The New A Class Chassis - includes tyre pressure sensors

The Chassis
  • Electromechanical speed-sensitive power steering fitted as standard

  • Innovative new spherical parabolic-spring rear axle ensures flawless dynamics and impressive comfort

  • Automatic adjustment of shock absorber force according to situation in hand

  • Latest generation of ESP® with tyre pressure loss warning as an option

A look at the chassis technology under the skin of the new A-Class provides further evidence that the new model differs fundamentally from its predecessor. Added to the mix are a series of new developments which play a major part in four outstanding characteristics of the new Mercedes compact car: active safety, driving dynamics, ride comfort and driving pleasure.

From the steering to the shock absorbers, the brakes to the rear axle, the Electronic Stability Program to the tyres, Mercedes engineers have taken a thorough look at all the chassis components, developed and incorporated innovative new systems and made telling improvements to already proven technology.

The result of this in-depth development and testing work can be seen and felt after just a few kilometres out on the road. The new A-Class takes the combination of dynamic handling and hallmark Mercedes comfort to a level above that offered by its predecessor, with both features enjoying equal importance.

In addition to these technological innovations, basic features of the outgoing model’s chassis have also been modified, some of them fundamentally. The Mercedes engineers have thus succeeded in laying the perfect foundations for further advances in terms of driving dynamics. Indeed, new dimensions for the car’s wheelbase, front and rear track width, wheels and tyres make a significant contribution to the sporty yet comfortable driving characteristics of the new A-Class.

The most important chassis data at a glance:

New A-Class* Outgoing model (standard version) Difference
Wheelbase 2568 2423 +145
Track width**
- front
- rear

1556
1551

1503
1452

+53
+99
Figures in millimetres; *Three and five-door versions; **For the A 150 / A 140 entry-level model

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Front axle: further modifications to McPherson technology

A McPherson front axle offers substantial advantages for front-wheel-drive vehicles and Mercedes-Benz has taken the development of this design to a new level. Where the wheel is normally located by the suspension strut, track rod and a camber arm with anti-roll bar, the front axle of the new A-Class can call on the additional benefits of rigid wishbones. These assume wheel location duties from the anti-roll bar, which is linked to the strut suspension by a torsion bar linkage and newly developed rubber mounts. This solution widens the scope of possible chassis settings – especially when it comes to enhancing elasto-kinematics and reducing road roar and tyre vibration - and at the same time facilitates the precise adjustment of the caster angle and camber. The wishbones each consist of two welded sheet steel shells and are attached to the frame-type integral support using large rubber mounts. Optimised sup-port heads have also been developed for the McPherson axle’s twin-tube shock absorbers.

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Steering: power assistance graded according to the speed of travel

Mercedes-Benz is introducing electromechanic speed-sensitive power steering for the first time in the new A-Class. Here, the power assistance is generated using an electric motor, which transfers it directly to the gearing of the rack-and-pinion steering mechanically rather than hydraulically. This newly developed technology impresses with a host of plus points: lower weight, more compact dimensions, in-creased effectiveness and a reduced energy requirement. However, the most important benefits are without doubt provided by the additional functions offered by this state-of-the-art steering system:

  • The electric motor is equipped with a micro computer which is integrated into the databus network of the new A-Class and regularly calls up the latest data on the travelling speed, engine speed, steering angle and torque at the torsion bar. This information is used to grade - or “parameterise”, as the experts would say – the level of power assistance according to the vehicle speed. This has the advan-tage of providing the driver with greater power assistance at lower speeds, making parking and manoeuvring far more comfortable than with a conventional power steering system. The speed-sensitive function is adjusted and programmed precisely for each body and engine variant of the new A-Class. The direct steering ratio and impressive dynamics of the newly developed system ensure precise steering feel. Active response characteristics aid the centring of the steering.
  • As the electric motor is only activated when the steering is in use, the system’s energy requirement is lower that of hydraulic steering – and that is reflected in reduced fuel consumption.
  • The electromechanic steering works without hydraulic oil, allowing for easier as-sembly and repairs. This technology also scores highly in terms of environ-mental protection, as oil disposal is not an issue.
The A-Class’ safety-enhancing and space-saving sandwich concept allows the steering gear to be positioned in front of the wheel centre. This helps to further improve the responsiveness of the steering, making the Mercedes compact car safe and easy to handle. Light and smooth handling characteristics and a pro-safety tendency to understeer combine to ensure enviable driving pleasure.

The steering column consists of an upper steering shaft mounted in the jacket tube and a lower two-piece steering shaft, whose constituent components slide against each other in a frontal collision in order to prevent the steering structure from pene-trating the interior. A universal joint connects the two steering shafts. The driver has various options when it comes to adjusting the attractively designed three-spoke steering wheel (diameter: 380 millimetres). A handle positioned underneath the steering column pulls out to allow full height adjustment (by +/- 20 millimetres), whilst the steering wheel is also available with additional reach adjustment (by +/- 30 millimetres) as an option.

As in other Mercedes model series, the steering wheel for the new A-Class is a central element in the state-of-the-art control and display concept. Thanks to the easy-reach multifunction buttons, the driver no longer has to take his or her hands away from the steering wheel in order to operate the radio, navigation system and car telephone or to access important information on the display in the instrument cluster. The standard-fitted multifunction steering wheel is made out of die-cast magnesium and is designed to deform as required in the event of a crash.

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Rear axle: new development allows exemplary directional stability

The rear axle of the A-Class is a new development whose description alone is enough to arouse the curiosity. The “spherical parabolic-spring rear axle” is the result of sev-eral years of complex development and testing work by Mercedes engineers and makes a key contribution to the exceptional driving dynamics and impressive ride comfort of the new A-Class.

The centrepiece of this intelligent construction is the forwards-arched axle housing, which takes the place of conventional control arms and serves to secure the wheels. The axle housing is fixed to the car body by separately arranged springs and shock absorbers, the anti-roll bar and a centrally located elastomer central bearing. A special linkage named after the brilliant British inventor James Watt is responsible for wheel location and support against lateral force. The two struts of the Watt linkage are attached to the rear section of the axle housing and connected by a rotating coupling in the centre to the car body.

The axle housing consists of two sheet steel shells, whilst the Watt struts are manu-factured using internal high-pressure forming processes and thus display outstanding rigidity. The axle components are protected from stone chipping by additional plastic coverings, which also offer aerodynamic benefits. The Watt coupling is a robust forged part.

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Precise wheel location and outstanding active safety at the critical limit

Precise wheel location and extremely effective anti-roll control are the major advantages of the newly developed rear axle. These two attributes make their presence felt even more strongly in an innovative vehicle concept such as the A-Class, whose particular centre of gravity would normally require a fairly stiff chassis setting. However, this special axle technology allows a more comfortable setting for the springs and damping – without detracting from the outstanding road-holding characteristics of the new Mercedes compact car. The camber angle of the wheels remains constant regardless of the driving situation, allowing higher lateral forces to be transferred be-tween the tyres and the road surface – particularly through corners – than is possible with a conventional link axle. This effect also influences the car’s tendency to under-steer, an important factor in the A-Class’ high level of active safety. Thanks to the innovative axle technology, the new model’s handling remains stable, sporty and pre-cise even when the car is carrying a load or operating at the limit.

A key attribute of the spherical parabolic- spring rear axle is the raised momentary centre of rotation – i.e. the pivot point around which the body leans to the side through corners. The distance between the momentary centre of rotation and the vehicle’s centre of gravity above it results in the lever arm, which influences the extent of the rolling motion. With the Mercedes rear axle, the momentary centre of rotation lies 280 millimetres above the road and therefore closer to the vehicle’s centre of gravity. The short lever arm prevents the rolling motion caused by vehicle lean.

The momentary centre of rotation remains constant in any situation out on the road. Even when the A-Class is fully loaded, this pivot point is only lowered by some 48 mil-limetres.

Due to the high momentary centre of rotation of the car’s rear axle and the low posi-tioning of the front axle, the roll axis of the new A-Class rises to the rear. This results in reduced body movement – a positive development which really comes into its own if the driver switches to a more dynamic driving style.

This innovative axle technology allows comfortable spring and damper ratios. The rear axle of the new A-Class features coil springs, single-tube shock absorbers and an anti-roll bar. The springs and dampers are positioned separately. The large elastomer central bearing, which links the axle housing with the car body, also ensures good structure-borne soundproofing and minimises vibrations.

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Shock absorbers: world premiere of the selective damping system

Another special feature of the A-Class chassis is an innovative selective damping system. Celebrating its world premiere in automotive construction, this new technology provides a straightforward yet extremely clever answer to a dilemma which has teased engineers for many a year. Should they favour a stiff shock absorber set-up in the interests of active safety and driving dynamics or is a softer, more comfortable setting the better all-round solution?

With the selective damping system in the new A-Class, Mercedes-Benz has killed these two birds with one stone. Working according to hydromechanical principles, this system achieves its aims without the need for complex sensors or electronics.

A brief insight into the workings of a modern shock absorber helps to clarify the func-tioning of this new development. After all, this is where the desired damping effect is generated, with wheel movements causing a valve system on the damper piston to compress oil. In so doing, flexible valve plates initiate the pre-defined resistance characteristics of the shock absorber.

With the new selective damping system, part of the oil flows through an additional valve body positioned above the damper piston itself. Inside is the control piston, which splits the valve body into two areas.

If the shock absorber experiences only minor vertical movement – consistent with a normal driving style – the control piston sits in a central position, keeping a bypass channel open, which clears the way for part of the oil to flow through the piston pin (see diagram). This oil moves past the damping valve, reducing the overall hydraulic resistance of the shock absorber. The effect is “softer” shock absorber characteristics, ensuring less road roar and tyre vibration.

If the shock absorbers are more active - during dynamic cornering or evasive manoeuvres, for example - the oil pushes the control piston in the valve body down-wards or upwards, automatically closing the bypass channel. The driver can thus rely on the full damping effect and the A-Class is stabilised to maximum effect.

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Brakes: reliable, strong and durable

In a further contribution to active safety and accident avoidance, Mercedes-Benz if fitting the new A-Class with disc brakes on the front and rear axles. The Mercedes engineers have adapted the diameter and thickness of the brake discs precisely to the improved performance credentials of the new car. For example, the front disc brakes of the A 150 and A 160 CDI measure 276 x 12 millimetres, giving them a diameter 16 millimetres greater than in the current A-Class. The powerful A 170, A 180 CDI, A 200 and A 200 CDI models are fitted with internally ventilated disc brakes (276 x 22 millimetres) at the front. The A 200 TURBO, meanwhile, comes equipped with 288 x 25-millimetre ventilated brake discs. A sensor on the right-hand calliper monitors brake pad wear, sending signals which are constantly analysed and assessed. If the thickness of the pads is reduced to a critical point, an indicator lamp lights up in the instrument cluster. The brake booster is another component to show a performance advantage over the outgoing model. The master brake cylinder of the 9-inch unit has a diameter of 23.8 millimetres (predecessor model: 22.2 millimetres).

Solid disc brakes with integrated duo servo brake for the parking brake function re-place the rear drum brakes of the outgoing model in all engine variants of the new Mercedes compact car.

An overview of the brake data for the new A-Class:

A 150
A 160 CDI
A 170
A 200
A 180 CDI
A 200 TURBO
A 200 CDI
Front axle
- Brake discs
- Piston diameter
- Disc diameter
- Disc thickness

Solid
57
276
12

Internally ventilated
57
276
22

Internally ventilated
57
288
25
Rear axle
- Brake discs
- Piston diameter
- Disc diameter
- Disc thickness

Solid
30
258
8

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The latest generation of ESP®: even greater precision, even more effective

The latest generation of the Electronic Stability Program ESP® makes a convincing impression with its even more precise and finely controlled interventions. This technology is fitted as standard in the new A-Class and is underpinned by a control unit which also contains the functions of the anti-lock braking system, acceleration skid control and Brake Assist. ESP® improves a car’s directional stability and reduces the danger of skidding under braking or acceleration or when coasting freely, by braking each wheel individually and/or adjusting the engine torque. The system also helps to shorten the ABS braking distance through corners and on roads where one side is slippery.

The hydraulic unit and electronic control module of the new generation of ESP® form a single compact unit. This means that the latest ESP® technology takes up around a third less space than its predecessor and also stands out with its lower weight. An-other new feature is the active wheel speed sensors, which are each equipped with a separate voltage supply and feed the ESP® computer even more precise data. A cen-tral point of this innovative system is the micromechanical rotational speed sensor, which records the movement of the vehicle around its vertical axis and thus recog-nises skidding movements. This sensor is located in the tunnel trim next to the park-ing brake lever and transmits its data via a separate databus system to the ESP® control unit.

For the new A-Class, Mercedes-Benz has dispensed with the button on the dashboard which allowed the driver to deactivate the acceleration skid control system. The new, intelligent control logic of the Stability Program uses data from sensors to recognise situations on the road where this function is required and automatically adapts the skid control function accordingly.

As before, the A-Class comes with the hydraulic Brake Assist system, which offers particularly notable advantages for compact cars with front-wheel drive. Where Brake Assist was previously linked up to a brake booster, the system now uses the ESP® infrastructure, making it more compact, lighter and more effective. The Brake Assist function is controlled by the high-pressure pump in the ESP® hydraulic unit. In emergency braking situations, this pump assists the driver by generating maximum pressure of 200 bar and thus helping to reduce the stopping distance. The necessary information is supplied by the wheel speed sensors, the stop lamp switch and two pressure sensors in the hydraulic unit.

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Tyre pressure loss warning based on ESP® technology

The highly efficient ESP® technology has the potential to incorporate additional functions, such as the warning system for the driver in the event of significant pressure loss in one of the tyres. Here, ESP® uses information from sensors monitoring wheel speed, which is largely dictated by the travelling speed of the car and the load and air pressure in the tyres. As the Stability Program permanently monitors the speed of the wheels and compares the values with each other, it also detects noticeable deviations.

In addition, the control unit automatically checks other road-holding variables such as lateral acceleration, yaw rate and wheel torque in order to reliably diagnose the loss of air pressure in a tyre. However, this system does not measure the actual air pressure in each tyre. If an insufficient level of pressure is detected, the warning “Tyre pressure, check tyres” appears in the central display of the instrument cluster. The automatic tyre pressure loss warning system comes as standard on A-Class variants fitted with 17 and 18-inch wheels and/or sports suspension, but is also available as an option for all other models.

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