Suspension: The closest connection to the road

Mar 8, 2011
It is the suspension that establishes contact with the road, and ride comfort and agile handling depend on it. The new Mercedes-Benz SLK offers a choice of three versions:
  • A conventional steel suspension comes as standard
  • A lowered sports suspension with harder springs and dampers ensures a systematically sporty driving experience
  • As an alternative a Dynamic Handling package is available. Its merits include a suspension with continuous adjustable damping. It has an electronically controlled fully automatic damping system, so the vehicle rolls away gently even on poor road surfaces, but still offers high driving dynamics
Also included in the Dynamic Handling package are a Direct-Steer system and the Torque Vectoring Brake developed by Mercedes-Benz.
Wheel location is the same for all three suspension variants: a 3-link suspension locates the wheels at the front, while at the rear this is done by a multi-link independent suspension which has already impressively demonstrated its unrivalled wheel location qualities in the preceding model. In the new SLK this has forged aluminium hub carriers, in order to reduce unsprung masses even further in the interests of even better roadholding. All models in the new SLK-Class also have a torsion bar stabiliser, which is attached directly to the body.
The standard suspension is a conventional steel suspension with wheel-locating front axle spring struts, a torsion bar, twin-tube shock absorbers at the front and single-tube shock absorbers with rear axle springs at the rear.
The optional sports suspension has a stiffer damper setup, shorter suspension springs and torsion bar stabilisers with a larger cross-section. The suspension is also lowered by ten millimetres. The roll control system has been reinforced to achieve even smaller roll angles when cornering at speed.
Suspension with automatic, continuously adjustable damping
The suspension with automatic, continuously adjustable damping, which is included in the Dynamic Handling package, sets new standards. It allows sporty and agile handling for maximum driving pleasure on winding country roads, but also cossets the occupants with excellent road roar and tyre vibration characteristics and a high level of stability on poorer road surfaces. An electronic system automatically and continuously ensures that the dampers are always set to suit the driving situation. The response time is normally only around 10 milliseconds.
Setting the basic characteristics of the suspension is still left to the driver, however, using a switch that enables him to choose comfortable or sporty damping characteristics. At the touch of a button he therefore has a suspension setup available which offers the same comfort as the standard suspension. If he chooses the sport setting, he is opting for a driving experience with a very high level of dynamic response.
Like the sports suspension, the suspension with variable damping is equipped with torsion bars with a larger cross-section, and the suspension is lowered by ten millimetres.
Brakes for sporty driving enjoyment
The well-proven, dual-circuit hydraulic braking system and the distribution of the two circuits to the front and rear axles has been adopted from the preceding model. All variants are equipped with single-piston floating calliper brakes all-round. In the entry-level SLK 200 BlueEFFICIENCY the calliper housings are of aluminium, while the more powerful engine variants use housings of composite aluminium at the front and aluminium at the rear.
The brake covers at the front axle were optimised in the wind tunnel for even more efficient cooling of the brakes and wheel bearings, and to protect the brake discs from road spray.
Brake disc dimensions
front, internally ventilated dia./thickness (mm)
rear dia./thickness (mm)
Torque Vectoring Brake: ESP® helps with the steering
The Dynamic Handling package includes the Torque Vectoring Brake system. This ensures an additional safety margin at the physical limits, makes the SLK even more agile and therefore enhances the sportiness of the roadster. The system achieves this with targeted, one-sided braking intervention by the Electronic Stability Program ESP® at the inside rear wheel when cornering. If ESP® detects a tendency to understeer, the Torque Vectoring Brake generates a defined turning or yawing moment around the vehicle's vertical axis within fractions of a second. Thanks to the different torque distribution that results, the SLK turns into the bend under precise control without any loss of handling dynamics. The advantage of this solution over complex mechanical components such as an active steering rear axle, additional multi-disc clutches or an active differential: the Torque Vectoring Brake can be implemented without an increase in vehicle weight and therefore no disadvantages in terms of fuel consumption.
Electric parking brake
As a new feature in Mercedes-Benz roadsters, the electrically operated parking brake in the new SLK helps in the design of a generous interior. Its switch is located in the dashboard, under the rotary light switch. Pressing this engages the electric parking brake, and pulling releases it. In models with an automatic transmission the brake is released automatically as soon as the engine is running, the transmission is in the
D or R position, the driver has a fastened seat belt and the accelerator is operated.
Direct and comfortable – the steering of the SLK
Like the preceding model, the base version of the new SLK has a constant-ratio steering system, however the ratio is more direct. This means that instead of 2.8, only 2.6 turns of the steering wheel are necessary from lock to lock.
The Dynamic Handling package includes a hydraulically assisted Direct-Steer system. This increases the agility and controllability of the SLK compared to the standard steering, and improves steering comfort when parking as less effort is required. It combines these attributes with predictable and safe steering characteristics at high speeds. As a result, the agile cult-roadster corners even more willingly and precisely, allows relaxed cruising on motorways and makes for even more exhilarating driving pleasure.
The new Direct-Steer system was developed from the familiar speed-sensitive steering that Mercedes-Benz offers for many of its models. In this case the steering effort depends on the vehicle speed – the effort required increases with the speed. In practice this means more steering comfort during parking manoeuvres and more safety at higher speeds, for example on motorways, where a greater steering effort ensures safe straight-line stability.
The new Direct-Steer system retains these positive attributes of speed-sensitive steering and adds a variable steering ratio that changes according to the steering angle. The ratio is indirect when the steering is centred, which ensures good straight-line stability and therefore a high level of safety. In contrast to other variable steering systems, the ratio of the Direct-Steer system increases rapidly after a steering angle of 5 degrees, becoming extremely direct from a steering angle of just 100 degrees. Only relatively small steering movements are then needed to make course corrections.
This has the following effect in practice: even at urban driving speeds, the driver needs to perform less pronounced steering movements, making changes in direction possible more rapidly and with less effort. The effect of the Direct-Steer system is even more impressive on winding country roads, where only small steering movements are sufficient to steer the vehicle. Even on tighter bends, the driver's hands hardly need to change their grip on the steering wheel. Rapid sequences of bends can be mastered almost intuitively safely and precisely, and with enormous enjoyment. This all adds up to considerably more agile handling.
The increase in driving dynamics is also aided by an increased yaw rate (steering response) at low to medium speeds, as this augments the effect of the steering movement. At high speeds this increase in yaw rate is small, which means that the car responds safely and predictably to steering movements.
Despite its remarkable effect, the key component of the Direct-Steer system is simply a steering rack. Its secret lies in the teeth cut into it. In the mid-area these are slightly wave-shaped, with varying flank profiles. To both sides the teeth are spaced at varying distances apart. The change in steering ratio is therefore produced by purely mechanical means. With this solution, Mercedes engineers have been able to dispense with the complex actuators and sensor systems used by other variable steering systems. The advantages include very low susceptibility to faults and low weight. Moreover, the system always responds predictably and in the same way, while other variable steering systems sometimes require rapid adaptation by the driver in rapidly changing situations.
ECO steering servo pump lowers fuel consumption
Driving straight ahead in the new SLK also means saving fuel. This is ensured by an ECO steering servo pump as standard in all SLK models. Its centrepiece is an electrically actuated proportional solenoid valve which variably opens a bypass aperture at the pressure connection to feed the steering gear with just the right amount of fluid. As the steering has a lower energy requirement when driving straight ahead, the throughflow pressure in the steering system can be minimised. Less energy is therefore consumed.
Mercedes-Benz SLK-Class, SLK 350, exterior
Mercedes-Benz SLK-Class: Suspension and drive system
Mercedes-Benz SLK-Class: Wheel location is the same for all three suspension variants: a 3-link suspension locates the wheels at the front, while at the rear this is done by a multi-link independent suspension with forged aluminium hub carriers.
Mercedes-Benz SLK-Class: Wheel location is the same for all three suspension variants: a 3-link suspension locates the wheels at the front, while at the rear this is done by a multi-link independent suspension with forged aluminium hub carriers.
Mercedes-Benz SLK-Class: Wheel location is the same for all three suspension variants: a 3-link suspension locates the wheels at the front, while at the rear this is done by a multi-link independent suspension with forged aluminium hub carriers.
Mercedes-Benz SLK-Class: Suspension and drive system