Variomatic TransmissionAmendments by Matthias Hiltpold
|Infinitely variable belt-drive by DAF|
The Variomatic should offer a relatively low cost alternative to the automatic gearbox for smallervehicles and/or engines with low torque. In 1958
the DAF company introduced a
development by Johan van der Brugghen, indeed, it was based on much older inventions. The simple operation and maximum torque over the largest possible speed range was the target. As a Continuously
Variable Transmission, the construction belongs to the infinitely variable transmissions and the principle is found, in a simplified form, in scooters and vehicles with a similar drive system. More modern
examples can be found here.
|Gear-ratio changes through shiftable tapered-disc pairs|
We are looking at the rear axle-drive from the front (see figure 1). First of all, the torque comes in from below (center) into the transfer case. Here it is distributed, without much gear-ratio, through conical gearwheels, to
the right- and to the left belt-drives. Of the front tapered-disc pairs, those on the inside are fixed, and the outside ones are movable through the engine vacuum, which can be clearly seen by the vacuum-pipe connection.
Of the rear tapered-disc pairs, the outsde conical discs are adjusted by the centrifugal force. The faster the car travels, the more the outside discs move apart and the closer the inside discs move towards each other.
The rolling diameter is then bigger there. When suddenly accelerated, then through the altered inlet manifold vacuum, the front discs move apart- and the rear discs towards each other. The trasmission-ratio increases
In addition, to the rear axle there is, on the left and on the right, another fixed gear-ratio. The torque to be transferred to the belt-drives, can thus be held lower. The compensation between the two driven wheels is only
possible through the slip in the tapered-disc drive, provides however, a limited-slip effect when driving on winter roads. Above, in figure 2, a part of the Variomatic with pendulum axle
can be seen. This causes a relatively high unsprung mass and frequent negative camber when driving. The construction shown in figure 1 allows the modern rear suspension of the later DAF- and Volvo models.
Drivers who are accustomed to the jerk when changing gears, find that the infinitely variable gearbox takes a little getting used to. Once the engine has reached mid-range RPMs, it will stay like that for a long while,
although the speed increases. This changes when one accelerates more strongly. The, otherwise possible corrrelation, between the RPMs and the speed is absent. Those who drive by ear, make life difficult for
themselves and find themselves possibly driving too fast. The engine brake in the Variomatic can be activated simply by pulling out the button on the dashboard. This affects a multiple sliding-sleeve in the engine
compartment, and cancels out the affect of the vacuum. Thereby, the RPMs remain just as high as when the throttle valve is opened. This button is pulled when braking is anticipated, e.g., when driving downhill, but also
when driving uphill, through the higher RPMS which are available after slowing down for a curve and then accelerating again.
Apart from the above, there is not really much to do. No clutch, and only one lever for driving forwards and reversing. This lever shifts the axle drive shaft coming from the engine in the front distributor casing, thus
causing a second conical gearwheel to mesh, which reverses the rotation direction back up to the maximum speed. To create an advantage for their cars, the Dutch developed a unique racing regulation called 'Slop-
car-racing'. In this case, the cars are raced, generally in scrap-yards, exclusively in reverse gear. The more straight sections the track has, the higher the chances were for the Variomatic, which can travel as fast in
reverse- as in forward gear. However, when cornering, it could get pretty dangerous for everyone. Driving in reverse, the steering has a negative reset force, which requires strong hands on the wheel, otherwise the car
will break-out. With the newly developed chain-belt transmission the possibility of driving fast in reverse, by the way, is prevented by the software. 11/10
|- ||Torque restriction with belt-drive, slipping is possible,|
|- ||The change-over from the customary gearbox takes getting used to,|
|- ||Jerky vehicle movement through the centrifugal clutch, paticularly in reverse gear,|
|- ||Costly repairs of the centrifugal clutch, sometimes before 100.000 kms.,|
|- ||Regular wear-and-tear/replacement of the belts (40.000 kms depending on stress),|
|- ||Regular adjustment of the disc separation gaps,|
|- ||High centrifugal mass in the gearbox, unfavourable for acceleration and consumption,|
|- ||Effeciency of the belt-drive not ideal.|