Latest in my series of unusual and often forgotten (and often deservedly so) means of varying ratios is the Beier variable ratio gear, or CVT as we’d call it today. It was designed by a Herr Beier, an Austrian gent whose name would therefore rhyme with Mayor, more or less.

Here’s a diagram, which unfortunately tells us absolutely nothing without a bit of explanation.

We’ll start by looking to the right. The sketch shows two shafts, each of which is equipped with several discs. The lower shaft provides the output and has discs which are able to move axially and are pressed together by the spring visible to the right. Each disc is machined so that the rim is its thickest part. In section then the rims look somewhat like jaws. The upper, input shaft has discs which interleave with the output ones and which are slightly conical in section with surfaces which are about 3 deg to the vertical. To proceed the input shaft is brought closer to the output shaft so that its discs force themselves between the output discs and come into contact with their raised rims. The output discs, we remember, are able to move along their shaft against spring resistance. The input discs are now able to turn the output discs, initially at approximately a 1:1 ratio. Now we can move the input shaft further towards the output, as shown on the left hand side of the shaft. The input discs still maintain contact with the raised rims of the output discs, but the contact point has now moved closer to the input shaft and the effective diameter of each input disc has now decreased. In simple terms a small wheel is now turning a bigger wheel and so reduction gearing has been set up. Continue to move the input shaft closer and the gearing reduces still further.

As each disc can transmit only very limited torque there are many discs and (not shown in the diagram) typically three input shafts clustered around the output shaft.

What’s not obvious is that the discs were not in mechanical contact with each other as oil jets endured that there was a thin film of oil between them which transmitted the torque.


Beier drives are used with electric motors, and more exotically they featured on the Napier Nomad two stroke opposed piston turbocompound diesel (with afterburning) aircraft engine from the 1950s.

The Beier drive was used on buses early in the 20th century but so far as I know nobody has tried it for automotive use since except Citroen who designed their own take on it in the early seventies with the aim of using it on the GS. Nothing came of it.