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The quattro drive has accompanied and changed the Audi brand over the years. In the past 30 years, Audi has not simply stuck to one system, but has continued to develop the technology and in the process revolutionized what we now call “all-wheel drive”.
"I can certainly drive more perfectly in the future with the quattro, but probably not faster any more"
Quote: Walter Röhrl
This quote comes from Walter Röhrl in 1984 after winning the Monte Carlo Rally. Walter Röhrl was never satisfied with himself, but he recognized that all-wheel drive was the biggest change in rallying. At this point, Audi was still using a rather simple all-wheel drive concept. A permanent all-wheel drive with two possible differential locks (center and rear).
The development was a steady evolution after the revolution of permanent four-wheel drive for production vehicles.
1987 - The Torsen differential
Torsen is an artificial word, formed from the two English terms: “Torque” and “sensing” - meaning “feeling force”.
But why do you need differentials?
On Youtube there is a good video explaining the necessity and the function of differentials:
Audi has been using the Torsen differential to distribute power between the front and rear axles since 1987. A system with which the power can be distributed between the two axes without the need for additional control units. Torque sensing means that the “perceived” loss of traction already means that the force is directed to the axle with more grip. A simple but ingenious system. It compensates for the differences in speed between the front and rear axles, but still ensures that the axle with more grip gets more power. The power is distributed up to 75% to the axle with the better traction.
But Quattro is not just Torsen
Audi does not change the name for the all-wheel drive, regardless of the form in which the power transmission takes place and so for the term “quattro” today you have to be prepared for four variants of the “quattro”:
- Haldex for vehicles with transversely mounted engines (Audi A3, TT)
- Self-locking center differential (vehicles with longitudinal engine)
- Crown wheel differential (sports models)
- Visco coupling (Audi R8 only)
Haldex for vehicles with transversely mounted engines (eg: Audi A3, TT)
For the transversely mounted units in the compact models, Audi chose a completely different technology - a multi-plate clutch that is electronically controlled and hydraulically actuated. She gave her debut 1998 in TT quattro and A3 quattro.
The coupling sits at the end of the cardan shaft in front of the rear differential - an installation position that also benefits the axle load distribution of the vehicle. In the basic distribution, the greatest part of the engine's power goes to the front axle. The controller constantly analyzes the driving conditions using a variety of data; if necessary, it initiates a redistribution of forces.
Inside the clutch is a set of fins that runs in oil bath. The metal friction rings lie in pairs behind each other - alternately one ring is fixed to the housing, which rotates with the cardan shaft, toothed, the other with the output shaft to the rear differential. The disk pack can be compressed by controlled hydraulic pressure. As the pressure increases, steplessly more torque flows to the rear axle, in some cases almost 100 percent.
To quickly build up the oil pressure, which can reach over 100 bar, serve two electrically driven pumps. In the current A3 and TT models, a reservoir that maintains oil pressure permanently provides for a faster redistribution of forces, taking only a few milliseconds
Self-locking center differential (Vehicles with longitudinal engine)
In the second-generation RS 4, Audi 2005 set the next evolutionary stage of its classic quattro drive. The new self-locking center differential, which is now active in many models with a longitudinal engine, remained true to the purely mechanical principle, but compared to the Torsen differential represents a significant advance.
In normal driving conditions, the force distribution between the front and rear axles is 40: 60 - this asymmetric-dynamic torque distribution leads to a sporty, rear-emphasized handling. If necessary, the center differential can redirect up to 60 percent of forces forward and up to 80 percent back. If one wheel of an axle should spin, it controls the electronic differential lock EDS by braking intervention.
The self-locking center differential is constructed as a planetary gear. A ring gear includes a sun gear; between them rotate cylindrical planetary gears, which are connected to the rotating housing. They distribute the drive torque asymmetrically - the slightly larger part flows over the ring gear, which has a larger diameter, and the output shaft connected to it to the rear. The smaller proportion reaches the smaller sun gear and goes from there to the front axle.
When the traction on an axle subsides, the helical shape of the gears and their oblique splines cause axial forces in the differential. They provide friction discs for a defined locking torque, which causes the deflection of the forces to the wheels with better friction coefficients.
The large SUV Q7 uses a special form of the self-locking center differential - it is integrated into a transfer case. The sun gear drives a secondary shaft via a chain, which leads past the transmission to the front axle. The chain takes over the transport of the oil, the usual oil pump is not necessary. The Q7's entire drivetrain has lost significantly in weight in the latest evolutionary stage. Regardless of this, the transfer case is extremely robust; it also allows a high ground clearance, which is important for off-road use.
Crown wheel differential (Sport models)
Exactly 30 years after the debut of the first quattro, Audi has introduced the latest evolution of its permanent all-wheel drive for longitudinally mounted front engines - the quattro drive with crown gear differential and wheel-selective torque control.
Inside the new center differential, which is used in the RS5, the A7 Sportback and the new A6, are two crown wheels that take their name from their gearing geometry. The rear wheel drives the cardan shaft to the rear differential, the front drives the output to Vorderachsdifferenzial. The crown wheels are engaged with four rotatably mounted differential gears. They are arranged at an 90 degree angle to each other and get their drive from the housing of the differential, so from the transmission output shaft.
During normal driving, the two crown wheels rotate as fast as the housing. Due to their special geometry, targeted unequal leverage effects result: in the basic distribution, 60 percent of the engine torque goes to the differential of the rear axle and 40 percent to the front.
If the moments shift because the grip on one axle decreases, different speeds and axial forces are created inside the differential - they cause the adjacent disk packs to be pressed together. The resulting self-locking effect now directs the majority of the drive torque on the axle with the better traction, up to 85 percent flow to the rear. In the opposite case - if the rear axle has less grip - this process takes place accordingly, now up to 70 percent of the torque flow to the front axle.
With this even wider torque distribution, the crown gear differential surpasses its predecessors - the traction is even better. The redistribution of forces and moments takes place without time delay and absolutely homogeneous, the active mechanical working method guarantees the highest efficiency and instantaneous reactions. Further strengths of the crown gear differential are its compactness and low weight - with 4,8 kilograms it is about two kilograms lighter than its predecessor component.
Audi couples the crown gear differential with an intelligent software solution in brake management, the wheel-selective torque control. It can specifically access each of the four wheels. The new system makes cornering even more precise and dynamic.
When cornering at high speeds, the software determines the optimum distribution of drive forces between all four wheels based on the steering input and the position of the accelerator pedal. If she realizes that the unloaded inside wheels will soon slip, she slows them down a little - a fine application of the pads to the disc with minimal pressure is enough.
Through the action of the differential, the wheels on the outside of the bend can bring more drive torque to the road. The support is smooth and continuous. The car remains noticeably longer neutral, the understeer when turning and accelerating is virtually neutralized, and the interventions of the ESP are made later and softer - if they are even necessary.
Viscous coupling (Audi R8 only)
The high-performance sports car R8 occupies a special position in Audi's range - including its packaging and drive. The mid-engine is placed longitudinally in front of the rear axle, the transmission directly behind it. It integrates a power take-off for a cardan shaft, which runs laterally past the engine to the front axle.
There, a viscous coupling takes over the force distribution between the front axle and the rear axle working with a limited slip differential. In regular driving, the clutch branches off only about 15 percent of the torque for the front axle - the R8 drives typical sports car rear-heeled. If the rear wheels slip, within a very short time further 15 percent will flow forward.
The main component of the viscous coupling is a set of circular discs, which are alternately interlocked: each one is connected via the housing with the cardan shaft, the next each with the output shaft to the front axle. The fins run in a viscous liquid.
If they rotate at very different speeds due to a loss of traction on the rear axle, the oil becomes tougher due to its internal friction. By taking along the other slats now reaches an increased torque to the output shaft to the front axle.
Quattro - a success story even after 30 years