Best Kept Secret: 1950 Zenith Big Twin

1950 Zenith Big Twin
Engine: 747cc air-cooled sidevalve 50-degree V-twin, 70mm x 97mm bore and stroke, 6:1 compression ratio, 24hp at 4,000rpm
Top speed: 63mph
Carburation: Single 1-inch (25mm) Amal Type 75/274 pre-Monobloc
Transmission: 4-speed Burman, chain final drive
Electrics: Lucas Magdyno
Frame/wheelbase: Single-loop tubular steel frame with bifurcated lower double cradle/56in (1,420mm)
Suspension: Druid girder fork front, rigid rear
Brakes: 7in (178mm) SLS drum front, 7in (178mm) SLS drum rear
Tires: 3.5 x 19in front, 4 x 18in rear
Weight (dry, est.): 385lb (175kg)
Fuel capacity: 3.6gal (13.6ltr)

Zenith [noun, usually singular] = pinnacle, apogee, summit [antonym, nadir, abyss].

That’s how the Cambridge Dictionary describes the name that British engineer Frederick W. Barnes, then 27, chose in 1905 to market an innovative two-wheeler with a low-slung frame and hub-center steering named the Bi-Car. Powered by a 3 horsepower German Fafnir engine, when launched at the Crystal Palace show that year it was said to be “a revolution in motorcycles.” Though the Bi-Car had a short life, its maker went on to experience both the zenith and the nadir of two-wheeled existence, including Brooklands race victories and twice breaking the World Land Speed Record. Moreover, Zenith rode out two world wars, and survived receivership as a consequence of the Depression before finally succumbing in 1950, simply because it could no longer source engines to fit its bikes. That’s quite a roller coaster ride, and yet even with that long history, the Zenith name is mostly unknown.

Zenith Motors

London-based Zenith Motors was a small manufacturer, but had significant success in speed contests throughout the Teens and Twenties. Manchester-born Freddie Barnes was the driving force behind it, a pioneering inventor who relished the stimulus of competition. After the Bi-Car, he began production of more conventional Zenith motorcycles in 1907 in his North London workshop with the Fafnir-powered Zenette, with a Druid girder fork and a triangular frame with optional scissor-action rear suspension. In 1908, to overcome the difficulty of climbing gradients in the days before motorcycles had gearboxes and when direct belt drive was the norm, Barnes developed and patented his own variable drive, the Gradua Gear system. Originally known as the Barnes Pulley, this provided a rider-selected continuously variable final drive ratio controlled via a handle and the ratio could be altered on the move. Other variable crank pulley systems of the day required the machine to come to a halt before the gearing was changed.

It did so by simultaneously changing the diameter of the crankshaft belt-drive pulley to alter the drive ratio while moving the rear wheel back and forth via worm gears within slots cut into the rear chassis fork, in order to maintain correct belt tension. A later hand crank system operated the mechanism via a vertical shaft and bevel gears, which made the Zenith Gradua almost impossible to beat in the increasingly popular speed hill climbs of the pre-World War I era. A Zenith rider could change gear while climbing the hill without stopping, while other competitors had to make do with a single choice of gear ratio. And while the change in the wheelbase was undesirable, it wasn’t a great issue at 1908 speeds. The system produced infinite variations in gearing from as low as 9:1 up to 3.5:1, but rival riders and manufacturers held this to be an unfair advantage, so many clubs excluded Zeniths from their events. Barnes was quick to capitalize on this ban, and registered a new trademark in 1911 emblazoned with the word “Barred” in bright red letters across the Zenith emblem!

Competition

The opening of the Brooklands circuit at Weybridge in 1907 acted as a magnet for firms such as Zenith, led by entrepreneurs like Barnes who relished demonstrating their products in competition. With increased demand for its products necessitating a bigger factory, Zenith Motors moved to Weybridge in 1908, just half a mile from the track. In March 1909, aboard a Gradua-equipped Zenette-JAP, Freddie Barnes set the very first standing-start record (18.63 seconds, averaging 12.89mph!) for climbing the steep Brooklands Test Hill, which every customer Zenith Gradua road bike was certified as having done, after stopping and restarting on the 25 percent gradient, something no single-speed bike could do. Barnes was also a mean racer, setting many speed records and successive Test Hill marks, as well as scoring many race victories at Brooklands on Zenith machines, and chalking up numerous successes in long-distance reliability trials.

Order the May/June 2018 issue of Motorcycle Classics to read more about the 1950 Zenith Big Twin. Contact Customer Service at (800) 880-7567 or contact us by email.