The new Mercedes-AMG SL: the suspension

Oct 28, 2021
Affalterbach

The SL 55 4MATIC+ is equipped as standard with a newly developed AMG RIDE CONTROL steel suspension with particularly light and capable aluminium shock absorbers including adjustable damping and lightweight coil springs. For the first time, a series-production Mercedes-AMG vehicle is fitted with a multi-link front axle with five links arranged entirely within the rim. This significantly improves the kinematics. The independent wheel control and wheel suspension elements enable high lateral acceleration values with minimal drive influences on the steering system. At the rear axle, a 5-link design likewise controls the wheels. The SL 63 4MATIC+ also celebrates the première of the innovative AMG ACTIVE RIDE CONTROL suspension with active, hydraulic anti-roll stabilisation. The system enables even better turn-in and load change behaviour with typical AMG dynamics, precision and feedback for the driver. At the same time, it increases ride comfort when driving in a straight line and over bumps.

To reduce unsprung masses, all suspension links, steering knuckles and hub carriers on the front and rear axles of the new SL are made of forged aluminium. The multi-link concept controls each wheel with the least amount of elastic movement. The high camber and track stability not only enables high cornering speeds, it also provides the driver with optimal road contact at the high cornering limit. This is reflected in excellent lateral dynamics and driving stability at high speeds as well as in the good-natured reaction to external influences such as crosswinds, bumps or jumps in friction coefficients. An extremely direct connection of the shock absorber on the rear hub carrier reduces vibrations and undesirable wheel load fluctuations. Independent wheel control and wheel suspension elements enable high lateral acceleration with minimal torque steer – also a comfort feature.

The lightweight coil springs are a new development: Special tempering has reduced their weight without reducing their performance. In the manufacturing process, the spring pad is glued onto the spring for the first time. This firm connection prevents wear and tear caused by dirt such as sand in the course of a vehicle's life. The spring does not corrode over the life cycle, and the maximum component load can consequently be increased with less weight. This saves around 0.2 kilograms per spring.

Another lightweight construction measure concerns the torsion bar stabilisers on the front and rear axles of vehicles without active anti-roll stabilisation. Thanks to a variable wall thickness, their weight can be reduced. For this purpose, the primary material is tailored to the load in the vehicle by means of a special drawing process. The maximum wall thickness is now only used where it is required due to the maximum load, in this case in the area of the rubber mounts.

Even more comfort and sportiness: adaptive adjustable damping with two valves

The SL 55 4MATIC+ features the latest generation of AMG adjustable damping as standard. This system works with two so-called pressure relief valves per damper. With the help of these stepless control valves, a significantly larger damping force spread can be realised: one valve controls the rebound damping, i.e. the force that occurs when the wheel rebounds, and the other controls the compression damping when the wheel compresses. The rebound and compression stages are controlled independently of each other. In detail, this means that the so-called soft characteristic of the shock absorber was reduced and the hard characteristic of the shock absorber could be made even more pronounced. The body thus responds more sensitively to impacts and isolates inputs from the road. At the same time, the further refined control of the rebound valve and compression valve (stepless and independent of each other) stabilises the body in the best possible way. This technology makes it possible to increase comfort on the one hand, but also to make the driving dynamics even sportier on the other hand.

The suspension control unit analyses data - including data from the acceleration and wheel path sensors - to adjust the damping force for each wheel in a few milliseconds to suit the situation. By using the two adjustment valves, the damper is able to provide damping force adjustment across the full range of wheel vibrations. Due to the special design of the valves, the damper reacts quickly and sensitively to changing road surfaces and driving conditions.

The driver can pre-select the basic set-up via the AMG DYNAMIC SELECT driving modes: at the touch of a button, the handling characteristics change, for example, from full dynamics in "Sport+" mode to smooth cruising in the "Comfort" setting. In addition, the tuning can be adjusted in three stages independently of the driving modes via a dedicated button.

For the first time in series production: AMG ACTIVE RIDE CONTROL with hydraulic, active anti-roll stabilisation

The SL 63 4MATIC+ features a completely newly developed hydraulic system, the innovative AMG ACTIVE RIDE CONTROL suspension, which is entering series production for the first time in a Mercedes-AMG model. Biggest innovation: Active hydraulic elements replace the conventional mechanical anti-roll torsion bars and compensate for rolling movements of the new SL in fractions of a second. For this purpose, the adaptive shock absorbers also have two hydraulic connections. One of them is on the compression side of the damper, the other on the rebound side. The connection of the damper chambers at all four wheels and the lines is made directly via the control valves of the adaptive dampers.

The intelligent hydraulic interconnection of the four suspension struts and the pressure regulation of the pump and switching valves allow a very wide roll rate with reduced roll movements at the same time. Figuratively speaking: Any torsion bar from zero to stiff can be realised automatically. In everyday life, this increases comfort because even one-sided bumps are individually compensated for. During dynamic cornering, the hydraulics also actively reduce loss of camber. Thanks to the resulting high camber stability, the roadster turns in very precisely.

When driving straight ahead, the system is opened completely depending on the driving mode and driving situation. The system compensates for individual obstacles that would otherwise lead to roll movements. Drivers and passengers experience a significantly more comfortable driving experience. The reduced roll when cornering pays equal dividends in terms of comfort and driving dynamics. The driving behaviour in the individual driving modes can also be spread even more between comfort and sport.

All dampers are hydraulically connected

The basic principle of the chassis is based on the fact that the compression or rebound sides of the individual dampers of the vehicle are interconnected. The compression stage of the front left damper is connected to the rebound stage of the front right damper via a hydraulic line. In addition, a line through the vehicle also connects to the compression side of the rear left damper and the rebound side of the rear right damper. Analogously, the other sides of the damper chambers are connected to each other via a second circuit. The simplified routings of the hydraulic lines each result in the shape of an "H".

In addition to the interconnection of the dampers, other components that are important for the function are used in the system. The rebound and compression sides of the dampers are directly connected via an electrically controlled 2/2-way valve, called a "comfort valve". A pressure accumulator is also attached to each of the damper compression sides. Both circuits are additionally connected to a central pump. It ensures that the hydraulic system pressure can be adjusted in both circuits. The connection between the pump and the circuits is made via 2/2-way valves. Pressure sensors are integrated downstream of the valves to monitor the system pressure in the respective lines. The possibility of freely selecting the system pressure opens up the degree of freedom to choose the size of the generated anti-roll support. The principle here is: The higher the system pressure, the greater the roll resistance to be expected.

The system works very efficiently as it uses the existing kinetic energy of the vehicle to positively influence the roll behaviour. The hydraulics of the AMG ACTIVE RIDE CONTROL suspension also feed the optional lift system for the front axle. If necessary, the front end lifts by 30 millimetres, making it easier to enter underground car parks or drive over speed bumps, for example. The lift system is operated via the multimedia central display or the AMG steering wheel buttons. The corresponding menu opens after pressing the area with the vehicle symbol on the control bar. Among other things, a position at which the lift system is used can be stored using GPS technology so that the front end of the vehicle is raised automatically when it passes the corresponding point again.

Limited-slip rear differential for optimised handling stability

As standard, the SL 63 4MATIC+ features an electronically controlled limited-slip rear differential that ensures outstanding traction and maximum driving safety in all driving conditions. For the SL 55 4MATIC+ it is available as part of the optional AMG DYNAMIC PLUS Package. Not only is the traction of the drive wheels on the rear axle further improved - the cornering speeds at the limit also increase. In addition, handling stability is optimised when changing lanes at high speed. The system operates with a variable locking effect in acceleration and overrun mode, and is perfectly tuned to the various driving conditions and road surface friction coefficients.

AMG high-performance composite braking system for short braking distances

The newly developed AMG high-performance composite braking system guarantees excellent deceleration values and precise control. It impresses with short braking distances, sensitive response and last but not least with high stability and durability - even under extreme stress. Among the comfort features of the vehicle are Hill Start Assist, priming and dry-braking in the wet. With the ignition switched off and the vehicle at a standstill, the transmission automatically selects the parking position "P". Meanwhile, the electric parking brake releases automatically when pulling away.

The composite brake discs are particularly light: Driving dynamics and turn-in behaviour benefit from the reduced unsprung masses. Brake disc (made of cast steel) and brake disc chamber (made of aluminium) are now connected with special pins and no longer with screws as before. This design saves space, which is used for even better brake cooling. The directional perforation is also new: In addition to the weight saving and better heat dissipation, this solution scores points with a faster response in wet conditions as well as better pad cleaning after braking.

The two SL variants with V8 engine are fitted with vented and perforated composite brake discs measuring 390 x 36 millimetres with 6-piston fixed callipers at the front, and 360 x 26 millimetres with 1-piston floating callipers at the rear. The SL 55 4MATIC+ has red-painted brake callipers, while the SL 63 4MATIC+ has yellow callipers.

As an option, both SL variants can be fitted with the AMG ceramic high-performance composite braking system, with brake discs measuring 402 x 39 millimetres on the front axle and 360 x 32 millimetres on the rear axle. It saves even more weight compared to the standard brake. At the same time, the ceramic discs withstand even extreme loads and keep braking distances permanently short - ideal for use on the race track, for example. Thanks to the bronze finish of the brake callipers, they are also immediately recognisable as absolute high-performance components. All pads are copper-free and therefore more environmentally friendly.

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