Transverse Systems
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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 friction 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 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) 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 Haldex LSC unit also requires additional maintenance, in the form of an oil and filter change every 60,000 kilometres (37,000 mi) (whereas the Torsen is generally considered to be maintenance-free). Another important disadvantage of the Haldex system is the requirement for all four tyres to be of 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.
Read more about this topic: Quattro (four-wheel-drive System)
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