribute to the exemplary Cd value include the following:

• The aerodynamically efficient shape of the front apron directs the airstream to the sides for an optimal flow around the front wheels. This component was extended to the rear so as to cover part of the engine compartment and prevent turbulence from developing around the engine itself. In the diesel models the engine compartment is fully encapsulated.

• Effective sealing around the radiator grille. This improves the airflow while making more efficient use of the cooling air.

• The exterior mirror housings, whose shape was optimised in the wind tunnel.

• The wheel spoilers reduce lift forces.

• The smooth underfloor panelling extending from the engine compartment to the rear axle. This ensures that the airstream is able to flow beneath the body without turbulences. The areas beneath the fuel tank and between the exhaust silencers are also covered with plastic panels.

• The rear apron is equipped with a special diffuser between the exhaust tailpipes.

• The optimised spoiler edge on the tailgate and the airflow breakaway edges on the rear side windows and tail lights influence the airstream so that it peels away from the vehicle as required .

Aero-acoustic analysis based on noise index

Aerodynamics is a discipline with many aspects. Research in the wind tunnel not only affects fuel consumption and handling safety, but also the level of comfort experienced by the vehicle occupants – the noise comfort. The airstream flowing around the car at speed can make itself felt in many ways, and impair travelling comfort with loud hissing, high-frequency whistling or sonorous booming. It is the task of aero-acoustic specialists to analyse and assess this background noise and eliminate the sources of intrusive sounds. The progress they have made is audible in the new M-Class.

The doors were a particular focal point for the aero-acoustic technicians. Care was already taken during the design stage to ensure that the outer and inner skins for a rigid, vibration-resistant structure by virtue of various reinforcing sections, and that the window frames in particular are extremely robust. In this way airstream-induced movement of the window frames at high speed was reduced by more than half. In addition the door edges feature a continuous double seal – and in some areas even a triple seal – to ensure that no wind noises are audible. The side windows also make a major contribution to noise insulation: the glass thickness has been increased by 0.3 mm to 4.1 millimetres compared with the preceding series.

The roof pillars, roof trim strips and exterior mirror housings were also subjected to aero-acoustic examination using special microphones, then contoured so that no disturbing wind noises are generated.

Mercedes engineers summarise the individual results of these aero-acoustic measures in the form of a noise index. This not only takes measured values such as volume and frequency into account, but also the subjective perception of noises which may not be loud but can certainly be irritating. The noise index for the new M-Class has been lowered to 50, about 50 percent down on the preceding series, and matches the exemplary level for the E Class. The new M-Class therefore occupies a leading position among off-roaders in terms of noise comfort as well.

Unimpaired visibility in the rain

Aerodynamics also makes an important contribution to driving safety with measures which allow the driver unrestricted visibility through the front side windows in the rain. Mercedes engineers achieved this by directing the rainwater to the rear, downwards or to the sides. In the new M-Class the side windows in the driver’s main field of vision remain clear because the rainwater hitting the windscreen is collected in drainage channels on the A-pillars, then conducted to the rear across the roof with the help of the slipstream and drained away downwards by a channel at the rear edge of the roof. This even works well during a heavy downpour. The housings of the exterior mirrors are designed in such a way that rainwater flows to the outside along an unobtrusive, continuous channel and drains away.

Windscreen wipers from the wind tunnel

The windscreen wipers of the new M-Class were also developed in the wind tunnel: they are known as aero-wipers. Instead of the articulated retention system used for conventional wiper blades, in which the rubber blades are claw-mounted, the aero wiper consists of a one-piece rubber section with an integral spoiler and externally mounted spring rails; these precisely follow the curve of the windscreen. The spring rails ensure an even distribution of wiper pressure along the entire length of the wiper blade, so that it always operates with the greatest possible contact pressure. The result is significantly better wiping quality, even in heavy snow. Dispensing with the conventional claw-mounted system, which is prone to icing in winter conditions, reduces the overall height of the wiper blade by almost half. This produces a noticeable reduction in windscreen wiper noise.

The new M-Class is equipped with an efficient two-arm wiper system with special kinematics: while the driver’s-side wiper arm moves around a fixed axis, its counterpart on the passenger side executes an additional lifting movement to wipe an even larger area of the windscreen. This provides the driver with optimal visibility. A rain sensor is standard equipment in the Off-Roader.

Bi-xenon headlamps with Active Light System and cornering light function

Good visibility means a high level of safety. In view of this, Mercedes engineers have also improved the headlamps of the M-Class. In addition to the standard halogen projection headlamps for main and low beam, which are automatically switched on or off by a sensor on the windscreen, fog lamps in the bumper panelling and ambient lighting in the mirror housings, other high-tech lighting systems are optionally available for the new Off-Roader. The bi-xenon headlamp package offers two additional functions which make driving even safer in the dark:

• Active Light System: The headlamps follow the steering movements of the driver and rapidly pivot to the relevant side when the car enters a bend. This improves road illumination by up to 90 percent: whereas the normal illumination range when entering a bend with a radius of 190 metres is around 30 metres, this increases by a further 25 metres with the new headlamp technology. Because the light distribution corresponds to the current steering angle, the driver recognises the course of a bend sooner than with conventional low-beam headlamps. Active headlamps work on both main and low beam.

• Cornering light function: This function is activated when the driver uses the indicators or turns the steering wheel, and illuminates the side area ahead of the vehicle to an angle of up to 60 degrees with a range of about 30 metres. In this way the cornering light function makes areas of the road visible which would remain dark with conventional headlamp technology when turning a corner. This means that pedestrians and cyclists can be seen clearly even in the dark. The driver also has better orientation when negotiating bends at slow speeds (up to 40 km/h). The oval lamps of the cornering lights are accommodated in the bumper panelling; they also function as fog lamps.

The equipment package containing bi-xenon headlamps, Active Light System and the cornering light function also includes dynamic headlamp range control and a headlamp washer system using high-pressure water jets.

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