quattro (all wheel drive system)

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quattro (four in Italian) is a registered trademark of Audi AG, for the all wheel drive (AWD) systems used only on Audi brand of automobiles.

quattro was first introduced in 1980 on the permanent-AWD Audi Quattro (often referred to as UrQuattro), and "quattro" has since been applied to all AWD models which Audi sells. Due to the trademark, the name is always spelled with a lower case "q".

Audi parent company Volkswagen AG has used different trademarks for their AWD vehicles. For Volkswagen branded cars, they initially used syncro, but more recently they use 4motion. Sister companies Škoda simply uses the nomenclature 4x4 after the model name, and SEAT uses 4. It is important to note that none of the above trademarks or nomenclatures define the type of AWD system, as discussed below.

[edit] Longitudinal systems

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

In that original quattro system, later found in passenger cars, the engine and transmission are situated in a longitudinal position. Torque is sent through the transmission to a mechanical centre differential which splits it between the front and rear axles. AWD was permanently active.

After 1987, the inclusion of a Torsen (TorqueSensing) center differential, instead of a manually-locking one, allowed engine torque to be automatically redirected to individual axles as driving conditions warranted. Under 'normal' conditions (i.e., grip is equal in both front and rear axles) torque is split between front and rear (50:50 in many, though not all, versions). In adverse conditions (i.e., when grip varies from front to rear), a maximum of 75-80% (depending on the transmission or model) 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 discernable notice to the vehicle occupants, to the axle that has more grip. Furthermore, unlike the various types of electronically operated differentials, Torsen has no requirement for electronic data from sources such as wheel speed sensors, therefore has an element of "fail-safe", unlike say Haldex, should one of the wheel speed sensors develop a fault. In comparison, viscous coupling and electronically controlled centre differentials that are used in other all-wheel drive systems are reactive since they only redirect power after wheel slippage has occurred. The advantage is felt under acceleration while turning since the power transfer between axles is less sudden, reducing the chance of spinning.

The Torsen-based quattro system also offers an advantage in the opposite function of distributing power to the wheels: engine-braking. When engine braking is used to slow the car down, the resulting loads on the front and rear axle are equally stabilized by the Torsen system in exactly the same way that engine torque is diverted - mechanically fully autonomous. This allows the spreading of the engine braking load on the tyres among the four wheels. The Torsen-based quattro-equipped vehicle is able to execute a more stable high-speed turn under deceleration with less risk of spinning 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-transmission assembly in a fore/aft position (longitudinally), the need arises to place the front axle behind the engine, leading to a common 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). The 'ideal' 50:50 weight distribution cherished by many driving enthusiasts, in other words, was not achieved. More recently, the 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) RS4. It was then used in the 2007-model B7 S4. It is planned to be incorporated in all future quattro-equipped cars which use longitudinal-mounted engine layout.

The torque split between left and right wheels has been achieved through the various iterations of the quattro system through manually locking differential (rear axle only), and, eventually, through open differentials with electronic differential lock (EDL). EDL is an electronic system that brakes just the one spinning wheel on an axle, therefore transferring torque across the axle to the wheel that does have traction.

[edit] quattro generation I

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

What: Permanent all wheel drive.

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

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

Open front differential, no lock.

¹ - ABS disabled when locked.

How: When two differentials are locked, up to 100% of torque can be transferred to either axle. The car will not move if one front wheel and both rear wheels lose traction.

[edit] quattro generation II

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

What: Permanent all wheel drive.

Torsen center differential, 50:50 split, automatically locking with up to 75% of torque transfer to either axle.

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

Open front differential, no lock.

¹ - ABS disabled when locked, automatically unlocks if speed exceeds 25 km/h (15 mph)

How: When rear differential is manually locked the car will not move if one front wheel and both rear wheels lose traction, but this is valid if 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 open differential e.g. if one front wheel is raised in the air the Torsen differential will not lock, all torque is transferred to the wheel in the air and the car will not move.

[edit] quattro generation III

Used on Audi V8 starting from 1990.

What: Permanent all wheel drive

V8 with automatic transmission:

Planetary gear center with electronically-controlled multi-plate locking clutch

Torsen differential rear.

Open differential front.

V8 with manual transmission:

Torsen differential middle.

Torsen differential rear.

Open differential front.

How: The car will not be able to move if one front and both rear wheels lose traction. The Torsen effect with a front wheel raised in the air will not happen on V8 with automatic transmission. If the front axle loses traction the multi-plate clutch is automatically locked transferring up to 100% of torque to the rear axle.

[edit] 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 sister vehicles; also the Volkswagen Touareg where they use separate transmissions, PTU's and front axles.

Manually locking rear differential was replaced by "electronic differential lock" (EDL) (Difflock imitation, detects wheelspin via ABS sensors and applies brakes to the one spinning wheel, thus transferring torque via open differential to the opposite wheel which has more traction)^[1]. EDL works at speeds up to 80km/h (50mph) on all quattro models (on non-quattro models: up to 40 km/h (25 mph).

What: Permanent all wheel drive.

Torsen center differential, 50:50 split, automatically apportioning up to 80% of torque transfer to either axle.

Open rear differential, electronic differential lock (EDL).

Open front differential, electronic differential lock (EDL).

How: In on-road conditions the car will not move if all four wheels lose traction. Torsen effect with one wheel in the air will happen on quattro IV because EDL will apply brakes to individual spinning wheel and Torsen differential will transfer the majority of torque to rear axle.

In off-road conditions (wheels in the air and an obstacle restricting the vehicle from moving forward) the car will still move when one front and one rear wheel lose traction. EDL will brake the single spinning wheel on an axle, transferring torque to the opposite wheel.

However, the Electronic Differential Lock is not a replacement for mechanical differential lock. If the braked wheels start to overheat, the EDL will temporarilly be disabled, and the car will end up stationary, spinning those wheels which have no traction.

[edit] quattro generation V

Starting from 2006 on Audi Q7 and B7 platform RS4. Will become the standard fitment on all future quattro Audis with longitudinal engine layout.

What: Permanent all wheel drive.

Torsen center differential, 40:60 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).

How: In on-road conditions the car will not move if all four wheels lose traction. Torsen effect with one wheel in the air will happen on quattro IV because EDL will apply brakes to individual spinning wheel, and Torsen differential will transfer the majority of torque to rear axle.

In off-road conditions (wheels in the air and an obstacle restricting the vehicle from moving forward) the car will still move when one front and one rear wheel lose traction. EDL will brake the single spinning wheel on an axle, transferring torque to the opposite wheel.

However, the Electronic Differential Lock is not a replacement for mechanical differential lock. If the braked wheels start to overheat, the EDL will temporarilly be disabled, and the car will end up stationary, spinning those wheels which have no traction.

[edit] Vectoring quattro system (quattro generation VI ?)

Audi's new vectoring quattro system, which will allow the dynamic allocation of torque to all four wheels^[2] 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.

[edit] BorgWarner

The Audi Q7, platform-mate of the VW Touareg and Porsche Cayenne, does not use the same underpinnings of either previous model. BorgWarner instead provides the AWD system for this more on-road-appropriate SUV.

[edit] Transverse systems

Since VWAG's first mainstream transverse engined vehicle in 1974, AWD has also been considered for their A-platform family of cars. It was not until the second generation of this platform that AWD finally appeared on the market. The mid-1980's 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" all 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 fed torque through itself to the passenger-side front wheel. At the rear axle, torque was first sent through a viscous coupling before reaching the final ratio gearset. This coupling was full of 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 A-platform, the viscous coupling has been dropped in favor of a Haldex electro-hydraulic limited-slip "coupler" or clutch. The Haldex unit is not a differential, and therefore cannot perform in the true sense like a differential. A Haldex unit may divert up to a maximum 50% of the torque to the rear axle as conditions warrant. Many people are confused with the torque distribution on Haldex systems. Under normal operating conditions, the Haldex clutch operates at rate of 5% 'grip' (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 the cars wheel speed sensors have determined that both front wheels have lost traction, the Haldex clutch can lock at 100% clamping force. However, when the Haldex locks at 100%, the torque between front and rear is exactly 50:50 - the front axle is ALWAYS driven, hence the 50:50 split. The torque split between left and right wheels is achieved with a conventiional open differential. If one side of the driven axle looses grip, then EDL (electronic differential lock) 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 four wheel drive system, the EDL controls front wheels only.

The main advantages of the Haldex system over the Torsen-based system include: a slight increase 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 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 unit also requires additional maintenance, in the form of an oil and filter change every 40,000 miles (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 wheel speed sensors.

[edit] Viscous Coupling

Important note: This AWD system was never used on Audi cars under the quattro badge.

The aforementioned viscous coupling AWD system was found in the Mk2 generation of transverse-engined A-platform vehicles, including the VW Golf and Jetta. It was also found on the Vanagon, Mk3 generation of Golf and Jetta, 3rd generation of Passat (which was based on the A-platform), and the 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 near the final drive. This AWD system was known as Syncro on all vehicles.

What: Automatic all wheel drive (on demand).

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

Open rear differential.

Open front differential.

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 100% of torque is automatically transferred to rear axle. 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 all 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 all 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.

[edit] Haldex

Starting from 1998, the Swedish Haldex unit replaced the viscous coupling. Haldex is used by Audi on the quattro versions of the Audi A3 and Audi TT, the Audi S3. It is also used by Volkswagen in the 4motion versions of the Mk4 and Mk5 generations of Volkswagen Golf, Volkswagen Jetta, and the R32, VW Sharan, 6th generation Passat (also based on the A-platform) and 5th generation Transporter (van). On the Audi's, the trademark holds true, and are referred to as quattro, whereas the Volkswagens receive the 4motion name. The Škoda Octavia 4x4 and SEAT Léon 4 and Alhambra 4 also used Haldex, being based off of Volkswagen models. Curiously, the Bugatti Veyron also utilizes Haldex, though with separate transmission, PTU and front and rear axles.

What: Automatic all wheel drive (on demand).

Haldex multiplate clutch with electronic control, acting as a pseudo center differential.

Open rear differential, EDL on some models (not proved).

Open front differential, EDL.

How: Normally front wheel drive vehicle. A Haldex unit may divert up to a maximum 50% of the torque to the rear axle as conditions warrant. Many people are confused with the torque distribution on Haldex systems. Under normal operating conditions, the Haldex 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 loose traction, the Haldex clutch can lock at 100% clamping force. However, when the Haldex locks at 100%, the torque between front and rear is exactly 50:50 - the front axle is ALWAYS driven, hence the 50:50 split. The torque split between left and right wheels is achieved with a conventional open differential. If one side of the driven axle looses grip, then EDL (electronic differential lock) 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 four wheel drive system, the EDL controls front wheels only.

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 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.

[edit] Haldex aftermarket applications

The Haldex centre coupling is often used as upgrade to an aftermarket AWD conversion on older FWD 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 VW Corrado or VW 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 VWAG car to control the Haldex centre clutch using the standard ABS 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 (TPS) calculations.

[edit] References

1. ^ VAG four-wheel drive systems and brand names.
2. ^ AudiWorld: Performance Preview: Exclusive Look at the B8 S4.

[edit] See also

• 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-ETS - a four wheel drive system from Nissan.

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