Definitions

traction wheel

Quattro (four wheel drive system)


quattro (meaning four in Italian), is the name used by Audi AG to indicate that four-wheel drive (4WD) technologies or systems are used on specific models of the Audi automobiles.

"quattro" is a registered trademark of Audi AG, subsidiary of the larger German automotive conglomerate, Volkswagen Group.

quattro was first introduced in 1980 on the permanent four wheel drive Audi Quattro (often referred to as the Ur-Quattro - "Ur-" being a German language augmentive for "original"). The term quattro has since been applied to all subsequent Audi 4WD 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.

Audi parent company Volkswagen Group (Volkswagen AG) has used different trademarks for their 4WD vehicles. Whilst Audi has always used the term "quattro", for Volkswagen (VW) branded cars, they initially used "syncro", but more recently, VW now only use "4motion". Related VWAG companies Škoda simply uses the nomenclature "4x4" after the model name, whereas SEAT uses merely "4". It is important to note that none of the above trademarks, or nomenclatures define the operation or type of 4WD system, as detailed below.

Longitudinal systems

Volkswagen Group has been developing four-wheel drive (4WD) systems almost since its inception during the Second World War. The Volkswagen Kübelwagen, Volkswagen Schwimmwagen and Volkswagen Kommandeurwagen were all military vehicles which required all four road wheels to be "driven", the latter being a 4WD Volkswagen Beetle. Their military, and four wheel drive experiences later aided them in designing the Volkswagen Iltis for the German military (Bundeswehr) in the 1970s. The Iltis utilized an early form of 4WD, which would later become synonymous with "quattro".

In that original quattro system, later found in road-going passenger cars, the engine and transmission are situated in a longitudinal position. Torque is sent through the transmission to a mechanical centre differential (commonly abbreviated to "diff") which apportions (distributes) the torque between front and rear driven axles. 4WD was permanently active.

After 1987, Audi replaced a manually-locking centre differential with the Torsen (TorqueSensing) Type 2 ("T2") 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 75-80% (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, therefore has an element of "fail-safe", unlike say 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 stabilised, in exactly the same way that engine "propulsion" torque is apportioned - mechanically fully autonomous. 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: 1) With placement of the engine and transmission assembly in a fore/aft position (longitudinally), the front axle is placed rearwards behind the engine, leading to a common (but unfounded) criticism of Audi vehicles: being nose-heavy (although this is common in cars where the engine is placed at the front of the vehicle). In other words, the 'ideal' 50:50 weight distribution cherished by many alleged driving enthusiasts was not achieved.

2) 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 < 80km/hr. 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.

3) While the standard (Type 2 or “T2”) Torsen supports a “static” torque ratio of 50:50 i.e. input torque is supported equally across both output shafts, the T2 has a Torque Bias Ratio (TBR) of 2:1 i.e. it allows twice the torque to be supplied to the most tractive output shaft than that that is available on the least tractive shaft (i.e. a torque split of between 33% to 66%). It is important to note however that, by nature, the T2 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 T2 Torsen in a centre differential installation, rather than being 50:50, will mirror the weight distribution (both static & 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 T2 with 2:1 TBR is more than sufficient in most conditions, Torsen T2 differentials with higher TBR's (4:1) are available and can further limit understeer by supporting a wider torque split. A better solution, however, is to directly apportion a torque split 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.

The Torsen T3 centre differential combines a planetary gear set with a Torsen differential in a compact package developed for centre differential installations. Unlike the T2 Torsen where the torque split is a nominal 50:50, in the T3 Torsen the torque split, due to the use of the planetary gear set, is an actual asymmetric 40:60 front-rear torque split (i.e. when grip is equal on both front and rear axles, 40% of torque is sent to the front axle, and 60% to the rear). As with the T2 Torsen, torque will be distributed dynammically depending on tractive conditions, but with a actual (rather than nominal) static bias. The T3 allows handling characteristics and vehicle dynamics more akin to rear-wheel drive-only cars. This asymmetric Torsen was first introduced in the highly acclaimed 2006-model (B7) Audi RS4. It was then used in the 2007-model B7 S4., as well as the S5 and Q7. It is planned to be incorporated in all future quattro-equipped cars which use longitudinal-mounted engine layout.

The torque split across axles, between left and right wheels, has been achieved through the various evolutions of the quattro system, through a driver-selectable manually locking differential (rear axle only), and, eventually, through open differentials with "Electronic Differential Lock" (EDL). EDL is an electronic system, utilising the existing Anti-lock Braking System (ABS), part of the Electronic Stability Programme (ESP), which brakes just the one spinning wheel on an axle, therefore allowing the transfer of torque across the axle to the wheel which does have traction.

Evolutions

quattro generation I

Used from 1981 to 1987 in Audi Quattro turbo coupé, Audi 80 B2 platform (1978–1987) (Audi 4000 in North American market), Audi 100 C3 platform (1983–1987) (Audi 5000 in North American market). Also, starting from 1984, used on the Volkswagen VW Passat B2 platform (VW Quantum in the US market) where it was known as Syncro.

System type: Permanent four-wheel drive.

Open centre differential, manually lockable via switch on centre console. ¹

Open rear differential, manually lockable via switch on centre console. ¹

Open front differential, no lock.

¹ - ABS disabled when locked.

How does the system perform: When all differentials are unlocked the car will not be able to move if just one wheel loses traction (is on ice or raised in the air). When both center and rear differential are locked, the car will not be able to move when one front and both rear wheels lose traction altogether.

quattro generation II

Starting from 1988 on older generation Audi 100 C3 platform and Audi Quattro turbo coupé until the end of their production, and on new generation B3 platform (1989–1992) Audi 80/90 quattro, B4 platform (1992–1995) Audi 80, Audi Coupé quattro, Audi S2, Audi RS2 Avant, C4 platform (1991–1994) Audi 100 quattro, Audi S4, earlier C4 platform (1995) Audi A6/S6.

System type: Permanent four-wheel drive.

Torsen centre differential, 50:50 'default' split, automatically apportioning up to 75% of torque transfer to either axle.

Open rear differential, manually lockable via switch on centre console located next to handbrake. ¹

Open front differential, no lock.

¹ - ABS disabled when locked, automatically unlocks if speed exceeds .

How does the system perform: When rear differential is manually locked, the car will not be able to move if one front wheel and both rear wheels lose traction altogether, but this is true only while all wheels are on the ground. Note that, due to the constructive feature of the Torsen (TORque SENsing) differential, if no torque is sensed on one axle the Torsen works as an open differential i.e. if one front wheel is raised in the air the Torsen differential will not lock, all engine power will be transferred to the wheel in the air, and the car will not move.

quattro generation III

Used on Audi V8 starting from 1990.

System type: Permanent or semi-permanent four-wheel drive.

V8 with automatic transmission:

Planetary gear centre differential with electronically-controlled multi-plate locking clutch

Torsen differential rear.

Open differential front.

V8 with manual transmission:

Torsen centre differential.

Torsen rear differential.

Open front differential.

How does the system perform: In on-road conditions the car will not be able to move if one front and both rear wheels lose traction altogether. Torsen effect with one wheel in the air still applicable to V8 with manual transmission, but will not happen on V8 with automatic transmission because center differential on this model offers 100% locking even when no torque is sensed on the spinning wheel.

quattro generation IV

Starting from 1996 on Audi A4 / S4 / RS4 (B5 platform), Audi A6 / S6 / RS6, Audi A8 / S8 with both manual and automatic transmissions. Also on VW Passat B5, where it was initially referred to as syncro, but by the time it reached US soil, it had been re-christened 4motion. Also used on the Volkswagen Phaeton and Volkswagen Group D platform sister vehicles; also the Volkswagen Touareg where they use separate transmissions, PTU's and front axles.

The manually locking rear differential from the earlier generations was replaced with a conventional open differential, with "Electronic Differential Lock" (EDL) (which, detects wheelspin via ABS road wheel speed sensors, and applies brakes to the one spinning wheel, thus transferring torque via open differential to the opposite wheel which has more traction). EDL works at speeds up to on all quattro models (on non-quattro models: up to .

System type: Permanent four-wheel drive.

Torsen T-2 centre differential, 50:50 'default' split, automatically apportioning up to 75% of torque transfer to either front or rear axle.

Open rear differential, Electronic Differential Lock (EDL).

Open front differential, Electronic Differential Lock (EDL).

How does the system perform: In on-road conditions the car will not be able to move if all four wheels lose traction altogether. Torsen effect with one wheel losing traction and spinning freely will not happen on quattro IV because EDL will apply brakes to the spinning wheel, and Torsen differential will transfer torque to another axle. However, in severe road conditions (wheels on ice or raised in the air and an obstacle restricting the vehicle from moving forward, on a slope) the car will not be able to move when one front and one rear wheel lose traction. EDL will brake the single spinning wheel on an axle trying to transfer torque to the opposite wheel, but because Audi's EDL action is too "soft" it is not able to brake the wheel to the full stop. The car will end up spinning one front and one rear wheels but not moving forward.

quattro generation V

Starting from 2006 on B7 Audi RS4 and the 2008 B7 Audi S4. Will become the standard fitment on all future quattro Audis with longitudinal engine layout.

System type: Permanent asymmetric four-wheel drive.

Torsen T-3 centre differential, 40:60 'default' split front-rear, automatically apportioning up to 80% of torque transfer to the front axle and up to 100% torque to the rear axle.

Open rear differential, Electronic Differential Lock (EDL).

Open front differential, Electronic Differential Lock (EDL).

Vectoring quattro system (quattro generation VI ?)

Audi's new vectoring quattro system, which will allow the dynamic allocation of torque to all four wheels will debut in the B8 S4. This will still use the 40:60 asymmetric Torsen centre differential, but will use an electronically controlled "Active Sport Differential" in the rear axle (instead of the conventional "open" differential with EDL). The front axle will still rely on an open differential with EDL.

BorgWarner

The Audi Q7, the platform-mate of the Volkswagen Touareg and Porsche Cayenne, does not use the same underpinnings of either previous model. BorgWarner instead provides the 4WD system for this more off-road appropriate SUV. however a Torsen Type 3 (T3) differential is used.

Transverse systems

Since Volkswagen Group's first mainstream transverse engined vehicle in 1974, four-wheel drive (4WD) has also been considered for their A-platform family of cars. It was not until the second generation of this platform that 4WD finally appeared on the market. The mid-1980s Mk2 Golf syncro, with its transverse engine and transmission positioning, still had most of its torque sent primarily to the front axle. Vehicles using this configuration therefore cannot be said to have a "permanent", or "full-time" four wheel drive system.

Attached to the transaxle is a Power Transfer Unit (PTU), which is connected to a rear axle through a propeller shaft. The PTU also feeds torque through itself to the front axle. At the rear axle, torque was first sent through a viscous coupling before reaching the final drive gearset. This coupling contained fiction plates and an oil just viscous enough so that pressure affected how many plates were connected and active (and therefore, how much power was being delivered to the rear wheels).

Starting with the Mk4 generation A4-platform, the viscous coupling has been dropped in favour of a Haldex Traction electro-hydraulic limited-slip "coupler" (LSC) or clutch. The Haldex Traction LSC unit is not a differential and therefore cannot perform in the true sense like a differential. A Haldex Traction unit may divert up to a maximum 100% of the torque to the rear axle as conditions warrant. Many people are confused with the torque distribution on Haldex-based systems. Under normal operating conditions the Haldex clutch operates a rate of 5% torque transmission. Under adverse conditions where the cars road wheel speed sensors have determined that both front wheels have lost traction the Haldex clutch can lock at 100% clamping force, meaning all torque is transersed to the rear axle. The torque split between left and right wheels is achieved with a conventional open differential. If one side of the driven axle loses grip, then the Electronic Differential Lock (EDL) component of the ESP controls this. EDL brakes a single spinning wheel, therefore, the torque gets transferred across the axle to the opposite wheel via the open differential. On all transverse engine cars with the Haldex-based four wheel drive system, the EDL only controls front wheels, and not the rear.

The main advantages of the Haldex Traction LSC system over the Torsen-based system include: a slight gain in fuel economy (due to the decoupling of the rear axle when not needed, thereby reducing driveline losses due to friction), and the ability to maintain a short engine bay and larger passenger compartment due to the transverse engine layout. A further advantage of the Haldex, when compared to just front wheel drive variants of the same model, is a more balanced front-rear weight distribution (due to the location of the Haldex center "differential" next to the rear axle).

Disadvantages of the Haldex Traction system include: the vehicle has inherent front-wheel drive handling characteristics (as when engine braking, load is only applied on the front wheels, and due to the reactive nature of the Haldex system and slight lag time in the redistribution of engine power), and the Hadex LSC unit also requires additional maintenance, in the form of an oil and filter change every (whereas the Torsen is completely maintenance free). Another important disadvantage of the Haldex system, is the requirement for all four tyres to be of nigh-on identical wear levels (and rolling radii), due to the Haldex requiring data from all four road wheel speed sensors. A final significant disadvantage is the reduction in luggage capacity in the boot (trunk), due to the bulky Haldex LSC unit necessitating a raised boot floor by some three inches.

Viscous Coupling

Important note: This 4WD system was used only on Volkswagen branded vehicles, and was never used on any Audi cars.

The aforementioned viscous coupling 4WD system was found in the Mk2 generation of transverse-engined A2-platform vehicles, including the Volkswagen Golf Mk2 and Jetta. It was also found on the Volkswagen Type 2 (T3) (Vanagon in the US), Mk3 generation of Golf and Jetta, 3rd generation of Volkswagen Passat B3 (which was based on a heavily revised A-platform), and the Volkswagen Eurovan.

Note that the Vanagon system was RWD-biased, due to the vehicle being rear-wheel drive by default; the engine and transaxle were in the rear, whereas the viscous coupling was found in the front axle near the final drive. This 4WD system was known as Syncro on all vehicles.

What: Automatic four wheel drive (on demand).

A viscous coupling installed instead of a centre differential, with freewheel mechanism to disconnect the driven axle when braking.

Open rear differential (mechanical differential lock optional on Vanagon).

Open front differential (mechanical differential lock optional on Vanagon).

How: Normally a front-wheel drive vehicle (except Vanagon, see above). In normal driving conditions 95% of torque transferred to front axle. Because viscous coupling is considered to be "slow" (some time is needed for silicone fluid to warm-up and solidify), 5% of torque is transferred to rear axle at all times to "pre-tension" the viscous coupling and reduce activation time. The coupling locks when slipping occurs and up to near 50% of torque is automatically transferred to rear axle (front in Vanagon). In on-road conditions the car will not move if one front wheel and one rear wheel lose traction.

The freewheel segment, installed inside the rear differential, lets rear wheels rotate faster than front wheels without locking the viscous coupling and preventing ABS from applying brakes to each wheel independently. Because of the freewheel, torque can be transferred to rear axle only when vehicle is moving forward. For four-wheel drive to work when reversing, a vacuum-actuated "throttle control element" is installed on the differential case. This device locks the freewheel mechanism when in reverse gear. The freewheel mechanism unlocks when the gear shift lever is pushed to the right pass the 3rd gear. The freewheel is not unlocked immideately after leaving reverse gear on purpose - this is to prevent the freewheel from cycling from locked to unlocked if the car is stuck and driver is trying to "rock" the car by changing from 1st to reverse and back.

Disadvantages of this four wheel drive system are related to actuation time of the viscous coupling. 1: When cornering under acceleration on a slippery surface, rear axle is engaged with delay causing sudden change in the car's behaviour (from understeer to oversteer). 2: When starting on a sandy surface, front wheels can dig into the sand before all wheel drive is engaged.

Haldex

Starting from 1998, the Swedish Haldex Traction LSC unit replaced the viscous coupling. Haldex is used by Audi on the quattro versions of the Audi A3 and Audi S3, and the Audi TT. It is also used by Volkswagen in the 4motion versions of the Mk4 and Mk5 generations of Volkswagen Golf, Volkswagen Jetta, and the Golf R32, Volkswagen Sharan, 6th generation VW Passat (also based on the A-platform) and Transporter T5. On the Audi's, the trademark holds, and are still referred to as quattro, whereas the Volkswagens receive the 4motion name. The Škoda Octavia 4x4 and SEAT Léon 4 and SEAT Alhambra 4 also used Haldex LSC, being based on Volkswagen Group models. Curiously, the Bugatti Veyron also utilizes Haldex, though with separate transmission, PTU and front and rear axles.

What: Automatic four wheel drive (on demand).

Haldex Traction LSC multi-plate clutch with ECU electronic control, acting as a pseudo center differential.

Open rear differential, no EDL.

Open front differential, EDL.

How: Normally front-wheel drive vehicle. A Haldex Traction LSC unit may divert up to a maximum 100% of the torque to the rear axle as conditions warrant. Many people are confused with the torque distribution on Haldex Traction systems. Under normal operating conditions, the Haldex LSC clutch operates at 5% (divide 5% between front and rear, and 97.5% torque goes to the front, and 2.5% goes to the rear). Under adverse conditions where both front wheels lose traction, the Haldex clutch can lock at 100% clamping force. This means, that since there is no torque tranferred to the front axle, all torque (minus losses) must be transferred to the rear axle. The torque split between left and right wheels is achieved with a conventional open differential. If one side of the driven axle loses grip, then the Electronic Differential Lock (EDL) controls this. EDL brakes a single spinning wheel, therefore torque gets transferred to the opposite wheel via the open differential. On all transverse engine cars with the Haldex Traction LSC four-wheel drive system, the EDL only controls front wheels, and not the rear.

Accompanied by EDL on front wheels, in on-road conditions the car will not move if both front and one rear wheels lose traction.

Again, due to limitations of Electronic Differential Lock (see quattro IV description above), in off-road conditions it is enough for one front and one rear wheel to lose traction and the car will not move.

The Haldex Traction system is more reactive than preventative, in that the front axle must lose traction and start to spin before the Haldex operates and sends torque to the rear axle. The Torsen's permanent 'full-time' even torque split under non-slipping conditions makes slipping less likely to start.

The Haldex Electronic Control Unit (ECU) disengages the Haldex clutch in the centre coupling as soon as brakes are applied to allow ABS work properly. When performing tight low-speed turns (e.g. parking) the clutch is disengaged by Electronic Control Unit to avoid "wind-up" in transmission.

Haldex aftermarket applications

The Haldex Traction LSC centre coupling is often used as upgrade to an aftermarket four-wheel drive conversion on older front-wheel drive Volkswagens. It is said to be capable of withstanding larger power outputs than the also commonly used viscous coupling system from a syncro vehicle.

The conversion is carried out by way of a viscous coupling rear axle and associated live suspension system from a syncro vehicle being fitted to a suitable project car (i.e. a Volkswagen Corrado or Volkswagen Golf), and then fabricating a custom bracket to hold the Haldex rear coupling.

Enthusiasts often either use the OEM ECU and engine management from a newer Volkswagen Group car to control the Haldex centre clutch using the standard ABS road wheel speed sensors - or can buy aftermarket controllers that supply the relevant pulse-width modulation to actuate the clutch and transfer drive to the rear wheels either via simple variable dial or based on throttle position sensor (TPS) calculations.

See also

References

External links

  • Audi.com corporate international portal

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