The technology of the new M-Class: Chassis in detail - Dynamism meets comfort
OFFICIAL PRESS RELEASE
Stuttgart, Germany, Jun 09, 2011 - All models in the new M-Class range offer excellent comfort and superb driving dynamics as standard – both on the road and away from the tarmac. Besides the steel suspension with selective damping, there are also some new chassis developments and innovative dynamic handling control systems available, which are designed to make driving the new premium SUV an even more effortlessly superior experience. These include the AIRMATIC air suspension with Adaptive Damping System (ADS), the ACTIVE CURVE SYSTEM for active roll stabilisation and the ON&OFFROAD package with six driving modes. A comprehensive package of measures has also been included to ensure that the M-Class is at the forefront of its segment for acoustic and vibration comfort too. To ensure that the driving experience continues to be a complete pleasure after hundreds of thousands of kilometres, the development engineers punish both chassis and body in practical trials staged worldwide and in series of selective laboratory tests. Some of the toughest examinations here include the body testing facility and the road-simulation test rig.
A high level of comfort, dynamic on-road handling and excellent off-road capability are ingrained into the new M-Class as standard. The chassis of the steel-sprung M-Class now includes selective damping for the first time, meaning the shock absorbers' compensatory forces are of a flexible nature, rather than having a rigid setting. During normal driving on moderately uneven roads or off-road excursions at low speeds, the system is tuned for a gentler ride to the benefit of the occupants' sense of wellbeing and the vehicle's off-road abilities. In order to keep the occupants feeling at their ease when driving at a brisker pace or performing abrupt evasive manoeuvres, the dampers switch to a firmer setting in such circumstances for a high level of handling stability. The M-Class driver is helped here by an electric steering system that provides the optimum level of power assistance to suit the particular driving situation. The system therefore makes light work of parking or off-road manoeuvres by maximising steering assistance. The SUV also scores here with the tightest turning circle in its market segment. The 180 degree turn is negotiated within a circle diameter of just 11.8 metres. As the speed increases, however, assistance is reduced in favour of greater directional stability.
For those who want to venture off surfaced roads, the ML 350 BlueTEC 4MATIC and ML 350 BlueEFFICIENCY 4MATIC models feature an extensive off-road specification as standard, which considerably enhances their capabilities when the going gets tough. Apart from 4MATIC permanent all-wheel drive and the electronic traction control system 4ETS, this also includes an off-road button that activates a special off-road driving mode:
• To improve traction, the wheel slip thresholds and the shift points of the 7G-TRONIC PLUS transmission are raised
• A flatter accelerator response curve enables more sensitive throttle control
• Off-road ABS ensures optimum braking characteristics, especially on loose surfaces
Start-off Assist and the HOLD function furthermore come to the driver's assistance when performing a hill start, while Downhill Speed Regulation, or DSR, enables the M-Class to automatically maintain the speed set with the cruise control lever on descents.
For anyone wishing to go exploring off the beaten track even more in the new M-Class, there is the optional ON&OFFROAD package.
Comfort system: AIRMATIC air suspension with ADS
The AIRMATIC full air suspension system including the Adaptive Damping System ADS offers a soft basic suspension setup with a low natural frequency, where full spring travel is available even when the vehicle is fully laden. This provides the basis for excellent ride comfort on road combined with noticeably superior handling stability. AIRMATIC compensates for variations in vehicle load and driving state, at the same time as acting as a level control system. Away from the tarmac, the air suspension increases the vehicle's off-road prowess by offering additional ride heights. The body can be raised or lowered with the engine running at any time, either automatically or at the driver's request with a simple turn of the rotary control in the centre console. The system is speed sensitive, meaning that it lowers the body at higher speeds
to minimise aerodynamic drag while increasing handling stability too. The air suspension system's basic components include:
• Air-filled spring struts on the front axle with integral ADS dampers
• Air springs with separate ADS dampers on the rear axle
• Electric compressor with central pressure reservoir and pressure sensor
• Air spring valves
• Electronic control unit
• Sensors for level control and damping control
The adaptive damping system ADS II is a fully automatic, electronically controlled system, which adapts the damping force at each wheel to suit changing requirements using a skyhook algorithm. In contrast to a passive system with a constant damper setting, the vehicle's motion is controlled uniformly in all driving situations, leading to improved handling stability and
a considerable reduction in body movement. The skyhook algorithm regulates the damping forces at each wheel so as to lessen the forces exerted on the body by the movement of the wheels. When there are low levels of body excitation, the M-Class drives with damping level 1 active. Should the body speed exceed a certain threshold, however, the system will switch to the skyhook algorithm and constantly alternate between the second and third damping levels by means of fast-acting solenoid valves in order to counter the body's rolling and pitching movements. Under even more dynamic handling conditions, level 4 is engaged, and when driving in Sport mode, level 4 damping is activated at all times. Damping control at each individual wheel takes place extremely fast – depending on the control command, the valves are capable of setting one of
the characteristic curves described here in less than 0.05 seconds:
Level 1 - Soft rebound / soft compression for comfortable ride characteristics, gentle body movements and little longitudinal and lateral acceleration
Level 2 - Soft rebound / firm compression (skyhook mode)
Level 3 - Firm rebound / soft compression (skyhook mode)
Level 4 - Firm rebound / firm compression: for minimising wheel load fluctuations when cornering and braking, high longitudinal and lateral acceleration for enhanced handling safety
The current driving state is determined using a steering angle sensor, 4 turning angle sensors und the road speed as derived from the ESP® signal and the brake pedal switch. Based on these signals, the control unit calculates the current damping forces required and actuates the appropriate damper characteristic curves. The driver is able to choose between Sport and Comfort modes manually by pressing a switch.
New dynamic handling control system: cornering without rolling
Active roll stabilisation in the form of the ACTIVE CURVE SYSTEM can be optionally combined with both the AIRMATIC air suspension with Adaptive Damping System (ADS) and the ON&OFFROAD package. This system uses active anti-roll bars on the front and rear axles, which it controls automatically as a function of the lateral acceleration, road speed and the ADS Comfort/Sport switch setting. The ACTIVE CURVE SYSTEM compensates for the roll angle of the body through bends, greatly increasing agility and driving pleasure in the process. The system has the additional effect of increasing handing stability and therefore safety, particularly at higher speeds. Further benefits of the ACTIVE CURVE SYSTEM include even greater ride comfort both when cornering and driving in a straight line, together with enhanced off-road abilities. The various control strategies in detail:
• Comfort is improved when driving straight ahead as the rotary actuators decouple the two halves of the front and rear anti-roll bars in this situation, meaning that the anti-roll bars are "open" and do not react to a stimulus on just one side, such as bumps or potholes.
• Increased ride comfort and more dynamic handling when cornering, because the system actively influences the anti-roll bars' torsional moments and twisting angles. The anti-roll bars' torsional moment furthermore remains constant in response to a stimulus on one side, such as is the case when driving over a pothole on the outside of the bend. Thanks to the individual control at the front and rear axles, distribution of the roll momentum can be varied, allowing the self-steering properties to be actively adapted to the prevailing driving situation. The handling characteristics are adjusted for extra agility when driving along country roads and for even greater stability on the motorway.
• When driving in extreme off-road conditions at low speeds, the two anti-roll bars at the front and rear axles are decoupled. This leaves the anti-roll bars "open", allowing greater axle articulation.
The key componentry of the ACTIVE CURVE SYSTEM comprises a belt-driven hydraulic pump and an oil reservoir in the engine compartment, as well as a valve block and active anti-roll bars at both the front and rear axles. In contrast to a passive anti-roll bar, the active variant is split into two in the middle, and the two halves are connected with one another by means of hydraulic rotary actuators. Using the CAN signals relayed to it by pressure sensors and a
lateral acceleration sensor, the electronic control unit regulates the hydraulic pressure.
The complex workings of the ACTIVE CURVE SYSTEM:
The hydraulic pump feeds oil to the system from the oil reservoir. The pressure control valves and directional control valves integrated into the valve blocks at the front and rear axles set the desired pressure and twist the active anti-roll bars in the appropriate direction for the driving situation. Inside the hydraulic rotary actuators that are built into the active anti-roll bars there are six oil-filled chambers, three of which are pressurised for each direction of travel, i.e. a left or right-hand bend. The front valve block additionally assumes the task of distributing the oil flow between the two axles, irrespective of load.
Full programme: ON&OFFROAD package for all eventualities
The ON&OFFROAD package for the new M-Class uses six different driving modes to optimise driving dynamics and handling safety by providing the optimum drive control needed to cope with the wide spectrum of operating conditions encountered both on and off the road. The added benefit compared to the Off-Road Pro Engineering package offered previously is plain to see: whereas before there was one optimum driving mode for the road and one for off-road, drivers now have the choice of an automatic mode, as well as two specially programmed off-road and three on-road modes.
The specific driving modes for all-wheel drive resolve a conflict of objectives that has previously hampered further development of 4MATIC and the 4ETS electronic traction system as well as ESP® and ASR. To take an example: on off-road tracks, it is important that the control systems respond very gently due to the low friction coefficients, but this makes a sporty setup virtually impossible. Conversely, a sporty basic setup impairs performance abilities in off-road terrain. In the past, the engineers always succeeded in finding excellent compromises which worked brilliantly under all conditions and continue to do so. The new ON&OFFROAD package with its specific driving modes, however, enables driving dynamics and handling safety to be optimised almost as far as is physically possible under all operating conditions by maximising networking between the individual control systems.
The system is operated using a rotary control on the centre console, which adapts the AIRMATIC settings and the drivetrain to suit the specific requirements. The six driving modes in detail:
• Automatic - covers the wide operating spectrum of everyday driving
• Offroad 1 - light terrain, tracks, driving over fields
• Offroad 2 - challenging off-road terrain with climbs
• Winter - for driving in wintry conditions on roads affected by freezing rain, snow or ice, or with snow chains
• Sport - for serious driving along winding roads
• Trailer - optimises start-off, manoeuvring and braking characteristics when towing a trailer
Despite so many options, the driver is never in any doubt about mode selection: the setting is made intuitively, while the driving mode selected is visualised in the multifunction display as well as in virtually photo-realistic quality on the screen of the COMAND system. If the driver selects the Winter mode, for instance, the display shows the M-Class in snow. The Sport setting displays a racetrack with red-and-white curbs, while images of off-road terrain with different profiles appear for Offroad 1 and 2. The display is interactive too: steering input and spring travel correspond to the actual values, while the graphics include additional information on the steering, gradient and tilt angles, the AIRMATIC level, the pre-selected speed for Downhill Speed Regulation (DSR), as well as the selected drivetrain settings including reduction gear and differential lock.
Irrespective of which basic mode is set, the reduction gear, differential lock, DSR and the AIRMATIC system's level control can all be adjusted individually. As with the basic settings for the ON&OFFROAD package, it is not possible for the driver to make mistakes here either. Illogical settings, which would have a negative impact on handling stability, safety or traction, are precluded. If case of doubt, the system will revert to automatic mode.
In terms of "hardware", the ON&OFFROAD package comprises an underguard, a two-stage transfer case with reduction gear, an inter-axle differential lock and enhanced AIRMATIC functionality that allows a maximum ground clearance of 285 millimetres and a fording depth of 600 millimetres.
Off-road data at a glance*
Haven of calm: optimum vibration and acoustic comfort
The new M-Class promises relaxed and therefore safe motoring under all possible (and even impossible) conditions. By paying close attention to detail every step of the way, the development engineers in the Mercedes Technology Centre have significantly improved what they call the NVH comfort level (Noise, Vibration, Harshness). This is a decisive parameter for the general sense of wellbeing on board, whilst also contributing to driver-fitness safety, especially on long journeys.
The basis for the high level of NVH comfort is provided by the very rigid SUV bodyshell of the M-Class. The development engineers were faced with some particular challenges here. For example, they had to make allowance for the optional panoramic glass sunroof's large aperture in the construction. The objectives set for drive comfort were no less ambitious. Substantial improvements have been achieved in this regard compared to its predecessor, thanks to features such as map-controlled, hydraulic engine bearings, a resized aluminium transmission cross member with integral vibration damper, as well as the overhauled 7G-TRONIC PLUS with twin-turbine damper and centrifugal pendulum. Potential noise penetration from the engine compartment through to the interior is minimised by a major assembly partition wall made from plastic and the intelligent use of soundproofing to suit requirements. Aero-acoustic enhancements include the vehicle glazing with its high-insulation acoustic windscreen, additional sealing in the side sections, plus the streamlined add-on parts such as the restyled exterior mirror housings.
The measures at a glance:
• Improved rigidity of the front end structure thanks to struts between the upper and lower levels of side members
• Hybrid construction of front module increases comfort at the same time
as reducing weight
• Cockpit cross member made from particularly rigid magnesium alloy
• Intelligent use of soundproofing to suit requirements: In the production facilities, insulation is applied to the bodyshell very precisely by computer-controlled robots. Insulation with varying mass distribution in parts is used in the firewall area in accordance with the potential noise penetration.
• Aluminium transmission cross member with integrated vibration damper on the diesel models
• Vibration-optimised 7G-TRONIC PLUS with twin-turbine damper, plus,
on the diesel models, centrifugal pendulum
• Map-controlled engine bearings on the diesel models
• Decoupled exhaust system on the diesel models
• Standard chassis with selective damping, as an option: AIRMATIC air suspension with Adaptive Damping System or AIRMATIC air suspension with Adaptive Damping System and ACTIVE CURVE SYSTEM dynamic
• Electric steering
• Major assembly partition wall made from plastic in order to minimise penetration of noise from engine compartment into the interior
• External noise minimised by acoustic encapsulation of engines
• High-insulation acoustic windscreen
• Additional aero-acoustic sealing in the side sections
Body testing facility: the M-Class on the road simulator
A totally dependable body and chassis systems that can cope with high levels of strain under even the most demanding conditions over hundreds of thousands of kilometres are particularly indispensable in an SUV such as the Mercedes-Benz M-Class, with its broad on-road and off-road performance spectrum. Besides practical tests with the prototypes, the engineers also subject the M-Class body to an exhaustive test of durability during the development phase that ranks amongst the toughest examinations in the world for body and chassis. Lasting several weeks, the load cycle covers a total of 3000 kilometres, which corresponds to an average of 300,000 kilometres of routine driving. This is because every kilometre driven on one of the state-of-the-art test rigs is 100 times more gruelling than in everyday driving. SUVs which come through this endurance test unscathed will be able to withstand the strain of a tough day's driving on and off the road in the M-Class.
The test programme comprises various "load spectra", which the engineers refer to as virtual test tracks. The individual courses are not dreamt up by the team of testers, however. Instead, real routes were digitised, including the "Heide" test with its tortuous pothole and cobblestone sections, special off-road torsion drives with maximum torsion levels, and corrugated tracks in Africa with high-frequency excitation. Computers orchestrate the sophisticated test rigs on the basis of this data. The rigs' servo-hydraulic cylinders mercilessly knock, jerk, shake and twist the bodies, just like on a real test drive. In all, 26 actuating cylinders arranged vertically and horizontally are responsible for stimulating the body on the test rig. A hydraulic system generates a pressure of 210 bar and pumps as much as 2000 litres of oil into the cylinders every minute. This enables the formidable forces of up to 20,000 Newtons to be produced, which shake the car thoroughly in quick succession at the computer's command.
To allow them to detect any damage early, the Mercedes engineers halt the test rigs after around 100 kilometres each time, or when one of the 150 or so sensors that permanently monitor the vehicle emits a signal to automatically shut down the facility. The body is then inspected meticulously for the tiniest sign of damage. The experts are highly experienced, so they know exactly where the critical points are and what they should look out for. Apart from the weld points and bonded connections, the panels along the force transmission paths between the chassis and body are also carefully scrutinised, as these areas have to withstand very high forces in an SUV when negotiating rough roads or torsion sections. The candidate is deemed to have passed the endurance test if its body shows zero signs of damage. Even the finest of cracks, which can only be spotted with the aid of special fluorescent colours and under ultraviolet light, are unacceptable.
Road-simulation test rig: an ordeal for the chassis
The road-simulation test rigs are just as punishing as the body testing facility. Applying identical load spectra over the same distance, the test engineers torture the M-Class chassis in all of its different variants here – from the basic steel suspension with selective damping to the AIRMATIC ADS air suspension to the version with the new ACTIVE CURVE SYSTEM. In contrast to the body testing facility, only the chassis is installed on the test rig here. The pivot points are fixed to the solid test rig frame, while the wheel hubs are connected to the servo-hydraulic cylinders using special mounting fixtures. This test setup allows the complete chassis to be examined in isolation, producing even more exact findings.
Compared to the testing for road-going saloons, the programme for a Mercedes-Benz SUV is far more rigorous. As with the body testing facility, the chassis is also subjected to all manner of different on-road and off-road load cycles. In contrast to real-life driving with prototypes, which continues to take place too, the constant laboratory conditions on the road-simulation test rig allow the engineers to analyse individual chassis parameters more effectively, and thereby take targeted action to correct any vulnerabilities found during development. The several weeks of laboratory testing staged over a distance of 3000 kilometres would take about half a year if carried out under real-life conditions.
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