by Philipp Deppe | 15.August 2012
Relaxed, comfortable and superior driving including a great deal of fun driving in all kinds of situations – these are the great strengths of the GL-Class. Compared with its predecessor, the new model is a winner with even better response and dynamic road handling.
The nimble way this full-size SUV with permanent four-wheel drive goes about its business is otherwise only to be found in much more compact and sporty off-roaders. The fact that the quality of the riding comfort could be increased to such an outstanding level despite all the GLs dynamic handling is one of the special achievements of the development team. Especially the NVH qualities (noise, vibration, harshness) of all the GL models are very convincing and are on a par with those of the S-Class. The high standards of comfort are derived from the combined result of a very rigid bodyshell, a perfected drivetrain, improved chassis and superb aeroacoustic finishing as well as the intelligent use of new-style insulation materials.
Floating along: Air suspension AIRMATIC ADS
The fully load-bearing air suspension system AIRMATIC with its adaptive shock-absorbing system (ADS) provides a comfortable basic springing with a low inherent frequency, although the full length of spring travel is also available when the vehicle is fully loaded. When driving on the road it provides outstanding comfort and clearly improved handling. AIRMATIC compensates various loads and driving conditions and at the same time acts as a level control. When driving way off the beaten track, the air suspension increases off-road mobility with extra riding height. The lifting or lowering of the body can be activated at any time when the engine is running and can either be done automatically or as the driver requires using the control buttons on the centre console. The system is speed-dependent and lowers the body at high speeds to minimise aerodynamic resistance while at the same time increasing driving stability. The main features of the air suspension are:
- Air-spring legs on the front axle with integrated ADS shock-absorbing function
- Air spring with separate ADS shock absorber on the rear axle
- Supercharger with central pressure tank and electric pressure sensor
- Air spring valves
- Electronic control unit
- Sensors for regulating height and shock-absorbing functions
The adaptive shock-absorbing system ADS is a fully automatic, electronically controlled system which adjusts the shock-absorbers on each wheel as required. Unlike a passive system with constant settings, the vehicle is stabilised equally in all driving situations which improves handling and leads to a perceptible calming of the body. If the response of the body movement is low, the GL-Class drives at level 1. When the speed of the body movement exceeds a certain level, the system goes into the so-called skyhook algorithm and switches as needed back and forth between the second and third shock-absorbing levels using its fast magnet valves in order to compensate the tilting and pitching motion of the body. In more dynamic driving situations, level 4 is activated and in the sport mode only level 4 is used. The regulation of each individual wheel takes place almost immediately. Depending on the command received, the valves can adjust to one of the characteristic curves mentioned in less than 0.05 seconds.
The specific driving situation is recognised by a steering angle sensor, four turning angle sensors and the driving speed from the ESP signal and brake pedal switch. With the aid of these signals, the control unit calculates the current shock-absorbing strength required and activates the appropriate characteristic curve for the shock absorbing system. Via a switch, the driver can preselect the required performance graph Sport or Comfort manually.
In the new GL-Class an electromechanically supported Direct-Steer system works with a variable steering ratio which becomes perceptibly more direct when the steering angle has an upwards trend. This gives the driver better feedback from the road so that he does not need to work so hard at steering. On the other hand, this also improves the handling and response of the vehicle making it more fun to drive both on very winding roads and in city traffic. In addition, electric steering gives speed-dependent steering support and thus provides clearly improved steering comfort over the whole speed range.
Greater vehicle dynamics with load sensor system – as standard
A patent is pending for the new load sensors developed by research engineers to improve driving dynamics even further. The system is able to recognise various vehicle loads carried by the GL and with the data received it is able to alter the regulating thresholds of the ESP® accordingly. Up to now, the ESP® always presupposed a vehicle loaded to the maximum with a complete roof load. The new load sensor system now intervenes only when required by the actual load carried and has a positive effect on vehicle dynamics.
An effective sensor system within the air suspension system AIRMATIC recognises the load situation at the time: The load carried by the vehicle leads to alterations in the axle loads of the front and rear axles. The difference in pressure between the front and the rear axle serves here as a reliable measurement for moving the height of the centre of gravity as the wheel pressures determined by the AIRMATIC pressure sensors are proportional to the wheel loads. The adjustment of the regulating threshold is effected according to the wheel pressures determined; the ESP interventions are carried out more sensitively and according to the actual load carried.
The ESP adaptation takes place after driving a short distance. When the vehicle starts driving, the load sensor system presumes that the vehicle is fully occupied with a full load. This means that safest driving situation possible is always guaranteed.
New driving dynamics system ACTIVE CURVE SYSTEM
The optional active tilting stabiliser ACTIVE CURVE SYSTEM can be combined either with the air-suspension system AIRMATIC with the adaptive shock-absorbing system (ADS) or with the ON&OFFROAD package. The system uses its active lateral stabilisers on the front and rear axles to regulate them automatically depending on the lateral acceleration, speed and position of the ADS Comfort/Sport switch. The ACTIVE CURVE SYSTEM compensates the roll angle of the body when driving round bends, thus providing a greatly improved vehicle response and more driving pleasure. At the same time, the system improves handling and safety when driving at high speeds. An improved ride both driving round bends as well as on straight roads and increased scope when driving off-road are among the other advantages of the ACTIVE CURVE SYSTEM. Individually, the control strategies are:
- Improved vehicle dynamics achieved when driving round bends as the starting torque and torque angle of the stabilisers are actively influenced. Furthermore, the starting torque of the stabilisers remains constant even when exposed to one-sided activation, for instance when driving over a pothole or round the outer periphery of a curve. The varying regulation of the front and rear axles distributes the roll moment variably so that under/oversteering characteristics are adjusted to the individual driving situation. When driving on country roads the handling is regulated so that it is particularly responsive and on motorways so that it is especially stable.
- In extreme off-road situations at low speeds, both of the lateral stabilisers are decoupled to allow an improved degree of axle articulation.
A belt-driven hydraulic pump and an oil container in the engine compartment together with one valve block each and active stabilisers on both front and rear axles are among the important components of the active roll stabilisers. In comparison with a passive stabiliser, the active stabilisers are divided in the middle and connected to each other by hydraulic actuators. Pressure sensors and a lateral acceleration sensor use CAN signals to pass on the required information to the electronic control unit for regulating the hydraulic pressure.
The ingenious ACTIVE CURVE SYSTEM uses its hydraulic system to pump oil from the oil container into the system. The pressure regulator integrated in the valve blocks of the front and rear axles and the directional valves set the desired pressure and turn the active stabilisers in the correct direction depending on the driving situation. The hydraulic rotary actuator integrated in the active stabilisers has six oil-filled chambers in its interior, and of these three are used to apply pressure for each driving direction, i.e. left and right-hand curves. In addition, the front valve block is responsible for distributing the oil flow between the two axles independent of load.
No stopping the GL off-road with new OFFROAD driving program
Every GL-model has an extensive standard off-road package for going on tours off the beaten track. In addition to its permanent four-wheel drive 4MATIC and electronic traction control 4ETS, this includes an off-road button which activates a special off-road driving program:
- Slip threshold of the wheels and the switching points of the 7G-TRONIC PLUS are raised to improve traction
- Flatter characteristic curve for the accelerator allows it to be regulated more sensitively
- Off-road ABS optimises braking especially on gravelly/sandy surfaces
In addition, the START Assist and the HOLD function support the driver when starting off on a hill. The Downhill Speed Regulation (DSR) automatically keeps GL-Class vehicles at the speed set using the Tempomat lever.
Extended range of programs with ON&OFFROAD package
The ON&OFFROAD package for the new GL-Class with its six driving programs improves both vehicle dynamics and handling safety as it provides the best drive control available for very varying operating conditions both on and off-road. There is a perceptible improvement in comparison with the Offroad-Pro Technik package which has been on offer for the G-Class up to now. Up to now there was one top driving program for on-road driving and one for going cross-country. The new package has one automatic program, two special off-road programs and three road programs at its disposal. The specific driving programs for four-wheel drive provide the solution to the conflicting aims which have made life difficult up to now for the further development and research of the 4MATIC, the electronic traction system 4ETS as well as ESP® and ASR. For instance: On off-road tracks it is important that the control systems regulate very gently because of the low friction values – but that makes it almost impossible to have a sporty tuning On the other side of the coin, a sporty basic tuning reduces the vehicle’s performance off-road. In the past, engineers have always found very good compromises which functioned – and still continue to function – outstandingly in every situation. However, the new ON&OFFROAD package with its six specific driving programs allows both vehicle dynamics and handling safety to be optimised even more for every driving situation.
The programs are activated via a rotary switch on the central console. At the same time AIRMATIC, ADS, drivetrain and electric steering are adjusted to suit the specific requirements. The six driving programs are:
- Automatic – covers the wide field of everyday driving
- Offroad 1 – for easy cross-country driving, tracks, field tracks, sand
- Offroad 2 – for more demanding off-road terrain with climbing sections
- Winter – for driving in wintry road conditions with freezing-over wet roads, snow, ice or with snow chains
- Sport – ambitious driving on very winding roads
- Trailer – optimises starting-off, manoeuvring and braking with a trailer
Despite the diversity, the driver is always in the position to make the right decision: It is easy to select the right program, the program selected is shown in the multifunction display in the instrument cluster as well as almost photo-realistically visualised on the COMAND screen. If the driver, for example, selects the program ‘Winter’, the display shows the GL-Class in snow; the program ‘Sport’ shows a racing track with red-white kerbs, ‘Offroad 1 and 2′ show cross-country formations with different contours. The depiction shown is also interactive: the steering angle and spring travel correspond to the real values. In addition the display gives additional information about the steering, climbing and tilting angles, the AIRMATIC level, the pre-selected speed for driving downhill (Downhill Speed Regulation, DSR) and the selected drivetrain settings with reduction gear and lock.
Reduction gear, DSR or the AIRMATIC level control system can all be selected individually regardless of the drive program chosen. Just as with the basic settings for the ON&OFFROAD package, the driver cannot make a mistake here either. Settings which are not logical and which have a negative influence on the handling of the vehicle, its safety or traction, are excluded. If there is any doubt, the automatic mode is activated.
From the “hardware” point of view, the ON&OFFROAD package comprises a steel underrun protection for the engine, fibre-reinforced underfloor panelling, a two-stage power transmission with reduction gears, a longitudinal differential lock and extended AIRMATIC functions allowing a maximum ground clearance of 306 mm and fording depth of 600 mm.
A premium S-Class ride with higher vehicle dynamics both on and off-road
Research and development engineers in the Mercedes Technology Centre have perceptibly improved the quiet-running and low vibrations of the new GL-Class by working extensively on the details, thus setting a clear sign in the market segment for luxury SUVs. In this regard, the model lies on a par with luxury saloons like the S-Class. The improvements in detail:
- Lower perception of noise in the interior due to the intelligent use of sprayable acoustic materials in the body-in-white, fibre-reinforced plastic partitions between equipment, arrangement of insulation as required for front partition between engine and passenger compartments
- Rigid front end design with V-strut, upper and lower levels of the side supports connected, light hybrid construction for front module
- Plastic engine supports
- Rigid cockpit cross-member made of high-strength magnesium alloy
- Highly insulating acoustic front windscreen, as an option acoustic side windows at the front
- Electric steering
- Reduction of the unsprung mass by using weight-optimised
- aluminium control arms and alloy wheels
- Aluminium Transmission Bridge with integrated vibration damper
- Air suspension AIRMATIC ADS, as an option with ACTIVE CURVE SYSTEM (dynamic tilt stabilising)
- Optimised body-in-white with new D-ring structure to increase torsional strength
- Optimised aeroacoustic
Once upon a time: More comfort = more weight
A perceptible improvement in riding comfort with a simultaneous reduction in the weight of the vehicle by 90 kg has been achieved by using innovative approaches. In this way the new GL-Class and all current automotive developments at Mercedes-Benz have made their farewells to an old conflict of aims: Up to now, any increase in comfort has been at the expense of the vehicle’s weight. The plastic engine support, the rigid cockpit cross beam made of high-strength magnesium alloy or the electric steering are among the outstanding examples of the “weight-optimised increase in comfort”. The highly insulating front windscreen and the optional side windows using multilayer glass technology give much better sound insulation, but weigh less than conventional multilayer glass. The weight savings on the axles and alloy wheels are also extremely impressive. The weight per axle is reduced by 18 kg by using high-strength aluminium alloys. In this way the “unsprung mass” – a decisive parameter with respect to comfort – is reduced. The lower the weight of the wheels and axles, the less they are activated when driving over potholes or uneven surfaces. As a result, suspension and shock absorbers are not required to work as hard for a calming effect. So, to put it simply: A reduction in wheel suspension activation and the ensuing calming means a more comfortable ride.
While looking for other ways of increasing riding comfort, the engineers made a find in the GL roof. Up to now, the automatic tailgate was operated by a hydraulic drive situated at the rear in the upper frame of the roof. In the new model, the hydraulic system has been replaced by an electric spindle. This may save “only” two kg, but they have a significant effect on the handling as weight-saving measures on the roof of a vehicle way above the centre of gravity are much more effective than comparable weight reductions, for instance on the underbody.
Source: Daimler AG
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