Unit construction

This sometimes includes the design of automobile engines and was often loosely applied to motorcycles with rather different internal layouts such as the flat twin BMW models.

Prior to unit construction, the engine and gearbox had separate casings and were connected by a primary chain drive running in an oil bath chaincase.

A driving factor behind the BSA/Triumph change was that Lucas[1] had declared an intention to abandon production of motorcycle dynamos and magnetos, and instead produce only alternators.

By contrast, Velocette, Matchless/AJS, and Norton motorcycles continued to be pre-unit construction (the former machines with pressed-steel primary cases) until the end of production in the 1960s and 1970s respectively.

[3] In 1914, ABC founder Granville Bradshaw designed a unit-construction horizontally opposed ('flat') twin for Sopwith Aircraft, who, at the time, also made motorcycles.

[8] In 1923, the advanced three-speed Triumph single-cylinder 346 cc sv unit-construction Model LS appeared, but did not sell well, and ended production in 1927.

[15] The 1947 Sunbeam S7, an advanced overhead-cam, longitudinal twin, unit construction motorcycle, designed by Erling Poppe, used shaft drive.

[17] They made the first twin-cylinder unit construction model in 1957 with the release of the 350 cc Twenty One 3TA (so named because it was approximately twenty-one cubic inches capacity).

The unit construction (in contrast to the separate engine and gearbox of the C10/C11 and c12) gave the family of motorcycles started by this model its familiar name.

The effects of such inexperienced maintenance led to a slightly undeserved reputation for unreliability - a well maintained and regularly serviced unit single will chug along for a very long time with no problems.

The warmer versions (such as the much-loved Starfire) were generally less robust, but their light weight, enjoyable handling and peppy engines meant that many people considered the hours of necessary maintenance a worthwhile trade-off.

The tunability and ready supply of these motors, combined with their compact and light(ish) construction has also made them a popular choice for modern "Classic" competition.

The continuation of the model until 1973 speaks well for the popularity and utility of this design, but also reflects badly on the forward-thinking and investment of the BSA management.