Vincent Comet
Vincent Comet Background & History
The Vincent Comet was a 500cc (499cc actual displacement) single built from 1935 through the 1955 model year. It’s history closely follows that of its bigger brothers, the 1000cc (998cc actual displacement) V-twin monsters, the Vincent Rapide and Black Shadow. The Series (A through D) that traced it evolution were the same for the single-cylinder Comet and the big V-twins. Why? Because those V-twins were built on the bones of the Comet.
This 1938 Vincent Comet shows what a prewar Series A looked like. Note the Brampton girder front forks.
Making a Single into a Twin
Prior to the war, Phil Irving, Vincent's chief engineer, was sitting at the drawing board with two tracings of the Vincent 499cc single-cylinder engine. As things got moved around, the top tracing roughly lined up withj the bottom one such that it made it look like a V-twin. With a little cajoling, he lined it all up and the Vincent V-twin was born. The likeness is so uncanny that the single looks the same as the V-twin except that it's missing the rear cylinder. The new 998cc V-twin was introduced in the 1936 Vincent Series A Rapide. It made 45 hp and was good for 100 mph, stellar at the time. It was produced in small numbers prior to World War 2. When it was reintroduced after the war in 1946, it had been completely reengineered and it was a monster. This in turn led to the Vincent Black Shadow, which was the hot rod version of the Rapide. But it all started with the lowly 499cc Vincent Comet single.
LEFT: The 499cc Vincent Comet single-cylinder engine. RIGHT: The 998cc Vincent Rapide V-twin. Note the similarities. Even the timing covers are almost identical. The V-twin looks like a Comet with one more cylinder stuck onto the back. That's what it was!
Vincent Comet Engine Design
Because the Vincent singles and V-twins shared so much architecture, the Vincent Comet’s specs will look very much like those of the Vincent Rapide and Black Shadow. A single camshaft sat high in the crankcase, operating short pushrods, and rocker arms in a classic OHV (overhead valve) setup, but with some striking differences from other brands. Vincent was the only company that employed two valve guides on every valve, on high, one low, to increase valve train stability and durability. The forked rocker arms actuated the valves between these two guides. The valve springs on the Series A were exposed and of the ‘hairpin’ type, just like the Series A Vincent V-twins. While the motoring public liked to think of the Vincent Comet as ‘half of a Rapide’, in fact it was actually closer in comparison to the high performance Black Shadow with its higher compression ratio of 7.3:1 (the Rapide had a 6.8:1 compression ratio) and the larger carburetor of the Black Shadow. Some things weren’t similar, however. For instance, the internal engine parts weren’t polished like the Black Shadow, nor were they selectively assembled from the best parts. Also, the Comet mounted is Burman BAP 4-speed gearbox with a set of aluminum brackets, rather than via the unit construction-method used on the V-twins.
1938 Series A Vincent Comet. Note the spring boxes under the seat supporting the rear suspension.
Vincent Comet Chassis Design
All Series A Vincent Comets came with Brampton girder front forks, while Series B and later machines got Vincent’s own “Girdraulic” girder front forks. All Vincents, single and V-twins alike, shared the same chassis, with Vincent’s proprietary cantilever rear suspension. This was considered an advanced design at a time when most motorcycles were still running rigid frames (no rear suspension). Triumph and BSA, often considered to be the leaders of the British motorcycles industry, didn’t ditch their rigid frames until 1953. Vincent launched the Comet with their cantilever rear suspension in 1935. Phil Vincent fancied himself as an innovator and certainly a purveyor of some of the finest and fastest motorcycles in the world. Mr. Vincent, and the Vincent product line certainly lived up to that standard. Cantilever suspension was very similar to Harley-Davidson’s Softail setup, where the entire rear section of the frame pivots like a swing arm. In Harley’s case it was to make the Softail look like a ‘hardtail’, that's slang for a rigid frame. Counter-intuitive perhaps, but it’s a look that is very popular among the chopper and Harley faithful. In Vincent’s case, it was the best solution they could come up with at a time when virtually no one had even heard of a swing arm before. The ‘rear frame member’ (RFM) on the Vincent Comet was mounted to a pivot on the back of the gearbox. At the top, the RFM was suspended by two almost-horizontal shock absorbers. The whole system was subject to some flexing under hard cornering, but it was the best system going at the time.
The Series A engine was a complex thing to look at. Check those chromed oil lines running up to the top end. Those would be gone by Series B.
Vincent Comet Series A through D
Series A was built from 1935 though the final year of civilian production before the start of World War 2, 1939. When civilian production resumed in 1946, the Vincent Comet was re-released as the Series B, after much development and refinement, the bulk of which was done during the war. The Series C is considered to be the ultimate Vincent Comet. And Series D came just as the company was ready to close its doors in 1955 and only one prototype Series D was built.
Series B and later Vincent Comets had engines that were completely redesigned. Note how clean they are now. These later Vincents always had muscular looking engines.
Vincent Comet Series B Changes
The biggest changes on the Vincent Comet came during World War 2, which became the transition between Series A and Series B. Like the V-twins, the Comet was completely re-engineered. The engine lost that gaggle of oil lines running up the right side of the cylinder, feeding the valvetrain. These oil lines were now all internal. For the first time, the valve springs were now fully enclosed within the rocker covers. The separate oil tank of the Series A was eliminated in favor of an ‘oil-in-frame’ setup with the frame’s backbone being a pressed steel box that doubled as an oil tank. The rear suspension was improved also. Series A machines suspended the RFM on two ‘spring boxes’ with a separate damper. Series B and later bikes got two actual dampened shock absorbers made by Girling.
This 1952 Vincent Comet is a Series C, regarded as the best of all the Series. Only 1 Series D would be made as a prototype in 1955, just as the factory closed its doors for good.
Vincent Comet Motorcycle Books
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