Quattro (four-wheel-drive system)

quattro (meaning four in Italian) is the sub-brand used by the automotive brand Audi to indicate that all-wheel drive (AWD) technologies or systems are used on specific models of its automobiles.

The word "quattro" is a registered trademark of Audi AG, a subsidiary of the German automotive concern, Volkswagen Group.

Other companies in the Volkswagen Group have used different trademarks for their 4WD vehicles. While Audi has always used the term "quattro", Volkswagen-branded cars initially used "syncro", but more recently, VW uses "4motion". Škoda simply uses the nomenclature "4x4" after the model name, whereas SEAT uses merely "4" ("4Drive" more recently). None of the above trademarks or nomenclatures defines the operation or type of 4WD system, as detailed below.

The Torsen T1 centre differential

After 1987, Audi replaced a manually locking centre differential with the Torsen (torque sensing) Type 1 ("T1") centre differential. This allowed engine torque to be automatically directed to individual axles as driving conditions and grip warranted. Under 'normal' conditions (where grip in both front and rear axles is equal), torque is split between front and rear with a 'default" 50:50 distribution in many, though not all, versions. In adverse conditions (i.e., when there is variation in grip between front and rear), a maximum of 67-80% (depending on the transmission, or model of Torsen diff) of the engine's torque can be directed to the front or rear axles. The fully automated mechanical nature of the Torsen centre differential helps prevent wheel slippage from occurring, by diverting torque instantly, without any discernible notice to the vehicle occupants, to the axle which has more grip. This method of operation can be described as proactive. Furthermore, unlike the various types of electronically operated differentials, Torsen has no requirement for electronic data from sources such as road wheel speed sensors; it, therefore, has an element of "fail-safe", unlike designs such as Haldex Traction, should one of the wheel speed sensors develop a fault. In comparison, viscous coupling and electronically controlled centre differentials that are used in other four-wheel drive systems are reactive, since they only redirect torque after wheel slippage has occurred. The advantage is felt under hard acceleration, including whilst cornering, since the torque transfer between axles is seamless, thus maintaining stable vehicle dynamics, and considerably reducing the chance of losing control of the vehicle.

The Torsen-based quattro system also offers an advantage in the opposite function of distributing torque to the road wheels, namely engine braking. When engine braking is used to slow the car down, with Torsen-based systems, the resulting "reverse-torque" loads on the front and rear axle are equally stabilized, in exactly the same way that engine "propulsion" torque is apportioned fully mechanically autonomously. This allows the spreading of the engine braking effect to all four wheels and tyres. The Torsen-based quattro-equipped vehicle is able to execute a more stable high-speed turn under deceleration, with less risk of losing control due to loss of grip in the front or rear axles.

The nature of the Torsen is akin to that of a limited slip differential in that, rather than actively allocating torque (as a computer controlled clutch can do), it supports a torque difference across the differential (the torque bias ratio/TBR), from the side with the least grip to the side with the most. Hence by nature the Torsen is limited in the amount of torque that can be supplied to the axle with the most grip by the torque available at the axle with the least amount of grip. Therefore, if one axle has no grip, regardless of the TBR, the other axle will not be supplied substantial torque. In the extreme, for a centre differential implementation, complete loss of traction on a single wheel will result in very limited torque to the other three wheels. Audi responded to this limitation for the first Torsen-equipped cars by adding a manually locking rear differential and then later replaced this feature with Electronic Differential Lock (EDL), which is the ability to use the individual wheel brakes (monitored by the ABS sensors) to limit individual wheel spin. EDL was implemented across both front and rear (open) differentials to operate at speeds less than 80 km/h. This has the effect of increasing torque from a single low-traction wheel hence allowing more torque to be passed by the Torsen to the remaining high-traction wheels.

While the standard (Type 1 or T1) Torsen supports a static torque ratio of 50:50; i.e., input torque is supported equally across both output shafts, the T1 has a Torque Bias Ratio (TBR) of 2.7–4:1; i.e., it allows about 3 to 4 times the torque to be supplied to the most tractive output shaft than that is available on the least tractive shaft or, a torque split of between 25% to 75%. However, by nature the T1 Torsen is locked under most circumstances (output shafts locked together). Only when the TBR is reached (i.e., there is a greater torque difference across the output shafts than can be supported by the TBR) do the output shafts turn relative to each other, and the differential unlocks. This characteristic results in a relatively free torque movement between both outputs of the (centre) differential, within the limits of the TBR. Thus the static torque distribution of the T1 Torsen in a centre differential installation, rather than being 50:50, will mirror the weight distribution (both static and dynamic) of the vehicle due to the traction available at either (front:rear) output shaft. In a standard car, this is desirable from the perspective of stability, acceleration and traction, but can be undesirable in terms of handling (understeer). While the standard quattro Torsen T1 with 2.7:1 TBR is more than sufficient in most conditions, Torsen T1 differentials with higher TBRs (4:1) are available and can further limit understeer by supporting a wider torque split. A better solution, however, is to apportion a torque split directly between both output shafts (front & rear) and for this reason Audi has adopted the Type 3 (T3) Torsen design in the latest generations of quattro.

quattro (meaning four in Italian) is the sub-brand used by the automotive brand Audi to indicate that all-wheel drive (AWD) technologies or systems are used on specific models of its automobiles.

The word "quattro" is a registered trademark of Audi AG, a subsidiary of the German automotive concern, Volkswagen Group.

Quattro was first introduced in 1980 on the permanent four-wheel drive Audi Quattro model, often referred to as the Ur-Quattro (meaning "original" or "first"). The term quattro has since been applied to all subsequent Audi AWD models. Due to the nomenclature rights derived from the trademark, the word quattro is now always spelled with a lower case "q", in honour of its former namesake.

"quattro" logo badge on the grille of an Audi car

Other companies in the Volkswagen Group have used different trademarks for their 4WD vehicles. While Audi has always used the term "quattro", Volkswagen-branded cars initially used "syncro", but more recently, VW uses "4motion". Škoda simply uses the nomenclature "4x4" after the model name, whereas SEAT uses merely "4" ("4Drive" more recently). None of the above trademarks or nomenclatures defines the operation or type of 4WD system, as detailed below.

The Torsen T1 centre differential

After 1987, Audi replaced a manually locking centre differential with the Torsen (torque sensing) Type 1 ("T1") centre differential. This allowed engine torque to be automatically directed to individual axles as driving conditions and grip warranted. Under 'normal' conditions (where grip in both front and rear axles is equal), torque is split between front and rear with a 'default" 50:50 distribution in many, though not all, versions. In adverse conditions (i.e., when there is variation in grip between front and rear), a maximum of 67-80% (depending on the transmission, or model of Torsen diff) of the engine's torque can be directed to the front or rear axles. The fully automated mechanical nature of the Torsen centre differential helps prevent wheel slippage from occurring, by diverting torque instantly, without any discernible notice to the vehicle occupants, to the axle which has more grip. This method of operation can be described as proactive. Furthermore, unlike the various types of electronically operated differentials, Torsen has no requirement for electronic data from sources such as road wheel speed sensors; it, therefore, has an element of "fail-safe", unlike designs such as Haldex Traction, should one of the wheel speed sensors develop a fault. In comparison, viscous coupling and electronically controlled centre differentials that are used in other four-wheel drive systems are reactive, since they only redirect torque after wheel slippage has occurred. The advantage is felt under hard acceleration, including whilst cornering, since the torque transfer between axles is seamless, thus maintaining stable vehicle dynamics, and considerably reducing the chance of losing control of the vehicle.

Audi quattro torsen diferential

Audi quattro Torsen centre differential

The Torsen-based quattro system also offers an advantage in the opposite function of distributing torque to the road wheels, namely engine braking. When engine braking is used to slow the car down, with Torsen-based systems, the resulting "reverse-torque" loads on the front and rear axle are equally stabilized, in exactly the same way that engine "propulsion" torque is apportioned fully mechanically autonomously. This allows the spreading of the engine braking effect to all four wheels and tyres. The Torsen-based quattro-equipped vehicle is able to execute a more stable high-speed turn under deceleration, with less risk of losing control due to loss of grip in the front or rear axles.

This configuration of the quattro system, however, does have some limitations:

With placement of the engine and transmission assembly in a fore/aft position (longitudinally), the front axle is placed rearwards behind the engine, which leads to the criticism of some Audi vehicles as being nose heavy. This results in a weight distribution of 55:45 (F:R).

The nature of the Torsen is akin to that of a limited slip differential in that, rather than actively allocating torque (as a computer controlled clutch can do), it supports a torque difference across the differential (the torque bias ratio/TBR), from the side with the least grip to the side with the most. Hence by nature the Torsen is limited in the amount of torque that can be supplied to the axle with the most grip by the torque available at the axle with the least amount of grip. Therefore, if one axle has no grip, regardless of the TBR, the other axle will not be supplied substantial torque. In the extreme, for a centre differential implementation, complete loss of traction on a single wheel will result in very limited torque to the other three wheels. Audi responded to this limitation for the first Torsen-equipped cars by adding a manually locking rear differential and then later replaced this feature with Electronic Differential Lock (EDL), which is the ability to use the individual wheel brakes (monitored by the ABS sensors) to limit individual wheel spin. EDL was implemented across both front and rear (open) differentials to operate at speeds less than 80 km/h. This has the effect of increasing torque from a single low-traction wheel hence allowing more torque to be passed by the Torsen to the remaining high-traction wheels.

Quattro AWD

While the standard (Type 1 or T1) Torsen supports a static torque ratio of 50:50; i.e., input torque is supported equally across both output shafts, the T1 has a Torque Bias Ratio (TBR) of 2.7–4:1; i.e., it allows about 3 to 4 times the torque to be supplied to the most tractive output shaft than that is available on the least tractive shaft or, a torque split of between 25% to 75%. However, by nature the T1 Torsen is locked under most circumstances (output shafts locked together). Only when the TBR is reached (i.e., there is a greater torque difference across the output shafts than can be supported by the TBR) do the output shafts turn relative to each other, and the differential unlocks. This characteristic results in a relatively free torque movement between both outputs of the (centre) differential, within the limits of the TBR. Thus the static torque distribution of the T1 Torsen in a centre differential installation, rather than being 50:50, will mirror the weight distribution (both static and dynamic) of the vehicle due to the traction available at either (front:rear) output shaft. In a standard car, this is desirable from the perspective of stability, acceleration and traction, but can be undesirable in terms of handling (understeer). While the standard quattro Torsen T1 with 2.7:1 TBR is more than sufficient in most conditions, Torsen T1 differentials with higher TBRs (4:1) are available and can further limit understeer by supporting a wider torque split. A better solution, however, is to apportion a torque split directly between both output shafts (front & rear) and for this reason Audi has adopted the Type 3 (T3) Torsen design in the latest generations of quattro.