Zenith The of vari­able drives

Old Bike Australasia - - ZENITH GRADUA -

The early years of mo­tor­cy­cles, or mo­tor bi­cy­cles as they were known back then, al­ways seems to amaze me. A re­cent brush with a Zenith Gradua had me fas­ci­nated and led me to re­search more about this mar­vel. This is a snap­shot of what I found.

Let’s be­gin at a pe­riod from the year 1900. The in­ter­nal (also re­ferred to back then as the ‘in­fer­nal’) com­bus­tion en­gine was very ba­sic with a very nar­row rev range. It was a mat­ter of tweak­ing the ig­ni­tion ad­vance, air and throt­tle con­trol to find the sweet spot and keep­ing it there. Th­ese days we have a wide range of us­able RPM and we use the throt­tle lib­er­ally to con­trol speed. Given that en­gines were best left at their op­ti­mum RPM, the trans­mis­sion sys­tem was de­vel­oped to ad­just the ground speed of the mo­tor bi­cy­cle to suit var­i­ous en­gine loads. Ini­tially th­ese trans­mis­sions were sim­ply di­rect drive but as years went on the virtues of vari­able gear­ing evolved. A sum­mary of this evo­lu­tion leads us to a pe­riod when the Zenith Gradua was the industry bench­mark.

Di­rect Drive Di­rect drive trans­mis­sion used a belt, chain or shaft to di­rectly con­nect the en­gine to the drive wheel. The de­com­pres­sion lever al­lowed the en­gine to turn over with min­i­mal fric­tion, al­low­ing the ma­chine to be pushed around with the en­gine stopped. A spe­cial start tech­nique was also nec­es­sary. The rider would ac­ti­vate the de­com­pres­sion lever to push the mo­tor­cy­cle to an ap­pro­pri­ate speed, jump onto the seat and then re­lease the lever to al­low the en­gine to start and off they went. Early ma­chines re­tained the pedal drive of the bi­cy­cle so that the rider could al­ter­na­tively pedal in­stead of run­ning to start. Th­ese ped­als

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