The technology of the new 4MATIC system: Latest generation of trend-setting all-wheel-drive system

Sep 11, 2006
Stuttgart
  • Power split between front and rear wheels in ratio of 45 to 55 percent
  • Multiplate clutch with 50 Nm locking force offers additional traction
  • Handling is clearly defined and always predictable
  • Compact, lightweight design is optimised for reduced friction losses
The Mercedes-Benz S-Class traditionally embodies the state of the art with regard to technical innovations – indeed, these have always made this top-of-the-range Mercedes the model for all passenger-car development. The same holds true for the new 4MATIC models, which are equipped with a trend-setting all-wheel-drive system of the latest generation. During the concept, design and development stages of this new drive technology, the principal objectives were:
  • Top traction performance on all road surfaces.
  • Retention of the outstanding handling characteristics and first-class ride comfort.
  • Performance and fuel consumption figures which are almost as good as those of two-wheel-drive vehicles.
  • Compact and weight-optimised design.
  • Attainment of the highest possible passive-safety standards.
In order to achieve these ambitious goals, the engineers and technicians of the Mercedes Technology Center (MTC) in Sindelfingen and Stuttgart-Untertürkheim decided to undertake a complete redesign of the all-wheel-drive system. The permanent 4MATIC all-wheel-drive system in the S-Class has a planetary-type central differential which splits the power between the front and rear wheels in a fixed ratio of 45 to 55 percent. The permanent all-wheel-drive system is active all the time, operates without the need for electronically controlled clutches and does not require any reaction time. As a result, the S-Class 4MATIC offers assured, clearly defined handling which is always predictable on all road surfaces – a clear advantage over other systems. On snow and ice in particular, the adhesion of all four wheels is maximised by a multiplate clutch whose locking force of 50 Nm ensures optimum traction. Complementing the high-precision mechanical compo-nents, the 4ETS and ASR electronic traction systems interact intelligently with the ESP® active safety system. The new 4MATIC all-wheel-drive system is designed in such a way that other Mercedes-Benz passenger cars will be able to benefit from it in the future.
Space-saving design with integrated transfer case
The power supply to the driven front wheels is provided via a transfer case which requires no additional installation space as it is integrated in the 7G-TRONIC automatic transmission. From here, a driveshaft supplies the power to the front axle. From the front differential, power is transmitted to the front left axle shaft by an intermediate shaft which runs through an encapsulated shaft channel in the engine oil sump.
The transfer case is notable for its compact design: by dispensing with one complete gear stage and by integrating the rear universal joint in the output gear, it is possible for the drive shaft to the front differential to run very close to the automatic transmission. Together, all these design measures allow identical floor as-semblies to be used as for vehicles with rear-wheel drive. As a consequence, cus-tomers who choose the new S-Class 4MATIC no longer have to compromise on space. Packaging the all-wheel-drive technology in this way also has the advan-tage that the results of all the relevant crash tests are almost identical to those obtained with rear-wheel-drive vehicles.
Noise and vibration comfort to typically high S-Class standards
Another benefit of this compact drive concept relates to the fact that the engine mounts are positioned on the new integral subframe and thus permit the use of extremely compact and rigid engine supports. This has a positive effect on noise and vibration comfort; on both counts, the 4MATIC models match the acknowl-edged high standard of the rear-wheel-drive S-Class. A no less important factor in this context is the high natural bending frequency of the entire engine-transmission unit, including the favourable integrated transfer case. The noise-optimised design of the oil supply system and all rotating components also plays an important part in this.
Additional all-wheel-drive-related fuel consumption just 0.4 litres
The inter-wheel speed difference is absorbed by means of a rear differential whose design (it is linked to the engine by a shortened propshaft) offers particular benefits with regard to fuel economy. The principle of the friction-optimised design of the rear differential has also been adopted for the newly developed front differential which has a diecast aluminium housing: by integrating the transfer case in the 7G-TRONIC automatic transmission it has been possible to dispense entirely with one gear stage and the associated seals. Optimum lubrication effi-ciency is achieved by sharing the oil circuit with the automatic transmission; low-friction bearings are also used. Together, these measures allow the S-Class 4MATIC models to return additional (NEDC combined) fuel consumption of just 0.4 litres per 100 kilometres – a figure so low as to be unrivalled – compared with the rear-wheel-drive models. Furthermore, the compact and weight-optimised design results in an all-wheel-drive weight premium of just 66 to 70 kilograms, depending on the engine – a clear advantage over all the competitor models. The low additional weight makes modified gear spacing and final drive ratios super-fluous – another factor which has a positive effect on fuel consumption and per-formance figures.
Multiplate clutch offers additional traction
In addition to the permanent all-wheel-drive system, an innovative multiplate clutch also helps ensure high traction in all road conditions. It is fitted to the central differential and produces a basic locking force of 50 Newton metres between the front and rear axles. If extremely low friction values – as encountered on ice, for example – are preventing one of the axles from transmitting any drive torque, the multiplate clutch helps by transferring the engine power from the axle which is turning faster to the one which is turning more slowly without any time-lag whatsoever. As a result, the wheels without traction are prevented from spinning. As well as enhancing traction, this system improves stability, too.
ESP®, ASR und 4ETS active safety systems completely redeveloped
This highly effective technology is complemented by the ESP®, ASR and 4ETS electronic active safety systems which have been completely redeveloped to take account of the specifics of all-wheel drive. The 4ETS traction control system uses a whole series of sensor data about the speeds of the different wheels, the steering angle as well as the vehicle's rotational motion and transverse acceleration. A micro-computer uses this data as a basis for correctly adjusting the automatic braking impulses in accordance with the driving situation. This improves traction when starting out from a standstill on a slippery surface and provides greater stability during challenging driving manoeuvres.
4ETS replaces conventional mechanical differential locks and is able to brake one or more wheels at the first sign of a loss of traction, thereby instantly increasing the drive torque to the wheels with best traction. 4ETS and ASR intervene almost imperceptibly as far as the vehicle occupants are concerned and combine optimum traction with maximum driving stability and excellent handling. At low speeds, 4ETS brakes up to three wheels, which provides the same effect as three differential locks and ensures maximum traction. At higher speeds, the system reduces the automatic braking impulses to ensure stability. As well as making the S-Class 4MATIC models extremely safe cars, this combination of ESP® and 4ETS with permanent all-wheel drive also opens up a new dimension of dynamic driving for motorists.
As with ESP® in model variants with rear-wheel drive, the all-wheel-drive vehicles also immediately warn drivers when they approach their physical limits. In this case, a yellow warning light in the instrument cluster begins to flash. This is a clear signal warning drivers to adjust their driving behaviour to the road conditions.
Four-link front suspension and multi-link independent rear suspension for optimum comfort and handling
Another key element which underpins the excellent driving dynamics and high ride comfort typical of the brand is the suspension design of the S-Class. Unlike the predecessor model series, the all-wheel-drive versions now feature the four-link front suspension which offers the best compromise between agility, precision and ride comfort. Equipped with the tried and trusted multilink independent rear suspension, the 4MATIC Saloons are characterised by handling that is always predictable and neutral with slight understeer. At the same time, the power-assisted rack-and-pinion steering makes for precise and effortless vehicle control. Thanks to its variable ratio (50.1 to 60.5 millimetres per steering wheel revolution), it works somewhat more indirectly in the middle of the range than at the extremes.
Comfort is considerably enhanced by the speed-sensitive feature that comes as standard: the lower the speed, the greater the power assistance. Below 200 km/h, the required steering input diminishes continuously as a function of the driving speed, so that the driver need apply only about a third of the maximum steering effort during slow parking manoeuvres. Variable centring is a special feature common to all S-Class models: the electro-hydraulic speed-sensitive servo is used to generate a centring moment that increases with the speed and gives the driver a secure and stable feeling in the straight-ahead position. In slow driving, this additional steering moment is not activated, so the benefits of the speed-sensitive steering can be fully exploited.
Standard: AIRMATIC all-round self-levelling air suspension
The standard-fit AIRMATIC all-round self-levelling suspension makes an important contribution to the high standard of ride comfort offered by the S-Class 4MATIC versions. When the vehicles are stationary, there is no difference between the ground clearance and ride height of the all-wheel-drive and rear-wheel drive models. On rough roads, where greater ground clearance is required, the S-Class driver can simply press a button to raise ride height by 30 millimetres. AIRMATIC is combined with the Adaptive Damping System (ADS). ADS constantly regulates the shock absorber force in line with requirements, taking the road conditions, the driving style and the car's load into account. The system is able to adjust the damping force at each wheel to the requirements of the prevailing situation within just 50 milliseconds. In doing so, the skyhook algorithm regulates the damping forces in such a way that the forces exerted on the car body by the movement of the wheels are reduced. Depending on the commands received, the ADS solenoid valves on the shock absorbers can actuate any of four different damper settings:
  • Stage 1: Comfortable ride with little body movement and low acceleration values, due to soft compression and rebound.
  • Stage 2: Skyhook mode - soft rebound setting and hard compression.
  • Stage 3: Skyhook mode - soft compression setting and hard rebound.
  • Stage 4: Hard compression and rebound setting to reduce wheel load fluctua-tions during dynamic driving.
When experiencing few body movements, the new S Class remains in the comfortable ADS Stage 1. If the speed of body movements exceeds a certain value, the system shifts to the skyhook algorithm and continuously switches between the second and third damping stages, with the help of fast solenoid valves, in order to offset body roll and pitching.
Another special feature is the S/C/M button in the centre console. The motorist can use this button to switch the saloon's characteristics from "comfortable" to "sporty", thereby influencing the suspension settings, ride height and transmis-sion programming. On 4MATIC models, the shift points for all gears are optimally adapted to all-wheel drive.
  • Comfort: In this mode, the ride height of the 4MATIC saloons is raised by ten millimetres over that of the rear-wheel drive versions, provided the speed re-mains below 80 km/h. When the speed exceeds 80 km/h, AIRMATIC automatically lowers the ride height by ten millimetres at both axles in order to improve stability and reduce drag. At more than 160 km/h, the body is low-ered by a further ten millimetres and the compression damping is changed to "hard". When the speed drops below 40 km/h again, the system returns the body to its normal height. The automatic transmission shifts gears at lower engine speeds and allows the S Class to move off in second gear.
  • Sport: In this mode, the ride height is the same when on the move as for rear-wheel drive S-Class models. At speeds exceeding 100 km/h, ride height is lowered by ten millimetres at both axles and is not raised again until the speed drops below 60 km/h. The shock absorbers are set to the "hard" compression setting from 40 km/h. Gear changes take place at higher rpm.
  • Manual: The automatic transmission can be operated manually by means of buttons on the steering wheel. The alterations to ride height and suspension settings take place in the same way as in "Sport" mode.
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