by Adrian-Liviu Dorofte

The new Mercedes-Benz CLA-Class: Chassis, Suspension and 4MATIC AWD System

XI. The chassis and suspension: In the service of dynamism

The chassis of the CLA features a McPherson front axle and an independent multilink rear suspension, featuring three control arms and one trailing arm per wheel. This means that longitudinal and lateral dynamics are virtually independent of one another. Direct Steer is standard for all versions.

A CLA-specific feature is the elastic decoupling of the rear axle carrier, which benefits ride comfort. The measures include rubber bearings vulcanised onto the anti-roll bar and the elastomer coating of the coil springs, which produces an effective noise de-coupling between the vehicle body and rear axle.

The connection of the damper to the vehicle body is via an aluminium bearing with a soft universal joint that contributes to reducing friction in the damper, and consequently helps responsiveness. Wheel carriers and spring links are made of aluminium, in order to reduce the unsprung masses.
Two suspension set-ups are available: the comfort suspension and the optional sports suspension for sporty yet comfortable handling. The latter entails lowering of the body by 15 mm (front) and 10 mm (rear).

Irrespective of the selected chassis, all CLA variants come with the Direct Steer system. This electromechanical power steering offers improved feedback to the driver in comparison to conventional systems and makes an important contribution towards overall efficiency, as the steering assist function only requires energy when the steering wheel is actually turned. Additionally it enables various steering assistance functions which are activated by the ESP® control unit. These include countersteering in case of oversteering, corrective steering when braking on road surfaces offering different levels of grip (split-friction braking), mitigation of the extent to which the front-wheel drive influences the steering and compensation for crosswind and road gradients.

With all engine variants, the CLA-Class comes with disc brakes on all wheels. The callipers on the rear axle and the brake boosters are made of aluminium. A particularly convenient feature is the HOLD function, which is familiar from the larger model series: when stopping, for example at traffic lights, the driver merely has to press the brake pedal slightly more firmly and the brake will remain engaged until they move off again. The brake is released automatically when the driver steps on the accelerator. On versions with manual transmission, the Hill Hold function automatically prevents the vehicle from rolling back unintentionally when moving off on a slope.

The CLA-Class is equipped with an electric parking brake, which operates by means of actuator motors acting on the callipers of the rear axle. The parking brake is activated via a button under the light switch on the left of the dashboard. This creates additional space in the centre console, as the handbrake lever is no longer required. When the button is pressed at speeds of over four km/h, the parking brake acts as an emergency brake, activating all four wheel brakes via the ESP®'s hydraulic unit.

In conjunction with the 7G-DCT automatic transmission, the parking brake offers a particularly convenient mode of functioning: when the driver accelerates sufficiently after fastening their seat belt, the parking brake is released automatically.

XII. The 4MATIC all-wheel drive system: Extraordinary agility and high traction reserves

The CLA from Mercedes-Benz is available with a new variant of the 4MATIC permanent all-wheel drive system featuring fully variable torque distribution. Similar to the other 4MATIC versions, this new development boasts high tractive power reserves and outstanding agility combined with superlative driving safety and energy efficiency.

The most innovative components of the new 4MATIC include the power take-off to the rear axle which is integrated in the 7G-DCT automated dual clutch transmission and the rear-axle gear unit with integrated, electrohydraulically controlled multi-disc clutch. This set-up enables fully variable distribution of the drive torque between front and rear axle. Additional benefits of this design are a lower system weight than is available from the competition and high efficiency. Similar to the front-wheel drive versions, all 4MATIC models also boast good energy efficiency.

The 4MATIC all-wheel drive system is a completely new development and was specifically adjusted to the requirements of the new front-wheel drive models.

Integrated power take-off unit (PTU) channels power to the rear axle

The new 4MATIC is combined with the 7G-DCT seven-speed automated dual clutch transmission. The power flow to the rear powertrain is provided by a compact power take-off unit (PTU) which is fully integrated into the main transmission and is supplied with lubricant from the latter's oil circuit. This configuration gives rise to substantial weight advantages of up to 25 percent over competitor systems which branch off power by means of an add-on component with its own oil circuit. Together with the friction-minimised tapered roller bearings, the PTU achieves an excellent level of efficiency.

Completely new development with torque-on-demand rear axle

The electrohydraulically actuated multi-disc clutch integrated into the rear-axle gear unit is responsible for fully variable torque distribution. Fundamental operating principle: when the multi-disc clutch is open, the car is driven almost exclusively by the front axle. When the clutch is closed, the rear axle comes into play. However, the drive torque can be shifted in fully variable mode between front and rear axle according to the given situation (torque on demand).

The system pressure to activate the rear axle is supplied in milliseconds by the rotor pump which is integrated into the rear-axle gear unit. The pump is activated automatically as soon as only minimal speed differences arise between front and rear axles. Pressure control is performed by a proportioning valve under the control of the ESP®.

Motoring enjoyment thanks to intelligent control systems

The 4MATIC is activated according to the basic principle of "as often as necessary, as rarely as possible". This means that when the underlying conditions allow, the all-wheel drive models run almost exclusively in particularly economical front-wheel drive mode. As soon as the driving situation requires, drive torque is channelled to the rear axle as appropriate. The reverse process takes place just as quickly: as soon as additional drive torque is no longer necessary at the rear axle – e.g. in case of heavy braking manoeuvres with ABS intervention – the rear powertrain is deactivated and torque is reduced to zero.

In case of impending understeer or oversteer under load, the drive torque is first of all distributed such as to stabilise the vehicle. Only if these measures fail to have a stabilising effect do the control systems adapted to 4MATIC conditions, such as ESP® or 4ETS, intervene to keep the vehicle on course.

With this strategy, the developers have managed to combine two key requirements defined in the performance specifications: optimum energy efficiency coupled with maximum driving dynamics and driving safety.

Individual: transmission modes to suit personal driving styles

The torque distribution ratio is also dependent on the activated shift program of the 7G-DCT dual clutch transmission. The ECO mode results in torque distribution of a conservative nature. Less torque is channelled to the rear axle, resulting in gentler handling and supporting an economical driving style at lower revs. In SPORT or MANUAL mode, the activation times are shortened and more torque is distributed to the rear axle in the interests of a dynamic, sporty driving style.

Credits: Daimler AG

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