The 2009 S-Class not only impresses with its exemplary ride and suspension comfort, but also meets the highest expectations in terms of driving dynamics and agility
This is in large measure due to various new and improved systems such as Direct-Steer, which varies the steering ratio according to the steering angle, and the modified Active Body Control (ABC) with crosswind stabilisation for the eight and twelve-cylinder models.
The basis for the exemplary ride comfort of the Mercedes-Benz S-Class is still the well-proven combination of a four-link front suspension, multi-link independent rear suspension and AIRMATIC air suspension. The intelligent combination of all systems, their precise coordination and their reliable interaction are the reason why Mercedes-Benz holds such a leading position with its flagship model.
More cornering enjoyment and steering precision thanks to the Direct-Steer system
The new Direct-Steer system was developed from the familiar speed-dependent 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 to them. This changes depending on the steering angle. The ratio is normal when the steering is more or less centred, which makes for good straight-line stability and therefore safety. The steering ratio changes very rapidly to become more direct as soon as the steering angle reaches five degrees, reaching its end value at a steering wheel angle of ninety degrees, where the steering feels very direct. Only relatively small movements of the wheel are then required to make course corrections. With the new Direct-Steer system, the number of turns of the steering wheel required from stop to stop is reduced by around 25 percent.
The perceived effect of Direct-Steer is however even more important than measured data. Even at inner-city speeds the driver needs to turn the wheel less frantically, with changes of direction faster and involving less effort. Winding country roads also become the natural habitat of the Mercedes-Benz luxury saloon equipped with Direct-Steer, as very small steering wheel movements are necessary to steer the car. 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.
Variable steering ratio by mechanical means
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. Starting from the neutral position for driving straight ahead, they are spaced increasingly further 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.
Torque Vectoring Brake: ESP® helps with the steering
Extra safety at the physical limits, as well as even more agility, is provided by the Torque Vectoring Brake in the 2009 S-Class – targeted, one-sided braking intervention at the inside rear wheel when cornering. If ESP® detects a tendency to understeer, the newly developed 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 S-Class 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. This Torque Vectoring Brake function is standard equipment in the 2009 S-Class.
The S-Class steers precisely when cornering thanks to targeted braking intervention at the inside rear wheel.
Active Body Control reduces the effect of a crosswind
Mercedes-Benz offers the Active Body Control (ABC) suspension system as an optional extra for the eight-cylinder S 450 and S 500 models with rear-wheel drive. It is standard equipment for the twelve-cylinder S 600. This system, which adapts the springs to the current driving situation within fractions of a second, has been improved even further for the 2009 S-Class by the suspension engineers with the addition of crosswind stabilisation.
The effect of crosswind is largely compensated by varying the wheel load distribution.
In strong crosswinds, and depending on the direction and intensity of the wind having an effect on the vehicle, this system varies the wheel load distribution in such a way that the effects of crosswinds are largely compensated or reduced to a minimum. For this purpose the ABC control unit uses the yaw rate, steering angle and lateral acceleration sensors of the Electronic Stability Program ESP® to vary the wheel load distribution via the ABC spring struts.
The system filters out weak crosswind signals, only coming into action when a certain threshold has been exceeded. In this case the driver is assisted by a change in the diagonal wheel load distribution, for example affecting the left front wheel and the right rear wheel. Specialists refer to this as Active Body Control crossover. The resulting steering effect is sufficient to reduce the effect of the crosswind.
Crosswind stabilisation is active at speeds above 80 km/h, when travelling straight ahead or taking gentle corners. If the driver takes purposeful and immediate corrective action himself, the function deactivates itself. Trials with test drivers, in which wind conditions akin to hurricane speed were simulated, elicited a positive response from all those involved.
Technical background to ABC: In this Mercedes-Benz active suspension system, the four spring struts are equipped with microprocessor-controlled plunger cylinders that can almost completely compensate for lifting, rolling and pitching of the body. The computer uses various acceleration sensors to obtain information on the current driving situation, and compares these data with those from the pressure sensors in the spring struts and the level sensors on the control arms. The system then computes the control signals, which the servo-hydraulic valves at the front and rear axle transform into precisely metered oil flows.
Once oil flows into the plunger cylinders, this modifies the tracing point of the steel springs integrated into the spring struts, generating the necessary force to counteract the body movements. Thanks to a constantly available hydraulic pressure of up to 200 bar, ABC is able to stabilise the body practically instantly, within fractions of a second. Short transfer lines and a fast processor enable the latest development stage of ABC to reduce body roll by 60 percent when cornering at speed.
At speeds between 65 and 140 km/h, ABC gradually lowers the suspension level by up to 15 millimetres to reduce aerodynamic drag. If more ground clearance is required on very poor road surfaces, the S-Class driver is able to raise the suspension level by 40 millimetres at the touch of a button.
AIRMATIC air suspension with Adaptive Damping System
AIRMATIC air suspension is included as standard in the S 350, S 400 HYBRID, S 450, S 500, S 350 CDI BlueEFFICIENCY and S 450 CDI, as well as the 4MATIC variants. Mercedes-Benz combines this with the Adaptive Damping System (ADS), which continuously regulates the shock absorber characteristics and takes the road conditions, driving style and vehicle load into account. The system adapts the damping forces for each individual wheel to the current situation within just 50 milliseconds.
Pressing the “S”, “C” and “M” key in the centre console enables the driver to individually adjust the vehicle level and suspension. In “Comfort” mode the body is automatically lowered by up to 20 millimetres from a speed of 120 km/h, and from 100 km/h in “Sport” mode, so as to improve handling, aerodynamics and fuel economy even further. In addition to this, the driver can change the transmission characteristics of the saloon from “comfortable” to “sporty”. In “Manual” mode the 7G-TRONIC automatic transmission can be shifted manually using the keys on the steering wheel.
The “S”, “C” and “M” keys are also on board if the S-Class is equipped with Active Body Control.
ADAPTIVE BRAKE: high-tech brakes for maximum safety
The Mercedes-Benz S-Class also represents the technological standard where its braking system is concerned: thanks to electronic control, the standard ADAPTIVE BRAKE allows control functions that enhance safety and comfort. These include priming of the braking system in critical situations: when the driver changes abruptly from the accelerator to the brake pedal prior to emergency braking, ADAPTIVE BRAKE increases the pressure in the brake lines to bring the brake linings close to the discs, so that they are able to enage instantly, and with full force when the brake pedal is operated. In this way the system supports the standard Brake Assist and Brake Assist PLUS.
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.
After the S-Class has been braked to a standstill, briefly pressing the brake pedal a little further is all that is required to activate the HOLD function. The car is then held by the brakes, even if the driver’s foot comes off the brake pedal. In this way ADAPTIVE BRAKE prevents the car from rolling forward inadvertently when stopped at traffic lights or stuck in stop-and-go traffic, and from rolling back when facing a slope. The HOLD function is deactivated automatically when the car moves off.
Extra-large brakes guarantee maximum deceleration
Large brake discs at the front and rear provide the technological basis for safe and reliable deceleration using ADAPTIVE BRAKE. Depending on the engine installed, the disc diameters range up to 360 mm at the front and up to 330 mm at the rear. The front brake discs are perforated for all model variants. The perforations shed moisture and dirt, which increases the brakes’ reliability and maintains their excellent deceleration values even in inclement weather conditions. A 9.5-inch tandem brake booster satisfies high standards for responsiveness and ease of use.
The S-Class also features an extremely easy-to-use electric parking brake as standard. Pressing a button on the dashboard is all that is needed to activate the brake. If the driver engages a gear and drives off, the brake is automatically released. The key components of this system are two separate servo drum brakes on the rear wheels and an electric motor with speed-reducing gear that applies the brake clips by means of cables. The electric parking brake can also be engaged if the engine is switched off.
Wide choice of wheel/tyre combinations
New light-alloy wheels ensure that the Mercedes-Benz flagship model impresses with an even more dynamic appearance. The V6 saloons are shod with 17-inch wheels in a seven-spoke design, while the V8 models have newly designed 18-inch wheels with five spokes. The twelve-cylinder S 600 is fitted with 18-inch five-spoke wheels of different widths as standard (front: 8.5-inch; rear: 9.5-inch).
The tyre pressures are monitored by the standard Electronic Stability Program ESP® or, on request, the electronic tyre pressure monitoring system. Should a puncture nonetheless occur, all S-Class models are equipped with the tyre sealing kit TIREFIT and an electric air pump. An emergency spare wheel is available as optional equipment.
Effortless progress in any weather: the S-Class with 4MATIC all-wheel drive
Mercedes-Benz offers 4MATIC all-wheel drive variants of the S 350, S 450 and S 500. The centrepiece is a centre differential with a planetary gear set. The permanent, defined torque distribution of 45:55 percent between the front and rear axles ensures reliable and highly predictable driving characteristics on all road surfaces. An integrated multi-disc clutch guarantees even more traction and optimal handling stability with the onset of slip.
Mercedes-Benz combines 4MATIC with the Electronic Stability Program ESP®
and the traction system 4ETS, which specifically brakes spinning wheels and transfers the torque to the wheels which still have good traction. The system uses sensor signals to meter the automatic braking impulses that increase traction when moving off on slippery surfaces and can improve handling in critical situations. 4ETS achieves the effect of conventional differential locks and provides more comfort than the technology used in other all-wheel drive vehicles.
Owing to its compact, lightweight, low-friction design, 4MATIC all-wheel drive has definite advantages over other systems in terms of weight, fuel consumption, comfort and passive safety. Depending on the engine variant, this all-wheel drive technology incurs an unrivalled, low weight penalty of only 66 or 70 kilograms.
Source: Daimler AG