Quattro (four wheel drive system) 757747 224015615 2008-07-06T23:07:17Z Lightbot 7178666 Units/dates/other {{lowercase|title=quattro}} {{For|other uses of "quattro"|Quattro}} {{Original research|date=April 2008}} {{Refimprove|date=January 2007}} <!-- AudiWorld, and other forums are not considered reliable sources --> {{Expert-subject|date=April 2008|Automobiles}} <br> [[Image:Audi quattro logo.JPG|thumb|250px|"quattro" [[logo]] badge on the front [[grille]] of an [[Audi]] car]] '''quattro''' (meaning ''four'' in [[Italy|Italian]]), is the name used by [[Audi|Audi AG]] to indicate that [[four-wheel drive]] (4WD) technologies or systems are used on specific models of the Audi [[marque]] of [[automobile|cars (automobiles)]].<ref name="Audi.comGlossary_quattro">Audi.com - Glossary [http://www.audi.com/audi/com/en2/tools/glossary/engine_driveline/quattro.html quattro]</ref> "quattro" is a registered [[trademark]] of Audi AG, who are a wholly owned subsidiary of the larger [[Germany|German]] [[automotive industry|automotive]] conglomerate, [[Volkswagen Group|Volkswagen&nbsp;Group&nbsp;(Volkswagen&nbsp;AG)]] (VWAG).<ref name="Audi.comGlossary_quattro" /> 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 Auto|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 [[World War II|Second World War]]. The [[Volkswagen Kübelwagen]], [[Volkswagen Schwimmwagen]] and [[Volkswagen Beetle#The military Beetle & production up to 1945|Volkswagen Kommandeurwagen]] were all [[list of military vehicles|military vehicles]] which required all four road [[wheel]]s 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 [[Bundeswehr|German military (Bundeswehr)]] in the 1970s. The Iltis utilized an early form of 4WD, which would later become synonymous with "quattro".<ref name="Quattro25">Audi of America Press Site [http://media.audiusa.com/article_display.cfm?article_id=9540 25 Years of Audi Quattro] 22 February 2005</ref> In that original quattro system, later found in road-going passenger cars, the [[internal combustion engine|engine]] and [[Transmission (mechanics)|transmission]] are situated in a [[longitudinal engine|longitudinal]] position. [[Torque]] is sent through the transmission to a mechanical centre [[differential (mechanical device)|differential]]<ref>Audi.com - Glossary [http://www.audi.com/audi/com/en2/tools/glossary/engine_driveline/centre_differential.html Centre differential]</ref> (commonly abbreviated to "diff") which apportions (distributes) the torque between front and rear driven [[axle]]s. 4WD was permanently active. [[Image:Audi quattro AWD system.jpeg|right|thumb|[[Audi]] quattro [[Torsen]] centre [[differential (mechanical device)|differential]]]] After 1987, [[Audi]] replaced a manually-locking centre differential with the [[Torsen]] ('''''Tor'''que'''Sen'''sing'') 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 [[wheelspin|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 [[wheel speed sensor|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 unit|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 force|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. 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 actually very common in the vast majority of 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. More recently, the latest Torsen T-3 centre differential has been adapted to give an asymmetric 40:60 front-rear 'default' 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). This 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#B7 RS4 (2006–2008)|Audi RS4]]. It was then used in the 2007-model B7 [[Audi S4|S4]].<ref name="Audi.comGlossary_quattro" /> 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|Anti-lock Braking System]] (ABS), part of the [[Electronic stability control|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.<ref name="EDL">Audi.com - Glossary [http://www.audi.com/audi/com/en2/tools/glossary/engine_driveline/electronic_differential_lock.html Electronic Differential Lock]</ref> ===Evolutions=== {{Disputed|date=April 2008}} {{Self-published|date=April 2008}} <!-- This whole section is a copy from the personal website of Zello [http://www.awd.ee/quattro.html], which is unreferenced and factually incorrect --> <!-- Tamer, this is your personal opinion. This section is based on a research performed by VW/Audi clubs members and reliable automotive magazines (not available on the internet). If you can come up with a better description of quattro systems here, you are welcome. Zello. --> ====quattro generation I==== {{Fact|Audi NEVER used any descriptive generations|date=April 2008}} Used from 1981 to 1987 in [[Audi Quattro]] turbo coupé, [[Audi 80]] B2 platform (1978–1987) (Audi 4000 in [[North America]]n market), [[Audi 100]] C3 platform (1983–1987) (Audi 5000 in North American market). Also, starting from 1984, used on the Volkswagen [[Volkswagen Passat|VW Passat]] B2 platform ([[Volkswagen Quantum|VW Quantum]] in the US market) where it was known as ''Syncro''. System type: Permanent [[four-wheel drive]]. Open centre [[Differential (mechanical device)|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 - [[Anti-lock braking system|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==== {{Fact|Audi NEVER used any descriptive generations|date=April 2008}} Starting from 1988 on older generation [[Audi 100#Audi 100 C3, 1983–92|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#Audi 80 (B3) (1987–91)|Audi 80/90 quattro]], B4 platform (1992–1995) [[Audi 80#Audi 80 (B4) (1992–96)|Audi 80]], [[Audi Coupé quattro]], [[Audi 80#The S2 and RS2|Audi S2]], [[Audi RS2 Avant]], C4 platform (1991–1994) [[Audi 100#Audi 100 C4, 1990–94|Audi 100]] quattro, [[Audi S4]], earlier C4 platform (1995) [[Audi A6#Third generation: (C6) A6 (2005-present)|Audi A6/S6]]. System type: Permanent [[four-wheel drive]]. [[Torsen]] centre [[Differential (mechanical device)|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 - [[Anti-lock braking system|ABS]] disabled when locked, automatically unlocks if speed exceeds {{convert|25|km/h|abbr=on}}. 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==== {{Fact|Audi NEVER used any descriptive generations|date=April 2008}} 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==== {{Fact|Audi NEVER used any descriptive generations|date=April 2008}} Starting from 1996 on [[Audi A4]] / [[Audi S4|S4]] / [[Audi RS4|RS4]] (B5 platform), [[Audi A6]] / [[Audi S6|S6]] / [[Audi RS6|RS6]], [[Audi A8]] / [[Audi S8|S8]] with both manual and automatic transmissions. Also on [[Volkswagen Passat|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 {{convert|80|km/h|abbr=on}} on all quattro models (on non-quattro models: up to {{convert|40|km/h|abbr=on}}. 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).<ref name="EDL" /> Open front differential, Electronic Differential Lock (EDL).<ref name="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==== {{Fact|Audi NEVER used any descriptive generations|date=April 2008}} Starting from 2006 on B7 [[Audi RS4]] and the 2008 B7 [[Audi S4]].<ref name="Audi.comGlossary_quattro" /> 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).<ref name="EDL" /> Open front differential, Electronic Differential Lock (EDL).<ref name="EDL" /> ====Vectoring quattro system (quattro generation VI ?)==== {{Fact|Audi NEVER used any descriptive generations - this is actually called "Torque Vectoring System, and has nothing to do with quattro|date=April 2008}} Audi's new '''vectoring quattro system''', which will allow the dynamic allocation of torque to all four wheels<ref>{{cite web |url=http://www.audiworld.com/news/07/b8-s4-exclusive-preview/ |title=AudiWorld: Performance Preview: Exclusive Look at the B8 S4}}</ref>{{Verify credibility|date=April 2008}} 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.<ref name="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. ==Transverse systems== Since [[Volkswagen Group]]'s first mainstream [[transverse engine|transverse]] [[internal combustion engine|engined]] vehicle in 1974, [[four-wheel drive]] (4WD) has also been considered for their [[Volkswagen Group A platform|A-platform]] family of [[automobile|cars]]. It was not until the [[Volkswagen Group A platform#A2|second generation]] of this platform that 4WD finally appeared on the market. The mid-1980s [[Volkswagen Golf Mk2|Mk2 Golf syncro]], with its transverse engine and [[transmission (mechanics)|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 [[Volkswagen Group A platform#A4|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 (mechanics)|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 {{convert|60000|km|-3}} (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=== {{Mergeto|4motion|date=April 2008}} ''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 [[Volkswagen Group A platform#A2|A2-platform]] vehicles, including the [[Volkswagen Golf Mk2]] and [[Volkswagen Jetta|Jetta]]. It was also found on the [[Volkswagen Type 2 (T3)]] (Vanagon in the US), Mk3 generation of [[Volkswagen Golf Mk3|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 [[Anti-lock braking system|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 (Vehicle maneuver)|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=== {{Disputed|date=July 2008}} {{Mergeto|4motion|date=April 2008}} Starting from 1998, the [[Sweden|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]], [[Volkswagen Passat#Mk6 (PQ46 platform) (2005-present)|6th generation VW Passat]] (also based on the A-platform) and [[Volkswagen Eurovan#T5|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 [[Anti-lock braking system|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==== {{Mergeto|4motion|date=April 2008}} 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|Volkswagen Group (Volkswagen AG)]] (VWAG) 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== {{commonscat|Audi quattro vehicles}} * [[4motion]] - [[Volkswagen]] branded four wheel drive system. * [[4Matic]] - a four wheel drive system from [[Mercedes-Benz]] * [[All-Trac]] - a four wheel drive system from [[Toyota]] * [[ATTESA]] - a four wheel drive system from [[Nissan]] ==References== {{Reflist}} ==External links== * [http://www.Audi.com Audi.com] corporate international portal {{Volkswagen Group brands}} {{Audi}} {{Audi (Europe) timeline 1970 to date}} {{Audi (North America) timeline 1970 to date}} [[Category:Audi]] [[Category:Four wheel drive system tradenames]] [[Category:Automotive technology tradenames]] [[bs:Quattro]] [[bg:Quattro задвижване]] [[de:Quattro-Antrieb]] [[ko:콰트로 (4륜구동)]] [[hr:Quattro pogon]] [[nl:Quattro (aandrijfsysteem)]] [[no:Quattro]] [[pl:Quattro (układ napędowy)]] [[sv:Quattro]]