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Zenith Gradua

1912 Zenith GraduaZenith Gradua1918 Zenith Gradua

Among the bikes favoured by sporting riders before World War I, Zenith bikes fitted with the factory's Gradua gear were among the first to offer a really practical variable drive ratio. For a few years they were so successful in hill climbs and other early events that they were barrer from single-gear competitions. The company took this as a compliment and even added the word 'Barred' to their badge.

The earliest pioneer bikes relied on direct belt drive to the rear wheel. Although their engines were designed for maximum flexibility, the fixed gear that resulted meant that a bike that could start off and pull up hills easily would have had a severly limited top speed. The alternative adopted by early sporting riders was to use an adjustable pulley which could be opened to give a similar effective diameter and hence a higher gear. The only problem this posed was that the belt tension was lost and it had to be shortened to suit. One early rider's manual devoted two pages to the process of climbing a hill and a similar amount to descending at speed afterwards.

Zenith changed all that with a belt drive system, the ratio of which could be varied on the move. Although technically limited, it was good enough to ensure the marque's success in many important competitions and having demonstrated the virtue of variable gearing it paved the way for even more practical alternatives.

Zenith was established in 1904 in Stroud Green, North London. Like many pioneers, they used proprietary engines, fitted to a bicycle-type frame with direct belt drive. But in 1908, designer Freddie Barnes developed the variable engine pulley. This had a pair of flanges that could be moved in or out by winding a handle while the bike was on the move. There were a number of limitations to this device. Unlike the chain drive that would eventually take over, belts were prone to slip in the wet, while the limited gear ratios still needed a large, flexible engine to make the best use of them. But with the JAP V-twins fitted to the majority of early Zeniths the theoretical limitations were overcome.

Improvements to the Gradua system followed including a variation in which a chain transmitted power to a gearbox firward of the engine. Transmission from there on was by Gradua belt drive.

Zenith continued in production from a new site in South London, making bikes powered by everything from a lightweight Villiers-powered two-stroke single to a 1100cc side-valve V-twin. Post-war, they restarted with a 750cc V-twin but in 1950 ceased production mainly because of the lack of suitable proprietary engine makers.

Zenith Gradua

  • Years in production - 1908-20
  • Engine - single-cylinder or V-twin side-valve four-strokes