The new Mercedes-Benz SL: Chassis and suspension - Best contact with the road
Powerful when accelerating, effortlessly superior at every speed, zappy on winding stretches – the new SL is a shining example, displaying perfect manners on all the world’s roads and in all driving situations. What’s more, it also pampers all the senses with any amount of fun at the wheel. For these emotive experiences SL drivers can thank the potent engines – and above all the sporty, agile tuning of the suspension – which is nevertheless very comfortable, being offered in the form of two different systems for the SL.
The SL features semi-active adjustable damping as standard. The optional active suspension system ABC (Active Body Control) is offered as an alternative.
The spring struts which come as part of the standard equipment combine a steel spring with a continuously adjustable damper. Its damping properties can be varied via an electrically controlled valve.
Continuous adjustment of the damping in accordance with the respective driving situation is taken care of fully automatically by control electronics which record the driving condition in question via sensors. The control unit uses the sensor data to determine the necessary damping forces for optimum comfort and handling. On this basis it steplessly regulates the damping valves at every wheel within a broad damping characteristic so that sprung mass vibrations such as lifting, rolling and pitching are minimised on uneven roads. As a result, the new SL gives its occupants a smooth ride even on poor roads, without relinquishing any driving dynamics.
The semi-active adjustable damping also rapidly counteracts sprung mass vibrations caused by the driver, by adjusting the damping characteristics. By way of example, the dynamic anti-roll and anti-pitch support compensates for pitch ensuing from braking and accelerating, and quickly and effectively stems rolling motion which is initiated above all by cornering or jerking the steering wheel during a dynamic evasive manoeuvre. Torsion bars at the front and rear axle help to reduce wide roll angles when cornering, and positively influence the understeer/oversteer characteristics.
Setting the basic characteristics of the suspension is still left to the driver, however, using a switch that enables them to choose from a broad spectrum ranging from comfortable to sporty damping characteristics. At the touch of a button they can therefore have a chassis and suspension set-up available which places more emphasis on comfort, for instance, whilst if they choose the sports setting, they are opting for a driving experience with a very high level of dynamic response.
Sports suspension in the AMG Sports package
A variant of this suspension is offered in the AMG Sports package. This sports suspension lowers the roadster by ten millimetres, the spring strut and torsion bars are configured to be ten percent more rigid and the parameterisation of the adjustable damping is selected in such a way that the sprung mass vibration is dampened more strongly – all measures which underline the vehicle’s sporty character.
ABC suspension for high levels of comfort
As an option the new SL is available with the unique active suspension system ABC (Active Body Control) as an alternative. It has been further improved for the new SL by the Mercedes-Benz chassis and suspension specialists. This system’s highly effective control elements are spring struts with a hydraulic steel cylinder arranged in line with the steel spring. When this cylinder retracts and extends, it actively adjusts the length of the spring and therefore the spring force in the spring strut. This eliminates the need for torsion bars at the front and rear axle.
With this technology an automatically controlled electronic system permanently adjusts the suspension to suit the driving situation and the road condition without any perceptible delay. This means that the active suspension system ABC can raise comfort to a high level, whilst simultaneously improving the roadster’s driving dynamics. This is possible as it almost completely compensates the vehicle body’s rolling and pitching movements when accelerating, braking and cornering. In addition to this, at the touch of a button it offers a unique, further increased spread of suspension settings between sports and comfort.
As the vibrations initiated by irregularities in the road surface are greatly reduced, the occupants in the SL experience a vehicle with perfect absorption capacity on poor roads. Above all it is alternating sprung mass vibrations (“cross-pitching”) which are extremely minimal.
Sprung mass vibrations which arise through the driver’s influence are also almost entirely ironed out – this is the case, for instance, when the body rolls during steering or when there is pitching during braking and accelerating. The occupants experience a vehicle which drives “as if on rails”, but without having to pay for this with the usual reduction in ride comfort which is typical of sports suspensions.
As the ABC system modifies the roll torque distribution depending on the driving speed, the driver experiences handling which changes with the driving speed. It varies from motoring pleasure where one can afford it in driving dynamics terms to directional stability where it is most needed. In other words, on winding rural roads the roadster moves with nimble agility, and on motorway stretches it stays safely on track and does not worry the driver with any nervous reactions.
The ABC system is also fitted with a hydraulic all-round level control system, thus ensuring a constant vehicle level irrespective of the load. When the engine is running, the driver can raise the level of the vehicle at the touch of a button, both when the vehicle is at a standstill and during the journey. Thanks to a new feature in the SL, if the level is raised by 50 millimetres in this way, the ground clearance for rough roads or ramps is increased. In addition, the SL fitted with ABC is automatically lowered by up to 13 millimetres at high speeds, thus lowering the centre of gravity and wind resistance – and therefore also the fuel consumption.
More track, less unsprung weight
The track and wheel suspension are the same in the case of both suspension variants. In favour of sporty, agile and handling which can be safely kept under control, the new SL has a considerably wider track than the predecessor (front axle plus 38 mm, rear axle plus 63 mm).
At the front axle, the tried-and-trusted principle of the 4-link front suspension has been adopted from the previous model series. It achieved superb results where comfort, dynamism, agility plus active and passive safety are concerned. Most importantly they are characterised by two individual links (a torque strut and a spring link), which are included in place of a lower wishbone. In conjunction with the ABC suspension no torsion bar is necessary.
In order to optimise the front axle even further in terms of weight, in the new SL the steering knuckle and the spring link are also made of aluminium. The lower front axle elements, the steering gear and the engine mounts are linked with an aluminium assembly carrier which is directly bolted to the body.
A lightweight construction was also systematically implemented at the rear axle, too. In order to reduce the unsprung masses, virtually all the suspension parts are made of aluminium, as is the subframe with load-matched panel thicknesses. The weight-reduced rear axle carrier consists of high-strength steel.
But the focus was not just on saving weight when developing the rear axle; the engineers have also optimised the kinematics and elastokinematics, with the objective of achieving more agility. Other results of the new configuration are lower forces, which considerably increases comfort. An additional support linking the rear subframe bearing to the bodywork also has a positive effect on the new SL’s ride comfort. It increases the introduction rigidity – a parameter whose properties include excellent road roar characteristics where NVH (noise, vibration, harshness) qualities are concerned. Thanks to these constructive measures relating to materials technology, the new luxury roadster offers a chassis with outstanding comfort coupled with excellent driving dynamics.
Like its predecessor, the new SL with adjustable damping also has an anti-roll bar, which is mounted directly at the rear axle and assembled together with the subframe, and serves to reduce the connection points with the bodyshell.
The chassis has a virtually neutral configuration, pleasing sporty drivers. In particular in the range from 60 to 130 km/h it reacts with enormous agility, and therefore facilitates a great deal of fun at the wheel on rural roads. In so doing the SL allows lateral accelerations which are impressive even for a sports car. Two different programs can be selected for the ESP®. The control interventions from the more sportily tuned variant allow side-slip angles during accelerated cornering – offering adept drivers plenty of motoring pleasure. At the same time it can be driven more easily and safely at higher speeds.
Electric Direct-Steer system for more driving dynamics
The broad spread of suspension settings is supported by a new electromechanical Direct-Steer system with speed-sensitive steering power assistance and a variable steering ratio that changes according to the steering angle. The ratio is very 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 decreases 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.
In contrast to the predecessor, the Direct-Steer system optimises the ratio course. This enables the new SL to shine with yet more agility, making cornering even more fun as a result. At the same time, through the new ratio course the maximum steering effort required when parking or turning is reduced by about 15 percent. This means that only 2.2 turns of the steering wheel from limit to limit are now necessary.
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 safely and precisely, and with enormous enjoyment.
Despite its remarkable effect, the key component of the Direct-Steer system is a simple 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 ingenious solution, Mercedes engineers have been able to dispense with the elaborate actuators and complex sensor systems used by other variable steering systems. The advantages of the Direct-Steer system therefore include very low susceptibility to faults and low weight. Moreover, the system always responds predictably and in the same way at each steering angle, while other variable steering systems sometimes require rapid adaptation by the driver in rapidly changing situations.
Instead of hydraulically, as is the case with the familiar speed-sensitive steering from Mercedes-Benz, the Direct-Steer system in the SL regulates the steering force electrically. This is taken care of by an electric motor – positioned axially parallel to the toothed rack – which transfers its power to the ball screw as a rotary movement via a toothed belt. The electric motor, drive unit, control unit and sensor system are integrated in a compact unit.
The control unit evaluates a large number of input variables in order to configure the steering torque optimally in accordance with the situation in question. A torque sensor determines the necessary assistance required at the steering-gear input shaft. In so doing the electronics take into account, for example, different vehicle axle loads. Due to the different engine weights, the characteristic maps for six-cylinder and eight-cylinder models are proof positive of the engineering artistry that lies behind the system.
Further advantages of the electric steering
- More active response: the electronics support the steering return, meaning that drivers hardly need to steer in order to return to the centre position.
- Lower consumption: as output is only called up when the driver is actually steering, the consumption falls by up to 0.3 l/100 km compared with the hydraulic Direct-Steer system.
- Active parking: with the electromechanical Direct-Steer system the Automatic Parking Control (APC) system can be offered in the SL for the first time.
Assistance for critical road surface conditions
A further assistance function has been realised in the new SL thanks to the electromechanical steering power assistance – the Steer Assist system: it can support stabilisation of the vehicle if the vehicle brakes on a road surface with different friction (µ-split) on the left and right, making it pull to one side. Drivers can find themselves confronted with this situation, for example, on a road surface which is covered with ice on one side. When braking, the vehicle then turns in the direction of the high friction – referred to by experts as a yaw impulse.
In this case the steering assistance system indicates to the driver via the steering wheel that they should countersteer in the correct direction in order to compensate for the yaw impulse caused by the difference in friction – until the vehicle has stabilised again. In order to do this the control unit evaluates the difference between the ABS pressures on the right and left, giving an indication of different road surface friction coefficients, and it also includes the ESP® data which register oversteer in this sort of situation.
By way of indication, the electric steering power assistance impresses a steering torque which supports the driver’s own reaction like a suggestion. This steering torque, which causes the driver to steer correctly through a light impulse, is hardly perceptible, and most of the time it is not even noticed at all. Nevertheless the assistance function is effective. However the steering assistance system merely provides an indication of the direction in which the driver should steer in order to stabilise the vehicle; it does not overrule the driver.
Brakes for sporty driving enjoyment
The new SL has adopted the proven braking system with a dual-circuit brake and the split of the two braking circuits between the front and rear axle from the predecessor model. At the front all the variants are equipped with newly developed 4-piston fixed-calibre brakes with an aluminium monobloc design and perforated brake discs. 1-piston combined floating brake callipers are fitted at the rear.
In order to be able to provide the required output from the braking system, the heat arising at the brakes must be removed as quickly as possible. This is why special attention was given to aerodynamically favourable and highly effective brake cooling. On the basis of different wind tunnel and road tests, the aerodynamics engineers developed a system comprising several components for targeted air ducting in the front axle area. A brake air channel has been integrated into the front engine compartment trim in order to guide the air over the front wheel spoiler in a targeted manner and steer it onto a guide blade. The guide blade fixed to the front suspension link diverts the air coming from the brake air channel over the brake discs to the exterior. Together with a front matched wheel spoiler, this has achieved effective cooling of the brakes. The newly developed brake cover plates consist of steel at the front axle and aluminium at the rear axle.
Brake disc dimensions
As a new feature in the SL, the electrically operated parking brake 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. It is also 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.
Under normal circumstances, irrespective of the duration of operating, the electric parking brake only closes the responsible combined floating brake calliper at the rear axle fully and for a sustained period when the vehicle is at a standstill or at speeds below 4 km/h. Provision has also been made for an emergency braking function: if the driver pressed the electric parking brake at a speed higher than 4 km/h, the SL brakes to a safe stop with the aid of the service brake. In this case, the emergency braking function is taken care of by the hydraulic unit of the ESP®, with the brake lights also lighting up.
The emergency braking is interrupted as soon as the control element is no longer kept depressed.
Torque Vectoring Brakes: ESP® helps with the steering
The new SL includes the Torque Vectoring Brakes system as part of its standard specification. This ensures an additional safety margin at the physical limits, makes the roadster even more agile and therefore enhances its sportiness. 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 Brakes generate 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 SL 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 is that the Torque Vectoring Brakes can be implemented without an increase in vehicle weight and therefore no disadvantages in terms of fuel consumption.
Wheels and tyres – a broad base for the SL
As part of its standard specification the SL 350 is fitted with 17-inch light-alloy wheels with 255/45 R17 tyres – options ranging up to 19-inch wheels are available for selection on request. The SL 500 is shod as standard with 18-inch wheels with mixed-size tyres (8.5 J x 18 ET 35.5 with 255/40 R 18 tyres at the front, and 9.5 J x 18 ET 47.5 wheels with 285/35 R 18 tyres at the rear).
Credits: Daimler AG
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