In 1999, with the introduction of the predecessor to the new CL-Class, Mercedes-Benz opened up new dimensions in handling dynamics: the flagship Coupé was the world’s first car to feature the Active Body Control (ABC) suspension system. This innovative system has been developed and improved even further for the new CL-Class.
Active Body Control resolves the traditional conflict of aims when configuring a passenger car suspension: should wheel vibrations caused by the road surface be kept to a minimum by a sports, stiff shock absorber configuration, or should the damping be made more comfortable and soft at the expense of handling safety and performance? Thanks to ABC, which is standard equipment in the new CL-Class, this conflict of aims is a thing of the past: the suspension configuration is automatically adapted to suit the current driving situation. A high level of comfort is accompanied by dynamic performance - or vice versa.
To this end, the four spring struts are equipped with microprocessor-controlled plunger cylinders which almost completely compensate yawing, pitching and rolling movements of the vehicle body. The computer receives information about the relevant driving situation from the various acceleration sensors, then compares this with the data from the pressure sensors in the spring struts and the level sensors in the steering control arms. The ABC system translates this information into control signals, which are implemented as precisely metered oil flows by hydraulic servo-valves on the front and rear axles. When oil flows into the plunger cylinders, these adjust the tracing point of the steel springs integrated into the spring struts, generating the forces necessary to counteract movements of the vehicle body.
Under pressure: body is stabilised within fractions of a second
Owing to the constantly available hydraulic pressure of up to 200 bar, ABC is able to stabilise the vehicle body immediately - in fractions of a second. The system operates in the vibration range up to five hertz, the range which causes pitching and rolling movements on uneven road surfaces, side-roll on bends and the typical body dive experienced during braking. Higher-frequency vibrations and wheel damping are the responsibility of passive twin-tube gas-pressure shock absorbers, which can be configured for excellent comfort.
Every component in the active suspension control system has been revised for the new CL-Class. The specialists have now arranged various components into groups, in order to improve the control functions with shorter connecting lines and reduce the installation space required.
On fast bends: body roll reduced by up to 45 percent
The major advantage of second-generation Active Body Control is its greater efficiency in reducing body movements according to the driving situation: the body roll angle during a dynamic driving manoeuvre is reduced by more than 45 percent – from the previous 2.2 to 1.2 degrees. When taking bends at speed – for example when leaving a motorway – this second-generation ABC suspension reduces the roll angle to just 0.7 degrees.
Other special features of Active Body Control include variable anti-roll control between the front and rear axles, which the system carries out automatically as a function of vehicle speed to combine agile handling with outstanding stability. The system also takes the vehicle load into account: on the basis of the plunger and spring travel, the ABC computer calculates the actual vehicle weight and takes it into consideration for the active control function.
At the touch of a button: individual suspension and transmission characteristics
In addition the new CL-Class features an S/C/M button in the centre console with which the driver is able to modify the overall characteristics of the Coupé from "comfortable" to "sporty":
- Comfort: In this mode the automatic transmission changes the gears at lower engine speeds, and the CL Coupé moves off in second gear. The ABC suspension operates in Comfort mode, comfortably smoothing out uneven road surfaces and bumps.
Sport: The automatic transmission makes full use of the engine speed range in this setting. The ABC suspension operates in Sport mode, with a stiffer damping effect on uneven surfaces and bumps. The characteristic map of the accelerator is also modified, so that the engine responds more rapidly when acceleration is required.
Manual: The automatic transmission is operated manually using gearshift buttons on the steering wheel, with significantly reduced gearshift times compared to "Sport" mode. The suspension configuration selected by Active Body Control is the same as in Sport mode, as is the characteristic map of the accelerator.
If the CL-Class is travelling on poor roads which make a higher ground clearance advisable, the driver is able to raise the suspension by 45 millimetres at the touch of a button. When travelling at high speed in Sport mode, ABC automatically lowers the suspension by up to ten millimetres to reduce aerodynamic drag and lower the fuel consumption.
Front axle: four-link technology improved even further
In addition to improving the ABC system, the Mercedes development engineers turned their attention to the choice of axle designs for the new luxury Coupé. The results of digital design and test procedures, as well as exhaustive practical trials, confirmed them in their view that four-link technology offers the greatest advantages for the front axle. Accordingly this was refined in detail – especially where comfort is concerned.
Wheel location at the front is by means of a so-called high-link axle. This consists of an aluminium wishbone at the upper level, a lower aluminium torque strut and a spring link of forged steel. The two levels are connected by steering knuckles. The fourth component, the one that gives the four-link axle its name, is the track rod; this creates the link between the front wheels and the transverse rack-and-pinion steering system.
Another major advantage of the Mercedes four-link axle is the design of the lower link level. This provides favourable axle kinematics, and is almost completely insensitive to vibrations caused by unbalanced tyres or fluctuations in braking force. It was on this positive basis that the engineers reconfigured the kinematics and elasto-kinematics of the new CL-Class, with a great improvement over the preceding series. Safety specialists at the Mercedes-Benz Technology Centre provided another important reason for choosing the four-link system: in a frontal crash it offers longer deformation paths at the lower link level than other axle designs, and is therefore able to absorb kinetic energy more efficiently.
Compared to the preceding model, the wider track (front + 24, rear + 29 millimetres) and the longer wheelbase (+ 70 millimetres) create other important conditions for the even better ride comfort and exemplary handling dynamics of the new CL-Class. The figures at a glance:
Steering: mid-centred for safe, stable handling
As before, the steering gear is located in front of the wheel centre and therefore supports the easily controlled understeer characteristic of the luxury Mercedes Coupé on bends. The servo-assisted rack-and-pinion steering has a variable ratio (50.1 to 60.5 millimetres per turn of the wheel), which is slightly more indirect in the mid-range than in the outer positions. The lower the vehicle speed, the greater the servo-assistance: below 200 km/h the steering effort is continuously reduced depending on speed, which means that the driver only needs to exert one third of the maximum steering effort when parking at low speed.
Variable mid-centring is a new feature in the CL-Class: the electrohydraulic, speed-sensitive servo valve is used to create a centring force which increases with the vehicle speed and gives the driver a feeling of safety and stability in the centre position. This additional steering force is not activated when the vehicle is at a standstill, enabling the advantages of the speed-sensitive steering to be fully exploited.
The telescopic steering column is electrically adjustable for height and reach, and also provides an automatic entry/exit aid as an additional convenience feature: once the driver has withdrawn the electronic ignition key, the steering wheel moves upwards to provide more legroom. The wheel then remains in this position until the key is reinserted into the ignition lock, when it returns to its preset position.
Mercedes-Benz has developed a four-spoke steering wheel with a diameter of 390 millimetres for the new CL-Class, whose diecast magnesium structure specifically deforms during a frontal impact to reduce the risk of injury to the driver. With its ergonomically located, illuminated control buttons, the steering wheel is a major component of the operating system in the new CL-Class. An electrically heated steering wheel is available on request.
Rear axle: made completely from aluminium
Because multi-link axle technology remains unsurpassed, the engineers at Mercedes have remained faithful to it in the new CL-Class, but have made certain detailed improvements: the rear axle housing is derived from the previous unit as an aluminium construction which is open to the front and features a bolted front cross-member. The major advantages of this new sub-assembly are lower weight, greater strength and more compact dimensions.
The use of uncompromising lightweight construction methods has enabled Mercedes engineers to reduce the weight of unsprung masses at the rear axle considerably, thereby improving both handling and ride comfort. Four of the five links are of forged aluminium, while the spring link is of single-skin sheet aluminium and the wheel carrier is of diecast aluminium. The rigidity of the wheel-locating components has also been increased.
The specialists in Sindelfingen also gave their attention to insulating vibrations which might be transmitted from the final-drive unit to the vehicle interior via the axle housing. Three large rubber bearings were developed to minimise this effect.
Braking system: high-tech for outstanding safety and comfort
ADAPTIVE BRAKE – this newly developed braking system familiar from the S-Class is also standard equipment in the new CL Coupé. It allows electronic control of the hydraulic, dual-circuit braking system and provides a number of support functions for additional safety and comfort:
- HOLD: Once the luxury Coupé has been braked to a stop, briefly depressing the brake pedal a little further activates this function. The CL-Class is then held in position by the brakes – even when the driver removes his foot from the brake pedal. In this way the HOLD function prevents the vehicle from rolling forward unintentionally at traffic lights or in stop-and-go traffic.
Hill holder: With this auxiliary function, ADAPTIVE BRAKE prevents the CL-Class from rolling backwards when the driver switches from the brake pedal to the accelerator to move off on an uphill gradient. When the sensor system detects that the Coupé has come to a stop on a gradient, the hill holder function is activated automatically and maintains a constant brake pressure for a short time to make moving off easier.
Priming: The braking system interprets rapid removal of the driver’s foot from the accelerator as an emergency braking situation. In this case ADAPTIVE BRAKE increases the pressure in the brake lines and brings the brake linings into light contact with the brake discs, so that they are then able to bite immediately and with full force when the brake pedal is depressed. In this way the system supports the functions of Brake Assist and Brake Assist PLUS.
Brake drying: In the wet ADAPTIVE BRAKE applies regular, brief braking impulses to wipe the film of water from the brake discs and ensure that the brakes are able to perform at their peak. The automatic brake drying function is always activated when the windscreen wipers of the CL-Class have been operating for a certain time. The driver does not notice the finely metered braking impulses.
Large-diameter front and rear brake discs create the technical basis for reliable deceleration with ADAPTIVE BRAKE. Depending on the engine version, their diameter is up to 360 millimetres at the front and up to 330 millimetres at the rear. The front brake discs are perforated. The perforations conduct water and dirt away, ensuring that the brakes operate reliably and with good deceleration performance in poor weather conditions. The figures at a glance:
4-piston fixed callipers
2 x 77 sq. cm
Int. ventilated, perforated
6-piston fixed callipers
4 x 120 sq. cm
Int. ventilated, perforated
1-piston floating calliper
2 x 38 sq. cm
4-piston fixed calliper
2 x 48 sq. cm
Parking brake: electric motor for added convenience
The new CL-Class is equipped with an outstandingly convenient, electric parking brake as standard. The brake is activated by simply pressing a button on the dashboard. The brake is automatically released as soon as the driver selects a gear and moves off. The major components of this new development are two separate servo drum brakes at the rear wheels and an electric motor with a reduction gear, which applies the brake drums by means of cables. The electric parking brake can also be applied when the engine is switched off.
Dynamic safety systems: new functions for even better performance
ABS, ASR, ESP®, Brake Assist – these well-proven dynamic safety systems developed by Mercedes-Benz are standard equipment in the new CL-Class, and now have a number of additional functions.
Mercedes engineers have enhanced the anti-lock braking system (ABS) with additional technology that makes braking even safer on surfaces which are slippery on one side. The system recognises these situations and controls the brake pressure in such a way that the driver is given optimal assistance in staying on track and braking efficiently. The ABS system in the new CL-Class also ensures more safety when braking on bends, as it distributes the braking forces to create a yawing moment which counteracts and stabilises the vehicle’s tendency to break away. The braking system uses data supplied by the ESP® sensors to detect these situations reliably.
The Electronic Stability Program also improves the functions of acceleration skid control (ASR): ESP® sensors supply online information about the slip at each wheel and enable the traction control to be activated even more precisely, particularly allowing much better progress on icy roads. What is termed “snow-chain logic” is another enhanced ASR function. On the basis of the drive torque and driving resistance, this system automatically detects whether snow-chains have been fitted and adapts the acceleration skid control accordingly.
When taking fast bends, a yaw moment control system now registers the stiffness of the tyres and improves ESP® control intervention if the CL-Class is fitted with winter tyres. The new, electronically controlled braking system ADAPTIVE BRAKE offers additional advantages because the brake servo unit is selectively controlled by electronic means, allowing ESP® to operate particularly sensitively and comfortably. The system is also faster and more efficient than before: ESP® automatically reduces the distance between the brake pad and the brake disc if there is a risk of skidding and the CL-Class needs to be stabilised by specific braking impulses.
Tyre pressure loss warning as standard
Tyre pressures are monitored by the Electronic Stability Program ESP® or a special control system based on radio technology:
- ESP® continuously compares the wheel rotation speeds, which mainly depend on the vehicle speed, vehicle load and tyre pressures. In addition the control unit automatically monitors other dynamic parameters such as the lateral acceleration, yaw rate and wheel torque in order to diagnose any pressure loss in a tyre reliably. It is therefore able to detect any significant deviations and inform the driver via the central display: "Tyre pressure, check tyres". This tyre pressure loss warning system based on ESP® technology is standard equipment in the CL-Class.
An optional electronic tyre pressure monitoring system is also available; it uses sensors in the tyre valves to monitor both the air pressure and temperature inside the tyre. The data are transmitted by radio to a special underfloor aerial at regular intervals. Intelligent software is used to identify the wheel positions automatically, so that the driver can be given specific information about the air pressure in each tyre via the central display.
To ensure continued mobility in the event of a flat tyre, Mercedes-Benz also offers run-flat tyres as an optional extra. These have self-supporting walls, and enable Mercedes customers to continue for a distance of up to 100 kilometres at a maximum speed of 80 km/h despite the flat tyre.