In order to drive the front wheels as well, the following components are needed: a differential gear and two drive shafts for the front axle, as well as a cardan shaft for connecting between the rear and front axle.
Buses "synchro" available with permanent or plug-in all-wheel drive.
Permanent four-wheel drive
A bus with permanent all-wheel drive has a so-called visco clutch between the front and rear axles. This clutch performs two tasks: 1. It distributes the power flow between the rear and front axles. 2. Equalizes small differences in the number of revolutions of the axles.
When cornering, there is a different number of revolutions of the wheels of the front and rear axles. Therefore, the distribution of forces must be flexible.
The visco clutch consists of a box filled with silicone oil, in which there are two friction discs tightly adjacent to each other, between which there is no mechanical connection. The discs are alternately connected to the cardan shaft and the drive shaft of the front axle. The force lock occurs through the viscous silicone oil, which reacts to speed differences between the discs. The difference in the number of revolutions between the rear and front axles causes the same difference in the number of revolutions of the discs. The greater the difference in speed, the more power is transferred from the rear to the front axle. The inclusion of the front wheels is soft and almost unnoticed by the driver.
On a normal, dry road, all wheels turn at almost the same speed. Then almost all the drive energy is taken by the rear axle. If, on a bad road, the rear wheels rotate somewhat more slowly than the front wheels, then the visco clutch immediately transfers more energy to the front axle. If the rear wheels spin, almost all of the power is transferred to the front wheels.
Plug-in all-wheel drive
The plug-in four-wheel drive consists of almost the same components as the permanent four-wheel drive, but does not have a visco clutch. Due to the high stresses that occur in the drive when turning and high speed, this drive should only be used on poor soils and at low speeds.
Unlike a visco-clutch bus, the front-wheel drive can be permanently disabled. The disengagement takes place via a disengaging clutch in the gearbox in front of the cardan shaft. This clutch is actuated by a special valve actuated by reduced pressure in the engine intake pipe.
As an option for heavy road conditions, Volkswagen offers buses with both versions of all-wheel drive a differential lock for the front and rear axles. These interlocks are also actuated by reduced suction pipe pressure.
The front differential of both four-wheel drive variants is filled with 1.5 liters of GL 4 SAE 80 gear oil. An oil change is not necessary. The oil is controlled from the side through the control hole.
Repair of the all-wheel drive must be carried out by special workshops.
1 - engine; 2 - clutch; 3 - gearbox; 4 —rear differential; 5 - rear differential lock; 6 - actuating element (only for vehicles with all-wheel drive); 7 - cardan shaft; 8 - visco clutch (only for vehicles with permanent all-wheel drive; 9 - front differential; 10 - front differential lock
A1 - rear-wheel drive
A2 - front-wheel drive
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