You have to go back to the 80s to find the first traces of the new generation four-wheel drive in Audi, BMW and Mercedes. The three have made their mark since the dawn of time, but they have chosen almost diametrically opposed technologies for all-wheel drive …
The principle of the Quattro is simple and dates back to 1980. A longitudinal engine, and a mechanical central differential which must be activated manually. Later, Audi switched to a mechanical central differential, always, but of the Torsen type (which is activated by the couple, we come back to this below). It is therefore a permanent four-wheel drive (4WD), with open front and rear differentials, which do not prevent the large differences in rotation between right and left wheel.
Progress on the Torsen (replaced today by what Audi calls the “Crown Gear”) with crowns and pinions allowed Audi to vary the Quattro: 40/60% of the front / rear torque in normal conditions, but in the event of loss of traction: up to 85% behind, or 70% in front. With the arrival of ESP and traction and traction sensors, it is also possible to also play on the left / right differences, in addition to the front / rear. Clearly, if 3 of the 4 wheels slip, the EDL (electronic differential), which is based on the ABS sensors, makes it possible to brake the wheel which slips to give traction to the only wheel which can still have grip. The central differential, meanwhile, plays its role by sending torque to the train which has a wheel still having traction.
Several Quattro …
Since nothing is easy in an automobile, there is not just one Quattro. Today there are 3. The first is the permanent Quattro with Torsen described in the previous paragraph, which is found on the longitudinal engines of the MBL platform. The second is a “fake” Quattro since it is a system manufactured by the Swede Haldex (bought by BorgWagner), and associated with transverse engines: Volkswagen Golf, Audi A3 … all the compact cars of the Volkswagen group are entitled to it. The big difference is that it is a predictive system, and not a passive one: sensors analyze in real time the loss of front or rear traction to adapt the torque to be sent to the two trains, via a clutch. multidisk that connects them.
There are several advantages to the Quattro “Haldex”: more compact (no need to have a longitudinal engine), lighter, and more responsive when the vehicle is stationary in a situation of precarious traction. Indeed, the Quattro at Torsen needs torque to activate, while the Quattro “Haldex” corrects (in a quarter of a second) the management of torque on reception of data from the sensors. On the other hand, it is not possible to have a distribution greater than 50/50: in short, where the Quattro Torsen can send up to 85% of the torque to the rear, the Quattro Haldex does not exceed 50% in back. A system that has its limits in sporty driving, but that we nevertheless find on Volkswagen Golf GTI, R, but also Audi S3 and RS3. Even if the German group has changed its “light” all-wheel drive (we saw it on the very recent Golf 8 R), it does not prevent that at high speed, the behavior is less incisive and more neutral than on a Standard Quattro.
Let’s not forget the latest Quattro system, the most recent: Quattro Ultra. It’s a classic Quattro with a longitudinal engine, except that the central differential is not a Torsen … but a multi-plate clutch! Clearly, it’s a mix of the other two Quattros, developed to reduce fuel consumption on large Audi sedans. With a multi-plate clutch, it is indeed possible to drive in 2-wheel drive and limit friction, while a traditional Quattro is a permanent four-wheel drive.
BMW developed the xDrive system more than twenty years ago. If the brand was already offering four-wheel drive road cars from the end of the 1980s, it was a very basic “passive” system by viscocoupling: in short, an oil that turns and locks the differential in case of Too large speed difference between front and rear axle. The xDrive is totally different. It is similar to what we find in the Volkswagen group with the Quattro “Haldex”, with one huge difference: the xDrive is able to send 100% of the torque on one of the two trains, while the Haldex is reminds, is limited to 50% at the rear.
Concretely, this means that the Haldex mixes the best of both worlds: a very flexible central differential, capable of operating in two-wheel drive (propulsion at BMW) and a compact system lighter than a fully permanent four-wheel drive and mechanical differential . Like the Audi Quattro, the xDrive operates at 40/60% in conventional conditions, but thanks to the sensors analyzing the data (steering, accelerator pedal, wheel speed, braking, etc.) from the computer, the system continuously adapts the torque distribution.
The “fake” xDrive
As the Germans don’t like to simply do anything, there are two different xDrives. The one associated with longitudinal engines, and the one found on the very latest 1 Series in traction and on the Mini. And there it is quite simply the famous Haldex four-wheel drive, described above! The Swedish giant indeed supplies several manufacturers, the Volkswagen group not being the only one …
Much like BMW and Audi, Mercedes worked early on on a four-wheel drive system for its passenger vehicles. Introduced on the W123, the 4Matic has greatly evolved. Starting off with a system with a central differential that could be locked in the event of loss of grip, and even a rear differential also with locking. Mercedes has greatly simplified things …
The 4Matic now differs from its counterparts, and wants to be less sharp. At BMW and Audi, it is the central differential which acts on the distribution of torque between the front and rear axles, either passively (Audi Quattro) or actively controlled (BMW xDrive). At Mercedes, the central differential is open: there is therefore no differential lock.
Under normal conditions, it offers a configuration of about 40/60 (variable depending on the model). It is the central “brain”, the electronic 4-ETS device, which takes care of managing the wheels independently when there is a loss of traction, by braking the wheel (s) that slip or lose grip. . ABS, ESP, 4Matic four-wheel drive and 4-ETS work together to optimize torque distribution and traction.
A small variant of the 4Matic has been introduced in recent years: the 4Matic +, with torque variation. This allows the front axle to be completely disengaged to switch to propulsion and have 100% of the torque at the rear. This has notably made it possible to offer a “drift” mode on certain AMG models.
Here again, at Mercedes, we must distinguish the 4Matic from the top-of-the-range models (from the “C” ranges) from the 4Matic from the compacts. This is very similar to the Haldex: compact, light, and disengageable with a multi-plate clutch which can engage the rear axle. The distribution can then go up to 50% of torque sent to the rear wheels, just like the Haldex. This system is found in particular on the A45 AMG Class.
Quattro, xDrive or 4Matic?
The answer is simple: it depends on the conditions! All have advantages and disadvantages, and no one system is great in all areas. Some are very efficient in action, but a little heavy and greedy, others are more compact, but more fragile. On paper, the 4Matic system still seems to be the least advanced of the three, in difficult conditions. But in normal road use, the driver will only see fire. It takes sporty driving and / or very complicated conditions to start seeing the differences between each technology.
- Efficient (especially at high speed, or once launched)
- Renowned for reliability and durability
- Can handle large torque values
- Heavier than a multi-disc system
- Generates greater overconsumption than its competitors
- Less compact
- Less efficient when conditions are very severe (zero grip of an axle when starting)
- Compact and rather light
- Efficient from start-up, on difficult terrain
- 100% of torque front or rear
- More “fragile” than a mechanical differential system
- Reactions a little slower than with a mechanical differential (Quattro)
- Very light
- Compact and generates little overconsumption
- Less efficient than a mechanical differential
- Use the brakes