The German automotive industry has traditionally been awe-inspiring for anyone who cares about the sound of the engine running and the sound of the asphalt tires. But which of the German automakers has been most successful in creating permanent all-wheel drive systems?
Fans of Audi quattro, BMW xDrive and Mercedes' 4MATIC are fighting hot battles on the Internet and beyond. At the same time, not everyone understands the principle of operation of a permanent all-wheel drive at all. Therefore, it makes sense to start from afar. As we know, four-wheel drive is when the torque from the engine is transmitted to both axles of the car.
Simply put, on all wheels at the same time. Now all-wheel drive is a familiar phenomenon that will surprise no one. But back in the 80s, many associated it almost exclusively with all-terrain vehicles. Often very highly specialized.
The Germans changed the world, or more precisely, the Audi engineers, who introduced their quattro system on road cars. At the same time, it would also be wrong to consider the specialists of the “four rings” as pioneers. It would be more correct to say that it was the appearance of the rally and road Audi quattro in the 80s that played a key role in popularizing the idea as such.
So what does four-wheel drive do? To begin with, it provides increased cross-country ability. We don't really need it on public roads, unless, of course, we are talking about the Lena motorway of the 2006 model. But there are other important aspects that should not be discounted. It is the four-wheel drive that allows the most efficient use of engine power in any driving mode. It significantly improves handling, especially if there is a slippery track ahead. Suffice it to say that the famous Bugatti Veyron 16.4, long considered the fastest production car in the world, has four-wheel drive. The same solution was used on its successor, the Bugatti Chiron.
There are four main four-wheel drive schemes: part-time, full-time, on-demand full-time and selectable. Each of them has its own advantages and disadvantages. Perhaps it makes no sense now to consider all four in detail. Let's dwell specifically on full-time, because our material is dedicated to it.
The permanent all-wheel drive concept implies that the engine is permanently connected to both axles by means of a differential. Some cars have a forced center differential lock, due to which they can become similar to cars with a plug-in all-wheel drive, when, under normal operating conditions, torque is transmitted to only one axle.
When creating the first cars with permanent all-wheel drive, the engineers faced a difficult task. The fact is that during the turn, the wheels of the car travel different paths. With a rigid connection of the axles, some wheels drove, while others slipped. In addition to the inconvenience of driving, this would also gradually destroy the car's transmission. For this reason, cars with permanent four-wheel drive received a transfer case with an center differential - a mechanism that serves to distribute power between the axles and allow them to rotate at different speeds. Slowing down one wheel automatically increases the speed of the other. In case of off-road driving, the differential has a lock - when it is activated, the revolutions on all wheels are the same, and the torque is determined only by the adhesion of the wheels to the road.
All of the above makes systems like xDrive a very versatile solution, but, alas, also not without drawbacks.Additional components, such as the aforementioned differential, make the system quite cumbersome and generally quite complex. There are other disadvantages as well. Due to the constant transmission of torque to all four wheels, you can, of course, achieve excellent grip on the track, however, fuel consumption in this case also increases.
Who is the first is right?
As we have already said, the pioneers here, with some reservations, can be called the Audi engineers. At the same time, quattro is not just a technical solution for a single model. Before us is a whole generation of systems that have been used on a wide variety of road cars. Germans don't like to officially share their quattros. An exception is the 2010 Audi RS5, which has been fitted with a new generation quattro.
The Audi quattro system is incredibly famous. On its basis, a group B rally car was created - the Audi Quattro (with a capital letter, so as not to be confused with the all-wheel drive system), which, by the way, confidently left all its opponents behind. The car has won two competitions in a row since its release. At the same time, he became the first rally car to use innovations in the rules that allowed four-wheel drive cars to be put up for competition.
Experts distinguish as many as six quattros and the quattro ultra system, which began to be installed on cars in 2016. Each of them is interesting in its own way, but now we will not dwell on all the advantages and disadvantages, but will try to assess the correctness of the chosen concept as a whole.
Audi uses permanent all-wheel drive with Torsen differential (TORque SENsing - torque sensing). It is a mechanical limited slip differential that uses a complex set of worm gears. It includes driven worm wheels and driving worm gears.
The main design difference is that worm gears can drive other gears in rotation, but cannot themselves be driven. This achieves a certain degree of differential lock. Depending on the size of the gear ratio and the design of the differential, the torque can be distributed in very different proportions.
Produced since 2005, the Audi Q7 crossover, which was built on the same platform as the Volkswagen Touareg and Porsche Cayenne, was equipped with a drive that has no ancestors among previous Audi models. For this model, BorgWarner has developed an all-wheel drive system, where it used a Torsen Type 3 (T3) differential.
The majority of Audi vehicles have a 50:50 torque split, thus providing near-front-wheel drive handling. In the sixth generation quattro system, the Torsen Type C center differential was replaced by a flat gear differential developed by Audi. This solution allows, if necessary, to transfer to the front and rear axles up to 70% and up to 85% of the torque, respectively. Together with advanced electronic systems, this gives excellent handling in almost any conditions: it does not matter whether it is about cornering, accelerating, braking, or even a combination of all of the above.
BMW's answer to the quattro system is the famous xDrive. It is a permanent all-wheel drive system that provides an infinitely variable, continuous and variable distribution of torque between the front and rear axles (depending on conditions). This system, in particular, has found its application on the X3 and X5 crossovers, as well as passenger cars of the 3rd, 5th and 7th series.
xDrive is nothing new. It is not in the "Bavarian" tradition to allow competitors from Audi to have trump cards up their sleeves. The first generation of this system has been in use since 1985. It had a torque distribution between the axles during normal driving in a ratio of 37:63 (37% to the front axle, 63% to the rear axle), as well as locking the center differential and the rear wheel differential using a viscous clutch (viscous coupling).
xDrive uses a typical BMW rear-wheel drive transmission scheme.The transfer case is responsible for the distribution of torque between the axles, which is a gear transmission of the front axle drive, controlled by a friction clutch. At the same time, the transmission of crossovers has not a gear, but a chain transmission.
When cornering with understeer, the friction clutch opens, and up to 100% of the torque is directed to the rear axle. When driving on slippery surfaces, slipping of individual wheels is prevented by locking the friction clutch and, if necessary, the electronic cross-axle locking of the DSC system. During parking, the friction clutch is fully disengaged, the vehicle becomes rear-wheel drive, which reduces the load on the transmission and steering.
xDrive is integrated into DSC (Dynamic Stability Control). DSC and xDrive interact via Integrated Chassis Management (ICM). The latter is also responsible for communication with the active steering system AFS (Active Front Steering).
In 2003, the fourth generation of the system was released. The distribution of torque between the axles during normal driving was 40:60. The center differential function rests on the shoulders of an electronically controlled multi-plate clutch. It is possible to redistribute the torque between the axles in the range from 0 to 100%, there is also an electronic blocking of cross-axle differentials.
Four-wheel drive for three beams
Like quattro with xDrive, 4MATIC has a long and rich history. The 4MATIC was first introduced in 1985. The system had a Ferguson-type locking center differential with 35:65 torque distribution and a 50:50 locking rear cross-axle differential. Differential locking was carried out by means of hydraulic clutches, which are activated using a fast-acting electro-hydraulic system.
Daimler-Motoren-Gesellschaft, which can be considered the distant ancestor of the Daimler AG concern, presented the Dernburg-Wagen truck back in 1907, which experts consider as the world's first all-wheel drive truck. The company was disbanded in 1926, following the merger with Benz & Cie.
4MATIC works in tandem with an electronic system for regulating the dynamic characteristics of a car, which includes an electronic stabilization system ESP and an electronic traction control system 4ETS (4-wheel Electronic Traction System). 4ETS interacts with Anti-lock Braking System (ABS), Traction Control (ASR) and Descent Speed Control (DSR). All this allows to achieve an optimal distribution of torque between all four wheels and maintain excellent traction.
The central structural element of the 4MATIC system is the transfer case, which carries out a stepless distribution of torque along the axes of the car. It consists of a dual planetary gearbox, spur gears and drive shafts.
The latest, fifth generation 4MATIC is a completely new development based on a front-wheel-drive architecture with a transversely mounted engine in front. The new generation is based on the principle of activating the system as often as necessary, and at the same time as rarely as possible. In other words, if the car can only be driven by front-wheel drive, there is no need for activation. In this case, the torque can be transferred to the rear axle in milliseconds.
It is believed that the 4MATIC system is very helpful if there is understeer or, conversely, oversteer, and uses the distribution of torque in order to stabilize the car.
Who is better?
So who did the best at implementing permanent all-wheel drive? It is difficult to name a clear winner, especially since Audi, BMW and Mercedes-Benz are constantly making some improvements based on many years of experience in operating their systems.In addition, all concepts, while similar, have quite significant differences. For example, Audi specialists used permanent all-wheel drive with a Torsen differential. Mercedes-Benz has created, in general, no less four-wheel drive, but also equipped its brainchild with a friction differential lock. Well, BMW at some stage began to operate a drive, where the front wheels are activated on demand using an electromechanical clutch.
With a 50:50 torque split in most Audi vehicles, the company has achieved near-front-wheel drive handling. In this case, for example, in the Audi A4, the rearward center of mass leads to the fact that the rear wheels receive 60% of the torque by default. The same applies to a number of sports cars, such as the Audi RS4. By the way, some consider the already mentioned A4 "not driven". A sharp start on it can be problematic, as the front end is not loaded enough.
The permanent automatic all-wheel drive that BMW uses is certainly very successful. At the same time, some drivers consider it to be insufficiently predictable: with limit slip, it is better not to rely on self-willed electronics, which can interfere with the process and upset the balance of the car. A number of experts believe that when using critical modes, the front wheels of the "Bavarians" could be more "active". In general, xDrive has learned to predict the situation well and to regulate the distribution of torque in advance. The rest is a matter of taste for motorists.
We can say that the engineers of Mercedes-Benz approached the issue most systematically. 55-60% of the moment is supplied to the rear wheels, therefore, although there is some "rear-wheel drive", the main emphasis is on driving safety, which, in general, is also very good. Compared to Audi models, although the center differential has friction discs, the moment is redistributed only at the start.
In general, as already mentioned, each of the systems has its pluses and minuses. And from this, in all likelihood, there is no escape. No one size fits all solution yet, although the future may come as a surprise. It cannot be ruled out that the creators of some hypothetical revolution in the world of permanent all-wheel drive will no longer be the Germans, but someone else, for example, engineers from Japan or the United States. But so far it is difficult to find a country that would feel more confident in such a difficult business as the automotive industry than Germany.