Zenith 'Gradua' motorcycle, 1920.
© Science Museum / Science & Society Picture Library
This motorcycle is representative of the large machines of the period which were often fitted to a sidecar and used for long distance touring. Fitted with a 5 hp V-twin JAP engine, the main technical feature of the Gradua is its unusual method of transmision, patented by E W barnes in 1907. It provides an infinite range of speeds between the top ratio of 3.5 to 1 and the bottom ratio of 6.75 to 1. The engine shaft carries an expanding and contracting pulley, in conjunction with which the rear wheel moves backward or forward regardles of the drive ratio selected. When the pulley is opened to its full extent the driving belt runs free on a flat loose ring at the botom of the groove, thus providing a 'free' engine, equivalent to being in neutral. No clutch is fitted.