From my very earliest designs for HPVs, even before the Trout, I have been interested in linear or elliptical drives. The advantages of these types of drives when applied to recumbent machines are reduced frontal, side and surface area and better forward vision, as well as the possibility of a more aesthetic shape.Reducing surface area may also lead to a lighter weight machine. The reduction of side area could also improve the side-wind stability of bikes with fairings.The cycloid drive was developed in the late 1980’s and was used in a number of machines including various forms of the Cycloid bike, Fortuna and the first K3.
The rolling eccentric sprocket produces an approximate sinusoidal movement at the crank.
As the chain remains at the same height during the pedal stroke, it can be fed through the frame tubes.
This version was only single speed but the leg load could be varied by using a shorter or longer stroke.
Cycloid side front and top view
Bagged cycloid,Kingcycle and Beanbag with tubby future World Champion
Cycloid FW Drive with Gears
Some lovely sketches by Martin Simpson
Although I won quite a few races with the bagged version of Cycloid, it did have its limitations.The single speed was ok for racing on velodromes and short circuits but not so good for longer hills. We did start making a geared version but it was not finished.
We have also found that although the drive is mechanically very efficient, physiologically it was not so good. We did a number of tests and found it was only about 90% as efficient as a circular drive. In the end we decided this inefficiency was caused because the main leg muscles were performing a 50% duty cycle compared with only about half that on a circular drive. Although it seems that this higher duty cycle should help efficiency, it doesn’t allow the muscles to recover and get rid of lactic acid build up, causing a severe burning sensation!