How Does It Work?
Mercedes-Benz 4Matic Solutions
Axel & Elaine Catton
Images Daimler Media
Ever since the automobile’s invention, people have been fascinated with the question of which wheels should move the car. The idea of, “What if we drove more than just the rear wheels?” came into play as early as 1898 with the Lohner-Porsche, which had an electric motor at each of the four wheels.
As far as Daimler is concerned, it was Paul Daimler, son of Gottlieb Daimler, who drafted Daimler-Motoren-Gesellschaft’s first all-wheel drive design as far back as 1903. As is often the case with technical development, it was the military that provided the impetus and the first Daimler to appear with all its wheels driven was an armored reconnaissance vehicle, which was tested successfully in 1905 before being delivered to the Austrian army. Powered by an 8.5-liter, 4-cylinder engine developing 50 horsepower at 1,000 rpm, it had a ground clearance of 13.1 inchess and an indicated climbing ability of 18 percent.
According to a German magazine of the day, Illustrierte Zeitung, “As trials demonstrated, these [vehicles] were capable of overcoming considerable gradients, even over rough ground and when fully laden. They are also capable of moving at considerable speed over different types of terrain.” In addition to four-wheel drive, the vehicles were fitted with a cable winch to haul themselves over obstacles. Further military contracts followed for all sorts of applications ranging from haulage to ambulances.
On the back of this knowledge, DMG turned its hand to an all-wheel drive car, building the Dernburg in 1907 – although calling it a car is a bit of a stretch. It was built on a commercial-vehicle chassis, had a wheelbase of 13.1 feet, a track of 56 inches and a ground clearance of 12.6 inches. It also had the innovative feature of having all-wheel steer as well. This automobile colossus seated two up front and four in the back, with a set of steps provided to overcome the somewhat awkward entry height of about 3 feet.
Subsequently, driving all four wheels was limited to military and off-road use, with complicated locking differentials that could put power directly to all wheels, but only when terrain and ground cover required.
These special-purpose vehicles aside, the first proper series-production passenger car with all-wheel drive was … an Audi? Wrong. Not a Subaru either, although they were earlier than the Audis. The long-defunct Jensen brand from England holds the distinction of being the first, with the 1966 Jensen FF (Ferguson Formula) featuring Ferguson all-wheel-drive and even a Dunlop Maxaret braking system, an early ABS. Only a little more than 300 were ever produced.
The early to mid-1980s were breakthrough years for all-wheel drive in passenger cars. Thirty years ago, in 1985, Mercedes launched its first all-wheel drive sedans with the 4Matic trademark name it adopted for the technology, the 1985 300E, 300D, and 300TE. Today, there are no less than 76 different Mercedes all-wheel-drive passenger-car models available. But what makes all-wheel drive so desirable, even for people who don’t live in the snowbelt or on the coast?
All-wheel drive in passenger cars is not only about traction and adverse weather; it is first and foremost about stability. As cars have become ever more powerful, it is even more critical today to get this power onto the road and into forward propulsion. A necessary element in forward acceleration is wheel spin. If you channel, say, 300 horsepower to the rear wheels and the road is ever so slightly damp, chances are your rear wheels will spin heavily before drying the surface enough to gain traction and translate that power into forward motion.
Splitting this power among all four wheels instead of two has two benefits. First, the amount of torque sent to each individual wheel is reduced, thus reducing the chances of wheel spin. Secondly, with both axles sharing the job of propulsion, the act of “pushing and pulling” results in better use of patches of road with more traction and overcoming areas of slippery surface.
In reality, what you will notice in a car with 4Matic versus rear-wheel drive is that, because the front axle is also pulling the car, even large amounts of power cannot cause the rear to step massively out of line. On the other hand, front-wheel-drive cars such as the CLA are transformed by all-wheel drive, which greatly reduces the tendency to over steer. All in all, a car with all-wheel drive is better balanced, safer to maneuver at the limits, accelerates faster and runs better in adverse weather. What’s not to like?
Admittedly, there is a weight penalty because you are carrying around an additional drive shaft and driven axle, which inevitably also results in slightly increased fuel consumption. So it makes sense to analyze the expected usage for your car before deciding on front-, rear- or all-wheel drive. Below, we have put together a general overview of the main differences in the three 4Matic systems available for Mercedes-Benz passenger cars today.
“Classic” 4Matic – for rear-drive architecture
This classic system is based on the rear-drive platform used for C-, E- and S-Class sedans, coupes and wagons, as well as the new V-Class. Here the engines are installed longitudinally, with the integral transmission and transfer-case unit providing a compact, lightweight arrangement. The standard torque split between front and rear axle in this setup is 45:55, which makes for very predictable handling.
How does it work?
A double-disc clutch with a basic locking torque of approximately 50 Nm (36.9 lb-ft) between the front and rear axle is fitted to the central differential. The clutch pack is permanently preloaded via a plate spring. Should one of the wheels spin, torque is transmitted to the slower axle through relative movement of the discs. This principle of variable torque shift brings with it a sustained improvement in traction and stability. Especially when the road is very slippery, the stabilizing effect of the prelock clutch is very noticeable. One important feature of this setup is that the transmission tunnel to the rear is of the same width as in rear-drive configuration. The rear constant velocity joint between the prop shaft and front axle was integrated into the transfer case’s output gear, which helps keep the gearbox narrow. The compact design also increases the natural flexibility of the engine and transmission assembly, which keeps noise and vibration levels low.
4Matic for front-drive architecture
The front-drive platforms of the A-, CLA-, GLA- and B-Class families start off with a different problem. Their base models don’t have a prop shaft to the rear axle, so the new setup must accommodate this major change.
This 4Matic system is activated according to the basic principle of “as often as necessary, as rarely as possible.” Whenever possible, the compact models run in front-wheel-drive mode, but as soon as the driving situation requires, drive torque is also channeled to the rear axle.
How does it work?
The power flow to the rear drivetrain is provided by a compact power take-off unit (PTU), which is fully integrated into the main transmission and supplied with lubricant from the latter’s oil circuit. This configuration has substantial weight advantages over systems with an add-on component with its own oil circuits. The new power take-off to the rear axle integrated into the 7G-DCT automated dual-clutch transmission and the rear differential with its integrated, hydraulically actuated multidisc clutch allow for fully variable distribution of drive torque between the front and rear axles. The two-piece driveshaft is decoupled in terms of vibrations. An axially moving joint at the front compensates for longitudinal movements of the engine-transmission unit, while an elastic, flexible coupling reduces the intrusion of high-frequency gearing vibrations at the rear differential. To minimize crash loads, the driveshaft is fitted with a telescopic element.
The electro-hydraulically actuated multidisc clutch integrated into the rear differential is responsible for the fully variable torque distribution. The fundamental operating principle is, when the multidisc clutch is open, the car is driven exclusively by the front axle. When the clutch is closed, the rear axle comes into play. However, the drive torque can be shifted fully between the front and rear axle according to the given situation (torque on demand).
AMG Performance 4Matic
As an option for AMG performance models – E63 and E63 S, as well as CLS63, S63 and S63 Coupe – Mercedes-AMG has developed a new AMG Performance 4Matic system. The main difference to the other 4Matic systems is the stronger rear bias in the torque split of 33:67 for an even more dynamic drive experience and better acceleration from a standing start.
How does it work?
Generally, the setup is similar to the 4Matic for the E-, S- and C-Class models. In combination with the AMG speedshift MCT 7-speed gearbox, the stronger rear bias improves traction in slippery and wintry road conditions. On the road, you can feel the benefits of the all-wheel-drive system in terms of traction and stability while still keeping the overall feeling of an extremely sporty rear-biased setup.