The suspension: Excellent dynamics without compromising on comfort

Oct 4, 2010
  • Premiere: electromechanical power steering
  • Agility: multi-link suspension with sporty tuning
  • Alternative: AIRMATIC air suspension
Supreme driving dynamics coupled with superlative ride comfort on long journeys were the terms of reference in developing the chassis for the new Coupé, whose design and functions are geared towards a stylish sportiness. To this end the chassis concept, which has already been excellently received by the trade press and the public in the new E-Class, has undergone a thorough revamp, adding a central new component: electromechanical power steering features in the new CLS as a world first in the upper vehicle segment. This pioneering innovation offers the engineers broad scope for new tuning options and will be introduced in a large number of Mercedes-Benz models in the coming years. In addition, this steering technology also makes an important contribution to the overall efficiency of the CLS: the steering assist function only requires energy when steering actually takes place, cutting fuel consumption by up to 0.3 litres/7 g compared to the previous model.
In the new steering system the steering gear and the supporting servo-motor form a single compact unit and are mounted as previously on an extremely rigid and low-weight integral support frame made of high-strength steel which is positioned in front of the wheel's centre line. As previously, the steering gear is designed as a direct-steer system, which means that the steering ratio becomes noticeably more direct over the steering angle. This enhances the vehicle's handling and agility substantially.
In combination with the sensors and the control unit the new steering system allows substantially greater degrees of freedom in configuring the forces which the driver is required to apply at the steering wheel in different driving situations – from parking to fast motorway driving. Active damping enhances the feeling of safety when driving straight ahead at high speed. The desired steering feel has been defined on the basis of an extensive study conducted by Mercedes' driving dynamics engineers (see section "Under the microscope: the Mercedes-Benz steering feel").
In addition to more precise steering and a more comfortable feel at the wheel, the new electric power steering – whose production costs are markedly higher than those for the hydraulic systems employed to date – offers a range of additional advantages. Power assistance remains available when the engine is stationary, for example – whereby a control algorithm ensures that the combustion engine is not prompted to start up in ECO start/stop mode when the driver merely plays with the steering wheel while the vehicle is at a standstill. The new steering system has also enabled implementation of the "Active Park Assist" automatic parking function (see section "Active safety") and relieves the strain on the driver on road surfaces which slope away towards the kerb.
The new steering system also assists with braking on a surface with vastly varying grip (split friction surface). Sensors recognise the impending yaw (the vehicle starts to turn towards the side of the lane with more grip). Through the superposition of steering moment the driver receives an impulse to countersteer. If he or she acts on this signal, the steering moment is withdrawn. Series of tests conducted with drivers have shown that deploying the Steering Assist on split friction surfaces has improved the directional stability and shortened the braking time.
The new CLS Coupé comes with a mechanically length- and height-adjustable steering column as standard. A variant featuring electric length and height adjustment with easy entry function is optionally available as part of the Memory package. The mechanical and electric versions of the steering column are height-adjustable by +/- 25 mm from the steering wheel's home position and length-adjustable by 15 mm towards the front and 35 mm towards the rear.
The suspension: a proven design in a new guise
As on the E-Class, the front suspension takes the form of a 3-link front axle with McPherson struts, which has been especially modified for the new CLS. The key feature of this modified suspension is two individual links (pulling strut and cross strut) in the lower link plane which are positioned at a steeper angle than on the E-Class, thus raising the roll centre to 90 mm. This results in enhanced anti-roll characteristics with the same anti-roll bar diameter and added negative camber on the outer wheel when cornering. This in turn provides for enhanced suspension comfort and enables higher lateral forces to be transmitted to the road surface. The stabiliser is connected to the suspension strut. The forward-sloping pulling strut is designed as a weight-optimised forged aluminium component, while the cross strut fitted in lateral direction is a weight-optimised forged steel component. The tie rod is defined as the third link, forming part of the rack-and-pinion steering system. The arrangement and design of the wheel control parts, and in particular the manner in which the lower A-arm is divided into two individual links, offer more favourable characteristics for the axle
kinematics than the fixed A-arm. The kingpin inclination is closer to the wheel centre. This provides for large longitudinal force leverage, thus minimising sensitivity to tyre imbalances and fluctuations in braking force.
The principle of the lightweight independent multi-link rear suspension mounted on a subframe has been adopted on the CLS from the new E-Class, on account of its unparalleled wheel control qualities. All components have been modified in comparison to the suspension's predecessor, for enhanced comfort and added driving dynamics. The new suspension has been designed specifically as a lightweight construction. Almost all wheel control parts are produced in aluminium, in order to reduce the unsprung masses. The axle's kinematics and elastokinematics have been optimised for greater agility and contribute substantially to the vehicle's enhanced comfort as a result of the new configuration and the attendant reduced forces. A further comfort measure is the use of an additional support for the front subframe bearing on the body, which leads to a marked increase in introduction rigidity, thereby addressing
the problem of noise vibration harshness (NVH).
Suspension: selective damping as standard
The standard steel suspension of the CLS is equipped with a selective damping system. The shock absorbers adapt to the given driving situation, reducing the damping forces automatically during normal driving with minimal actuation of the shock absorbers and increasing them as necessary up to maximum level during dynamic cornering or evasive manoeuvres.
The spring struts on the front axle consist of cylindrical, lateral force compensating coils springs, dual-pipe shock absorbers and newly-developed three-phase head bearings. The front axle stabiliser is connected to the spring strut, which also assists actively in controlling the front wheels.
AIRMATIC: air suspension with continuously variable damping system
The optionally available AIRMATIC air suspension (standard for the CLS 500 BlueEFFICIENCY) is combined with an electronically controlled continuously variable damping system which controls each wheel individually. A total of seven sensors monitor the driving situation and body status and relay information to an electronic control unit which additionally processes information on vehicle speed, steering wheel angle, braking torque and engine torque to determine the ideal shock absorber characteristic. The system adjusts the damping force for each wheel individually according to the given road or driving conditions. The extremely fast damping adjustment enables high driving dynamics without compromising on comfort. In evasive manoeuvres, for example, the damping forces are adapted immediately to the vehicle's driving dynamics. The driver is additionally able to alter the vehicle's characteristic by means of the damping programme switch in a broad range extending from comfortable to sporty handling.
Automatic ride-height control is a feature of the AIRMATIC system. The level of the vehicle does not alter as a result of loading, ensuring that outstanding ride comfort is maintained when the vehicle is loaded. Lowering of the ride height at higher speeds reduces fuel consumption and enhances driving safety. The driver is additionally able to increase ground clearance on rough roads or on difficult upward slopes by pressing the level adjustment button.
The aerodynamic lift has been reduced considerably in comparison to the
predecessor model, further enhancing braking stability at high speeds. The front-axle lift coefficient, which is crucial to driving stability at high vehicle speeds, is on a par with that of the previous CLS.
Brakes: with Hill-Start Assist and brake drying function
With the ADAPTIVE BRAKE, the new CLS offers an innovative braking system incorporating additional assistance functions for even greater comfort and safety. One example of this is the practical HOLD function: after braking to a standstill, briefly pressing the brake pedal a little further is all that is required to activate this function. The car is then held by the brakes, even if the driver's foot comes off the brake pedal. The HOLD function is deactivated automatically when the car moves off.
If the driver's foot moves abruptly from the accelerator to the brake pedal before an emergency stop, the brake system increases the pressure in the brake lines and brings the pads into contact with the brake discs so that maximum braking power is available as soon as the driver hits the brake pedal. This priming of the brakes allows the system to assist the standard-fit Brake Assist system.
ADAPTIVE BRAKE also has safety benefits in the wet: the system briefly applies the brakes at regular intervals to wipe the film of water from the brake discs and ensure that the brakes are able to perform at their peak. The finely metered brake pulses are imperceptible to the driver. This automatic brake drying function is always activated when the windscreen wipers have been operating for a certain length of time.
Finally, ADAPTIVE BRAKE assists the driver when starting on an uphill slope. When the sensors detect that the Coupé is stopped on an uphill slope, Hill-Start Assist is activated automatically, briefly holding the brake pressure at a constant level so as to prevent the car from rolling backwards.
Even more effective brake discs and brake booster
The brake system helps to reduce the unsprung masses through the use of floating (four- and six-cylinder models) or fixed front callipers with housings in aluminium or aluminium composite design and rear floating callipers with aluminium housings. Large front and rear brake discs provide an ideal technical basis for ensuring safe and reliable deceleration in any driving situation. The front and rear discs measure up to 360 millimetres and 320 millimetres in diameter respectively, depending on the engine version. To cool the brakes the Mercedes engineers have developed two-piece aluminium cover plates which have undergone testing in the wind tunnel. These ensure that the cooling air flows from the underbody into the wheel arches and subsequently on to the brake discs while the vehicle is in motion.
Wheels and tyres: 19-inch size available ex-factory for the first time
The four- and six-cylinder variants of the CLS feature 17-inch wheels fitted with 245/45 R 17 tyres as standard. The CLS 500 BlueEFFICIENCY is factory-fitted with 18-inch wheels and tyres in size 255/40 R 18 – this combination is available as an option for four- and six-cylinder models. Alternatively, 18-inch wheels fitted with tyres in size 255/40 R 18 at the front and 285/35 R 18 at the rear are available for all models. The 19-inch wheels which are available for the first time lend the CLS an even sportier appearance. These wheels are fitted with tyres in size 255/35 R 19 at the front and 285/30 R 19 at the rear.
The electronic stability programme which features as standard also monitors the tyre pressure and displays a warning in case of any sudden loss of pressure in a tyre. For this purpose the system continuously compares the rotational speeds of the wheels, which are primarily dependent on the vehicle's speed, the load and the tyre pressure. The control unit additionally refers to other driving dynamics-related ESP® sensor signals, such as lateral acceleration, yaw rate and wheel torques, in order to diagnose pressure loss in a tyre. In this way the system is able to establish any deviations, informing the driver accordingly on the central display.
Mercedes-Benz CLS-Class, chassis
Mercedes-Benz CLS-Class, chassis