All-wheel drive for all – tailor-made solutions
Mercedes-Benz introduces a new generation of the 4MATIC permanent all-wheel drive system with fully variable torque distribution. This totally new development is based on the front-wheel drive architecture with transversely mounted engines. Like the other 4MATIC versions, it boasts high tractive power reserves and outstanding agility accompanied by optimum driving safety and energy efficiency. The new 4MATIC will complement the four all-wheel drive versions which have been available to date. This will ensure 4×4 technology tailored to the respective vehicle categories – from the new compact class and the models of the C-, E-, S-, CLS- and CL-Class to the comprehensive range of SUVs and ATVs covering the GLK-, M-, GL-Class and G-Class. The new 4MATIC will also boost driving enjoyment in the two particularly dynamic Mercedes-Benz CLA and CLA 45 AMG models.
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. Similarly to the front-wheel drive versions, all 4MATIC models also boast good energy efficiency.
The 4MATIC all-wheel drive is a completely new development tailored to the requirements of the new models with front-wheel drive. The future all-wheel drive line-up from Mercedes-Benz will be heralded in the coming year by the CLA-Class and the particularly agile and powerful CLA 45 AMG. While the technical basis is identical on all versions, the AMG models feature a modified power output to the rear axle.
Integrated power take-off unit (PTU) channels power to the rear axle
The new 4MATIC is being combined with the 7G-DCT seven-speed automated dual clutch transmission which was premiered in the B-Class. The power flux to the rear powertrain is provided by a compact power take-off unit (PTU) which is fully integrated in the main transmission and is supplied with lubricant from the latter’s oil circuit. This configuration gives rise to substantial weight advantages in comparison to rival 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.
The low system weight of the new 4MATIC is not attributable solely to the PTU. Other weight-stripping measures include a compact rear-axle gear unit, inductively hardened rear-axle side shafts in hollow design and weight-optimised propeller shafts. The system weight of the new 4MATIC is up to 25 percent lighter than the all-wheel drive versions offered by the competition.
The two-part cardan shaft is vibrationally decoupled, with an axially moving joint at the front compensating the movements of the engine-transmission unit and preventing transfer to the rear powertrain. To minimise crash-induced stress, the cardan shaft is fitted with a crash joint.
Completely new development with torque-on-demand rear axle
The electrohydraulically actuated multi-disc clutch integrated in the rear axle gear unit is responsible for fully variable torque distribution. Operating principle: when the multi-disc clutch is open, the CLA is driven almost exclusively by the front axle. When the clutch is closed, the rear axle comes into play. This means that 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-type pump which is integrated in the rear-axle gear unit. The pump is activated automatically as soon as only minimal speed differences apply between front and rear axle. Pressure control is performed by a proportioning valve under the steering of the ESP.®
Intelligent control systems for more enjoyable driving
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 CLA 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 understeering or oversteering 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 CLA stable. 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.
The torque distribution ratio is also dependent on the activated shift programme of the 7G-DCT dual clutch transmission. The ECO programme 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. On the AMG versions, adaptation of the 4MATIC controller takes place in accordance with the 3-stage ESP.®