The New Mercedes-Benz SLS AMG Roadster: Perfect open-top performance - VI
Chassis and braking system: Perfect racetrack performance and the long-distance comfort for which a Mercedes is known
The motor sports derived technology to be found everywhere in the powertrain of the Mercedes-Benz SLS AMG Roadster is also evident in the suspension and braking system. The aluminium suspension with double-wishbone axles is designed for uncompromising lightweight construction and outstanding handling dynamics. The new AMG RIDE CONTROL sports suspension with variable, electronically controlled damping is available on request. The AMG high-performance braking system is also available in a newly developed, highly stress-resistant ceramic version, with composite brake discs all-round.
The commitment of Mercedes-Benz and AMG to building a highly desirable super sports car that combines consummate racetrack performance with hallmark Mercedes long-distance comfort has given rise to an ingenious suspension layout. All four wheels are located on double wishbones with a track rod, a technology that has proven itself in motor racing right through to Formula 1. With a double-wishbone axle, the wheel location and suspension functions remain separate; the spring/damper struts are supported on the lower wishbone. The double-wishbone concept with its high camber and track rigidity positively locates the wheel with minimal elastic movements, providing the driver with an optimum sense of road contact when driving at the limits.
The kinematics are determined by the differing lengths of the control arms and the position of the control arm anchorage points on the chassis and hub carriers. The broad base of the control arms transfers the wheel forces to the rigid steel subframe at the front axle, and into the compact and rigid, cast nodal points of the rear-end structure. The wishbones, steering knuckles and hub carriers at the front and rear are made entirely from forged aluminium – substantially reducing the unsprung masses; this configuration also notably improves the suspension response. The suspension configuration of the SLS AMG provides an optimum balance between low body roll and a taut basic setup with a good level of comfort.
Outstandingly individual: the AMG RIDE CONTROL sports suspension
The AMG RIDE CONTROL sports suspension with variable, electronically controlled damping is available as an optional extra. Depending on the driving situation, speed and vehicle load, this system modifies the damping characteristics almost instantly, automatically reducing the angle of body roll. For the driver this means extremely fast, imperceptible adaptation between maximum agility and optimal ride comfort – to suit the driving style and road conditions.
A small key with a major effect: pressing the shock absorber symbol also allows individual and convenient adjustment of the damping characteristics. Three stages are available: "Comfort", "Sport" and "Sport plus". The currently active mode is shown in the AMG main menu. "Comfort" is automatically active when the engine is started, and provides a sensitive suspension response with soft damping characteristics. "Comfort" is recommended for a comfort-oriented driving style with moderate speeds, for example in urban traffic or on motorway stretches with speed limits. But even on poor, uneven road surfaces, "Comfort" mode will be the mode of choice for many Roadster drivers owing to its rather softer damper settings.
The key in the AMG DRIVE UNIT is pressed once to activate "Sport" mode. The left control lamp in the key and the display in the AMG main menu confirm manual activation of "Sport" mode to the driver. In "Sport" the shock absorbers respond more firmly depending on the driving situation, which not leads to improved roadholding when driving in a sporty manner, but also effectively reduces body roll and pitching. In most driving situations, "Sport" mode is the best compromise between satisfactory suspension comfort, low body movements and high agility.
"Sport plus" is activated if the driver presses the key again, and the right control lamp lights up in confirmation. In additional to higher damping forces, this mode has special control algorithms configured for a decidedly sporty driving style e.g. on uneven road surfaces – perhaps high-speed laps on a circuit. The driver is able to store his preferred setup using the AMG key in the AMG DRIVE UNIT. Sustained pressure on the AMG symbol stores the currently active suspension mode and selected transmission mode. Brief pressure will then quickly and conveniently activate the preferred combination.
Electronics monitor numerous parameters
The electronic damper control is based on acceleration sensors which register dynamic vehicle body and wheel movements, both absolutely and in relation to each other. Other factors relevant to the driving dynamics, such as drive and braking moments, the steering angle and lateral/linear acceleration are continuously registered and evaluated. The current vehicle speed, wheel rotation speeds and selected gear ratios are also continuously analysed. High-performance control electronics in permanent dialogue with the engine and transmission control unit ensure that the forces of the four newly developed shock absorbers are adapted practically instantly.
Depending on the selected suspension mode, different control configurations and parameter weightings are active to strike the best possible balance between comfort and dynamic performance. Electromagnetically actuated valves in the shock absorbers control the oil flow between the lower and upper chamber. In "Comfort" mode more oil flows through the valves, and less in "Sport" and "Sport plus" mode.
Long wheelbase and wide track
The long wheelbase of 2680 millimetres not only results in outstanding straight-line stability but also low wheel load shifts, significantly reducing the vehicle's tendency to dive and squat when braking and accelerating. The broad track width – front 1682, rear 1653 millimetres – ensures lower shifts in the wheel loads from the inner to the outer wheel when cornering, enabling the tyres to retain more grip. The large caster angle of 11.6 degrees significantly increases negative wheel camber when cornering, and also improves tyre grip – this also ensures outstanding stability when braking heavily while cornering.
Weight-optimised "Flow Forming" AMG light-alloy wheels
The weight-optimised axle design is complemented with AMG light-alloy wheels produced by the new "flow forming" process. During this production process, hot forming in the area of the rim base makes for a denser material structure, thereby improving long-term durability. This also means that lower wall thicknesses can be used. The weight saving of around 1.1 kilograms per wheel compared to conventional light-alloy wheels reduces the unsprung masses, further improving the handling dynamics and suspension comfort.
The AMG light-alloy wheels in size 9.5 x 19 inches at the front and 11.0 x 20 inches at the rear are shod with 265/35 R 19 size tyres at the front and 295/30 R 20 at the rear. Four variants are available: AMG light-alloy wheels in a 7-spoke design or a 5-twin-spoke design, or alternatively two low-weight forged wheel variants in a 10-spoke design. With an approx. 14-percent lower weight than the "Flow Forming" wheels, these reduce the unsprung masses even further. The weight saving per wheel is 1.8 kilograms. Tyres developed exclusively for the SLS AMG ensure top performance and excellent grip.
A tyre pressure control system which continuously monitors the pressure in all four tyres is standard equipment, with the data for each wheel shown in the display. During a journey, four wheel sensors on the insides of the light-alloy wheels send out continuous radio signals with information about the air pressure, tyre temperature and direction of rotation. A control unit evaluates the data and informs the driver in the event of a deviation from the nominal values. In this case the display shows the message "Check tyres!", or in case of a sudden pressure loss "Attention. Tyre defect!". A yellow warning lamp also lights up in the instrument cluster.
Power-assisted rack-and-pinion steering for reassuringly direct feedback
With a constant mechanical steering ratio of 13.6:1, the rack-and-pinion steering of the SLS AMG Roadster provides reassuringly direct feedback and meets the high expectations for a super-sports car. The servo assistance is speed-sensitive, improving the feedback to the driver as the speed increases – an indispensable factor when driving straight ahead at high speed. Perfect linear transfer of the steering angles is made possible by the constant velocity joint integrated into the steering system. The chosen geometry also provides plenty of scope for the performance-enhancing headers in the sports exhaust system.
Installing the steering gear on the subframe in front of the engine allows a particularly low engine position. The subframe is of high-strength steel, and is rigidly bolted to the side members. It not only optimises steering precision, but is also a key component in the front-end crash structure.
AMG high-performance braking system with composite technology
High deceleration performance, precise pedal feedback, perfect fade-resistance and superior safety reserves are the key attributes of the AMG high-performance braking system. Ventilated, grooved and perforated brake discs all-round in size 390 x 36 millimetres at the front and 360 x 26 millimetres at the rear ensure deceleration at the highest level. Brake discs in race-tested composite technology are installed at the front axle, with the grey cast iron discs radially and axially floating, and fixed to an aluminium bowl via stainless steel connections. This sophisticated technology ensures highly efficient heat conduction, and therefore outstanding fade-resistance even with a highly dynamic style of driving.
The use of aluminium also saves weight, and in the case of the brakes this has particular advantages in terms of agility and suspension responsiveness because of the considerable reduction in unsprung masses. Six-piston fixed callipers at the front and four-piston fixed callipers at the rear allow a generous brake lining surface area of 2 x 120 sq. cm and 2 x 58 sq. cm. The optionally available, red-painted brake callipers are a new feature.
High-performance ceramic composite braking system as an option
AMG ceramic composite high-performance brakes are available as an option, and are instantly recognisable thanks to the "AMG Carbon Ceramic" logo on the brake callipers. Thanks to the special materials and production technology used to manufacture the discs from carbon-fibre-reinforced ceramic in a vacuum at 1700 degrees Celsius, the ceramic discs are much harder. This not only increases the service life many times over compared with a grey cast iron disc, but also their resistance to extreme loads and heat. The result is extremely short stopping distances, an exact pressure point and much higher fade resistance even under extreme operating conditions. The larger ceramic discs – front: 402 x 39 millimetres; rear: 360 x 32 millimetres – also feature a composite design and are connected with a floating radial mount to an aluminium bowl.
The ceramic brake discs are 40 percent lighter in weight than the conventional, gray cast iron brake discs. The further reduction in unsprung masses not only improves handling dynamics and agility, but also ride comfort and tyre grip. The lower rotating masses at the front axle also ensure a more direct steering response – which is particularly noticeable when taking motorway bends at high speed. The ceramic discs are also corrosion-proof, and therefore unaffected by deicing salt and brine. Six-piston fixed callipers with a brake lining area of 2 x 154 sq. cm. are installed at the front, with four-piston fixed callipers and a lining area of 2 x 73 sq. cm. at the rear.
Braking system data for the Mercedes-Benz SLS AMG at a glance:
Both AMG high-performance braking systems feature precisely configured cooling ducts: the airflow entering the apertures in the front apron is conducted directly to the front brake callipers, helping to lower peak temperatures in the brake fluid – for example when driving at very high speeds on a racetrack.
Efficient anti-locking system and 3-stage ESP®
The specification of the braking system includes an anti-locking system with demand-controlled brake distribution. When braking on bends, a yawing moment is produced which counteracts and stabilises the vehicle's tendency to turn on its own axis. The braking system uses the data from the ESP® sensors to recognise such situations reliably.
The Electronic Stability Program is designed as a particularly sophisticated 3-stage ESP® with a sports function – a speciality exclusive to AMG. Perfectly configured to match the outstanding driving dynamics of the Roadster, 3-stage ESP® has three different settings. Using the ESP® key in the AMG DRIVE UNIT, the driver is able to choose between "ESP ON", "SPORT Handling Mode" and "ESP OFF" – the currently active mode is shown in the display of the AMG instrument cluster. In "ESP ON" mode, there is braking intervention at one or more wheels and a reduction in engine torque if the car starts to become unstable.
Briefly pressing the ESP® key activates "SPORT Handling Mode". By means of over/understeer intervention accompanied by engine torque adjustment, this mode allows even more dynamic handling manoeuvres such as the appropriate drift angles. All the functions of ESP® are available as soon as the brake pedal is depressed.
Sustained pressure on the ESP® key activates "ESP OFF". In this case there is no dynamic control intervention whatsoever, and the engine torque is no longer reduced. "ESP OFF" should only be used by highly experienced drivers on closed racetracks. In this mode too, all the functions of ESP® are available as soon as the brake pedal is depressed. In all three ESP® modes the traction logic of the acceleration skid control (ASR) is active. As soon as a drive wheel begins to spin, traction is significantly improved by specific braking intervention and interaction with the standard, mechanical multi-plate differential lock. This enables the engine power to be transferred to the road even more efficiently when driving in particularly dynamic style.
Hill-holder function improves comfort and safety
Numerous functions of the high-performance braking system also improve comfort and safety, for example the hill-holder function. When the sensor system detects that the driver of the SLS AMG Roadster wants to move off on a gradient, the hill holder function is activated automatically and maintains a constant brake pressure for a short time to prevent the car from rolling backwards. This gives the driver enough time to move his foot from the brake to the accelerator without first engaging the parking brake.
When the driver suddenly switches from the accelerator to the brake pedal prior to emergency braking, the braking system increases the pressure in the brake lines and applies the pads to the brake discs, so that they can grip instantly with full force when the brake pedal is pressed. The system supports the standard Brake Assist by means of this 'priming'. Further standard features include the brake-drying function, which uses brief braking impulses to ensure that the film of water on the brake discs is removed in the wet, thus considerably improving the responsiveness of the brakes.
* Official photo and details courtesy of Mercedes-AMG GmbH *
Copyright © 2011, Mercedes-Benz-Blog. All rights reserved.