the forward/neutral/reverse switching is done in the primary gearbox.
the input shaft has a gear which is engaged in 2 gears, that are placed on the output shaft.
(it is a single shaft with the two ends sticking out of the housing. take note that one of the gears is not fixed to the shaft!!)
the 2 gears rotate in opposite directions. the selector lever moves a bush that is fixed on the output shaft (can slide but cannot turn) and the bush engages in either the left or right gear, or neither of the two.
due to the opposite turning direction of these gears, changeover from one to the other gear means the output shaft also reverses direction.
The way this works has never changed since the late fifties, up to 1991.
Volvo did add another function to the box, the "p" position. this puts a blocking lever into a separate tooth gear. really quite simple.
up to the DAF 44/55, the secondary variomatics were separate units, each driving one wheel. this meant that no differential was needed because of the possibility to correct by changing the ratio of each belt.
the older system also has disadvantages:
- the belts are deformed when the wheel moves, reducing lifespan
- at low speeds the belts start slipping in a tight bend, car gets jumpy
- high throttle may cause rear wheels to 'step aside'
- suspension with old-fashioned swing-arms
the 'de Dion' axle of the DAF 66 was a genuine improvement, the differential being the only 'real' disadvantage.
the volvo 300 series system is almost identical.
