Syncro
Syncro (today: 4Motion ) is a brand for all-wheel drive in vehicles from Volkswagen . Since the VW subsidiary Audi achieved great success in racing with its Quattro models in the early 1980s , all-wheel drive has also become socially acceptable in cars. The Syncro drive concept is directly related to the Audi systems. A prototype shown at the IAA in 1983 was called the Passat Variant Tetra .
First generation VW Syncro
With the first-generation Syncro drive, the differential gears could not be locked. This means that the drive forces are evenly distributed as long as all wheels have even grip. If an axle or a wheel does not have traction, the vehicle cannot transmit any drive power. The axle or the relevant wheel spin. The use of an anti-lock braking system is possible here without any problems, as there is no permanent force-fit connection between the axles. When the cardan shaft is removed , the car cannot move because the transfer differential directs the rotary movement to the free output.
Second generation VW Syncro (Audi from 1983, VW from 1985)
With the second generation Syncro, switchable differential locks were introduced, which make it possible to move forward with reduced grip due to the season , too much power or very slippery ground. The differential locks can be inserted here one after the other in a meaningful sequence. In the first switching stage, the medium is blocked differentially in order to provide both axles with full drive power. However, if one wheel spins on the front axle and rear axle, no drive force is transmitted. With an additionally locked rear axle differential, which means a rigid through drive from the engine to the rear axle, a drive can also take place under very adverse circumstances. The front axle differential cannot be locked.
Since all the wheels of the vehicle turn at the same speed in this configuration and the different arc lengths per wheel can no longer be compensated for by differences in speed when cornering (see differential gear), the drive train becomes tense (twisting of the shafts and joints) and individual wheels move faster / slower than the ground below them (block / spin). This leads to increased wear on the wheels and drive train and unsteady handling when cornering.
A disadvantage of locking a differential manifests itself in an area of use that is restricted to snow, loose ground or on wet roads. The driving behavior changes very strongly and it is no longer possible to experience a clear limit area . When cornering with locked differentials on a dry road, the drive train becomes tense and the wheels “rub” over the asphalt, which causes increased tire wear. The use of the anti-lock braking system (ABS) is possible, but is disabled when the differentials are locked, as selective braking intervention by ABS is no longer possible. If the cardan shaft is removed, the front wheels can only be driven when the center differential is locked.
VW Syncro with viscous coupling
The viscous coupling , a development by Steyr-Daimler-Puch Fahrzeugtechnik for vehicles with all-wheel drive based on the Passat 32B , has a cylindrical cell filled with silicone oil in which the wing discs rotate. Half of the discs are connected to the front axle, the other half to the rear axle. The discs are arranged alternately. If there are speed differences during normal travel, for example due to loose ground, cornering etc., the blades in the cell rotate at different speeds and automatically establish a positive connection between the two axles. The all-wheel drive with viscous coupling is a cost-effective “hang-on” solution to convert an existing single-axle drive into a four-wheel drive with relatively little design effort. The center differential is omitted because of the viscous coupling, which is connected to the output shaft of the original gearbox either directly or via a bevel gear. An anti-lock braking system control is possible because the rear axle is automatically decoupled by a freewheel when coasting. When reversing, in some models a switch is activated on the rear axle by pressing a switch, which bypasses the freewheel so that all-wheel drive is also available when reversing. If the cardan shaft is removed, it is only driven to the front wheels; the front axle drive and transfer case outlet are rigidly connected. The VW T3 syncro bus does not have such a freewheel, but was still offered with an optional three-channel ABS (front wheels individually and rear axle). Because of the rear engine, only the rear axle is driven when the cardan shaft is removed.
VW 4Motion (Syncro successor system)
The successor to the VW Syncro, called 4Motion , works according to a different system. The torque distribution to the front and rear axles is solved differently depending on the installation position of the engine. Vehicles with transversely installed engines have a primary front-wheel drive, with the rear axle connected by a multi-plate clutch developed by the Swedish company Haldex . This system is often referred to as " Haldex " for short . In the case of a longitudinally installed engine, the drive torque coming from the gearbox is first introduced into a central locking differential of the Torsen design (type A) (" Torsen " for short ), which distributes the torque evenly and permanently to both axles with good grip. Both principles differ mainly in the degree of blocking. If required, the Haldex coupling achieves significantly higher locking values than the Torsen central differential, but only the latter guarantees a classic permanent all-wheel drive that is independent of control electronics.
VW Syncro with Torsen limited slip differential (4Motion)
The Torsen limited-slip differential - Torsen by TORque-SENsing , "torque sensing" - was initially only installed by Audi ( Quattro drive ), later also in the 4Motion system from Volkswagen. The Torsen - differential is the center differential and sits right on the gearbox. It divides the drive torque between the front and rear axles in a 50:50 ratio with good grip on all wheels. A simple open differential is not used here because it is always distributed 50:50, even with poor grip conditions. This means that if one axis hardly transfers any torque to the ground (when it is slippery), the other axis will only receive this low torque. A Torsen limited-slip differential compensates for this disadvantage slightly. Due to the high internal mechanical friction, this differential can direct more torque to the stationary (i.e. better gripping) axle. The specific value depends on the degree of locking and the grip conditions. The rear axle lock is switched semi-automatically. It is switched on manually via a button and switched off electronically at speeds over 25 km / h. The rear lock should only serve as a starting aid.
VW Syncro with Haldex coupling (4Motion)
The 4Motion system with Haldex clutch is an electronically controlled, wet-running multi -disc clutch. In contrast to the viscous coupling , torque is transmitted through the contact pressure of the lamellas, not through shear friction in the highly viscous silicone oil. This has the advantage of higher transmittable torques and shorter response times.
The force distribution during normal driving, i.e. the vehicle drives straight ahead on a flat stretch at a constant speed, is 100: 0. The entire engine power is transferred to the front axle. If the electronics recognize changing road or load conditions, for example when cornering, towing a trailer, driving uphill or downhill, it changes the force distribution. The control range of the electronics extends from the aforementioned 100: 0 to 50:50. In the case of increased tractive effort, up to 100% of the engine output is transferred to the rear axle.
If the wheels on the front axle do not transfer any power to the road, for example when the road is slippery or tires are spinning, the entire engine power is briefly transferred to the rear axle. The front wheels continue to turn due to the longitudinal locking effect of the Haldex coupling, but no power is transmitted to the road.
Anti-lock braking systems and other vehicle dynamics controls can also be used here. Like the first-generation Visco clutch, the Haldex clutch can also transfer power to the rear axle when reversing. When the cardan shaft is removed , the drive is only on the axis of the gearbox flanged to the motor. The Haldex coupling was developed by VW in cooperation with the Swedish company Haldex .