The E-Class Cabriolet features AGILITY CONTROL suspension with selective damping system as standard. The shock absorbers adapt to the current driving situation automatically:
when driving straight-ahead, i.e. when the shock absorber impulses are low, the damping forces are lower, making for more comfortable driving characteristics; but when cornering at speed or performing evasive manoeuvres, when there is more body roll, the damping forces are raised to the maximum.
This effect is achieved by purely hydromechanical means: the core components of the AGILITY CONTROL shock absorber system are a bypass channel in the shock absorber’s piston pin and a control piston moving within a separate oil chamber. When linear travel of the shock absorber is low, the control piston forces oil through the bypass channel to produce a smaller damping force at the actual damper valve. If the excitation of the shock absorber is greater, the control piston moves to its limit position so that oil ceases flowing through the bypass channel, meaning that the full damping force is available. In this way, the chassis and suspension meet stringent requirements in terms of road roar, tyre vibration and agility – without ever compromising on driving safety.
In addition, the long wheelbase (2760 mm), the wide track (front 1538 mm/rear 1544 mm) and the deep location of the engine positioned a long way back are key ingredients for an agile driving experience. The axle load distribution between front and rear is exceptionally well-balanced at almost 50:50 percent. Then there is the low vehicle centre of gravity for the Cabriolet. The dynamism of the open-top E-Class is also enhanced by the standard-fit 235/45 R 17 tyres.
Front suspension: three links and McPherson struts
In the interests of favourable axle kinematics, superior vibration characteristics and enhanced safety, the lower link level of the three-link front axle with McPherson struts consists of two separate elements which act as torque and cross struts. Both are made of aluminium. As well as offering more precise wheel location, this design has the particular advantage of compensating vibrations caused by tyre imbalance and fluctuating braking forces better than rigid wishbones. It also provides longer deformation paths in the event of a frontal collision. The third front-axle link is the track rod which connects the transversely installed steering gear to the wheels.
The spring struts consist of cylindrical, transverse force-compensating coil springs, double-tube shock absorbers and three-phase head bearings. In order to further optimise ride comfort, a starkly upright position has been chosen for the spring strut so as to reduce the forces acting on the torque strut bearing. As a consequence, it was possible to reduce the bearing rigidity, which has a positive effect on the chassis’ tyre/road contact characteristics and rolling characteristics – for example when driving over expansion joints running across the road. The anti-roll bar is connected to the spring strut, which is likewise actively involved in locating the front wheels.
Subframe: robust high-strength-steel construction
The front-axle components, steering gear, engine and transmission are pre-mounted on a subframe. This subframe is made from high-strength steel and is bolted to the side members of the bodyshell, which also makes it a key element of the crash structure at the front end of the new Cabriolet. During a frontal collision, the subframe creates a separate load dissipation path which effectively absorbs the impact energy.
Direct steering for agile handling
The rack-and-pinion steering system for the Cabriolet operates with a ratio of 14.5 and is therefore relatively direct. Positioning the steering gear 80 millimetres in front of the wheel centre makes for predictable self-steering characteristics with a slight tendency to understeer. The steering gear housing and valve body are made of aluminium, while the rack is forged from high-strength steel.
Drivers can adjust the steering column to their liking within a height adjustment range of +/- 25 millimetres and a reach adjustment range of 35 millimetres – well in excess of the standard in this vehicle category. A fully electrically adjustable steering column is included if the optional Memory package is ordered.
To greatly enhance occupant safety, the steering column has been designed to collapse telescopically by up to 100 millimetres when hit by the driver with a certain degree of force in the event of a frontal collision. This extended deformation path reduces the forces exerted on the driver’s head and chest area.
Speed-sensitive power steering as an option
As standard equipment for the E 350 CDI BlueEFFICIENCY or an optional extra for the other models, Mercedes-Benz offers speed-sensitive power steering, which increases assistance when driving at low speeds. Up to a speed of 200 km/h, the steering effort is continuously reduced as a function of vehicle speed, which means that only one third of the maximum steering effort is required when parking at slow speed. Variable centring is another new feature adopted from the S-Class: the electro-hydraulic speed-sensitive servo valve is used to generate a centring moment that increases with the speed and gives a secure and stable feeling in the straight-ahead position. At slow speeds, this additional steering moment is not activated, so the benefits of the speed-sensitive power steering can be exploited to the full.
The power steering pump features an electrically controlled solenoid valve which controls a bypass opening and, as a result, allows metering of the oil quantity in line with requirements. This setup reduces the pressure and oil quantity when driving straight ahead, for example – the power steering pump operates with a substantially reduced output and, consequently, helps to save fuel.
Rear axle: modified independent multi-link suspension
Mercedes-Benz’s independent multi-link suspension, which has proven itself time and again over a period spanning more than 25 years, has been further modified for use in the new E-Class Cabriolet in order to meet more stringent requirements in terms of comfort and agility. Modified components include the subframe, subframe carrier, struts and hub carriers/steering knuckles. In keeping with the lightweight design concept, wheel location components such as struts, hub carriers and the metal sections of the elastomer bearings are largely made of aluminium. The rear-axle subframe is made of high-strength steel.
Optional extras: sports suspension and Dynamic Handling package
Alongside the standard-fit AGILITY CONTROL suspension with adaptive damping system, the E-Class Cabriolet offers two further options for adapting the suspension to the driver’s individual requirements. One of these optional extras is the AGILITY CONTROL sports suspension with speed-sensitive sports steering, stiffer springs, tauter shock absorbers and stronger stabiliser torsion bars. When cornering or performing evasive manoeuvres at speed, this sporty suspension tuning translates into noticeably further reduced body roll. AGILITY CONTROL sports suspension is already included in the AMG Sports package.
Further optional equipment available includes a Dynamic Handling package (standard for the E 500 Cabriolet), which solves the classic suspension-tuning trade-off by offering a choice of two modes: Sport or Comfort. These modes allow continuously-variable electronic control of the shock absorbers based on seven sensor signals.
The system adjusts the damping force for each wheel individually and fully automatically, depending on the current road conditions and driving situation. When driving normally on poor road surfaces, a soft damper setting is selected to give the occupants maximum comfort while maintaining excellent handling stability and safety. If the driver decides that more brisk progress is called for, however, the shock absorber settings are continuously adapted to meet the need for more dynamic performance.
The driver can pre-program the principle vehicle characteristics at the push of a button. In Sport mode, the hydraulic forces of the shock absorbers are increased so as to allow even better directional stability and road adhesion at high speeds as well as reduce understeer at speeds of up to 120 km/h.
Speed-sensitive sports steering with variable centring is also part of this package. To enhance agile handling, the steering ratio has been reduced from 14.5 to 13.5; the steering torque is increased at speeds above 80 km/h to provide a more sporty steering feel. Furthermore, the system adjusts the accelerator pedal characteristics in Sport mode in order to ensure more spontaneous engine response. If the Cabriolet is fitted with an automatic transmission, the gear-change characteristics are also altered, while the gear-change times are shorter and the driver can change gear more quickly by using the shift paddles on the steering wheel.
Body Index: outstanding agility without compromising on ride comfort
In order to allow comparison of the suspension characteristics, Mercedes engineers have developed the “Body Index” concept – a composite formula for handling dynamics, which is calculated from the readings for various typical driving manoeuvres. The Body Index is an indication of how well the suspension is linked to the body, how closely the car hugs the road and how nimble the handling is through fast bends. In other words, how taut the suspension settings are. The higher the figure, the sportier, i.e. firmer, the suspension characteristics.
The Body Index for the E-Class Cabriolet with AGILITY CONTROL suspension fitted as standard is 2.0 and, thanks to adaptive shock absorber control, is achieved without compromising on ride comfort. If the Cabriolet is equipped with the Dynamic Handling package, the Body Index is between 1.89 (Comfort) and 2.06 (Sport).
Brake system: with HOLD function, Hill-Start Assist, brake drying function and priming function
With the ADAPTIVE BRAKE (fitted as standard), the E-Class Cabriolet offers an innovative braking system incorporating additional assistance functions for enhanced comfort and safety. One example of this is the practical HOLD function: after the two-door model has been braked 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. In this way, the ADAPTIVE BRAKE prevents the car from rolling forwards inadvertently when stopped at traffic lights or stuck in stop-and-go traffic and from rolling backwards when facing up a slope. The HOLD function is deactivated automatically when the car moves off.
If the sensors detect that the car is stopped on an uphill slope, Hill-Start Assist is activated automatically. If the driver’s foot then moves from the brake pedal to the accelerator pedal, the brake pressure is kept constant for a brief period, giving the driver enough time to move off without the car rolling backwards down the hill.
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 already brings the pads into slight 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. This automatic brake drying function is always activated when the windscreen wipers have been operating for a certain length of time. The finely metered brake pulses are imperceptible to the driver.
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 344 millimetres and 300 millimetres in diameter respectively, depending on the engine version.
For brake cooling, the Mercedes engineers have developed two-section aluminium cover plates which have been tested in the wind tunnel and ensure that, whilst driving, the cool air flows from the underbody into the wheel arches and, finally, to the brake discs.
Control systems: electronics for safe motoring
The E-Class Cabriolet features anti-lock brakes, acceleration skid control, Brake Assist and the Electronic Stability Program as standard. For ESP®, Mercedes-Benz has developed control logic that offers the driver even more effective assistance in cornering situations involving understeer: precisely measured brake pulses sent to up to three wheels make it easier for the car to turn safely into bends.
As an option, Mercedes-Benz can equip the Cabriolet with a pivoting trailer coupling, whose ball head needs to be neither fitted nor removed again. Instead it pivots away beneath the body together with its electrical socket, so that it is concealed from sight when not in use. The maximum towing capacity is up to 1800 kilograms (braked).
ESP® Trailer Stabilisation, which is activated if a trailer coupling is fitted, detects dangerous oscillating movements of the trailer and initially restabilises it automatically by sending precisely measured brake pulses to the front wheels of the towing vehicle. If these oscillating movements persist, the system applies the brakes to reduce the car’s speed automatically whilst reducing the engine torque until the trailer is stabilised.
The Electronic Stability Program monitors the air pressure in the tyres indirectly too, and warns the driver if there is a sudden loss of pressure in one of the tyres. To this end, the system continuously compares the rotational wheel speeds, which mainly depend on the vehicle speed, vehicle load and tyre pressures. The control unit also consults other dynamic ESP® sensor readings, such as the lateral acceleration and yaw rate, to help it diagnose tyre pressure loss. The system is therefore able to detect any deviations, and informs the driver accordingly via the central display.
To ensure continued mobility in the event of a flat tyre, Mercedes-Benz also offers run-flat tyres as an option. These are available in 245/45 R 17 format and feature self-supporting sidewalls, enabling Mercedes customers to continue driving for a distance of up to 50 kilometres at a maximum speed of 80 km/h, depending on the vehicle load.
The range of wheels and tyres for the new E-Class Cabriolet offers a wealth of individualisation options. Standard equipment for the Cabriolet includes 235/45 R 17 tyres, while 235/40 R 18 front tyres and 255/35 R 18 rear tyres are available as optional extras (standard for the V8 Cabriolet). The AMG Sports package with special wheel design also includes 235/40 R18 (front) and 255/35 R18 (rear) tyres.
Source: Daimler AG