For the vehicle design where the vehicle's skin is used as a load-bearing element, see Monocoque.
Unit construction is the design of larger motorcycles where the engine and gearbox components share a single casing. 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 the adoption of unit construction, the motorcycle engines and gearboxes typically had separate casings and were connected by a primary chain drive running in an oil-bath chaincase. The unit construction design retained a similar chain drive but continued to use separate oil reservoirs for the engine, gearbox, and primary drive. Triumph and BSA were already using cast non-ferrous alloy chaincases and started converting to unit construction in the 1950s. A driving factor behind the BSA/Triumph change was that Lucas 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. In reality, the casings were not really "unitary," as the crankcase section was vertically divided in the middle and no oil was shared between the three portions. In the 1960s Japanese motorcycles introduced the now-familiar horizontally split clamshell which has become almost universal. Modern horizontally split four stroke engines invariably use single oil reservoir (whether wet- or dry-sump) but, while this simplifies matters, it is arguable that the previous system of having different types of oil for engine and gearbox is preferable. The BMC Mini was an early example of a car with the "gearbox-in-the-sump;" but this practice of using a single oil reservoir, which has become the norm for motorbikes, is generally undesirable for cars and trucks. Two stroke "total-loss" bikes always have separate oil for the gearbox, as engine oil is burned along with the fuel.
Advantages and disadvantages
The advantages of unit construction are:
the combined unit contributes to the stiffness of the entire motorcycle. unit construction may enable the engine unit to become a stressed member of the frame. the primary chaincase can be lighter and more compact. the primary chaincase can be shorter, so that wheelbase may also be shorter. engine and transmission are now much more accurately aligned to each other in the frame, improving primary drive life. the engine is cleaner and fashionably modern in appearance (compared to earlier designs). A significant disadvantage is that there is no longer any tension adjustment possible of the chain drive between engine and transmission, and tensioning (which is almost certainly still required) must be over a rubber-faced steel slipper. This is quiet and the tensioner does not wear greatly. This change to unit construction meant that it was no longer possible to choose a gearbox from a different manufacturer (e.g. a close-ratio unit for racing) and to send worn gearbox units to be rebuilt.
History
Early history Alfred Angas Scott, founder of The Scott Motorcycle Company, designed a motorcycle with unit construction for the engine and gearbox. Production of the motorcycle began in 1908. In 1911, Singer offered motorcycles with unit-construction 299 cc and 535 cc engines. In 1914, ABC founder Granville Bradshaw designed a unit-construction horizontally opposed ('flat') twin for Sopwith Aircraft, who, at the time, also made motorcycles. In 1919, Harley-Davidson introduced the Model W Sport Twin with a unit construction flat-twin. In 1920 Carlo Guzzi constructed the G.P. 500 prototype with unit-construction. This was followed in 1921by the first Moto Guzzi production model, the "Normale", also a 500cc unit-construction motorcycle. In 1921, an expanding Bianchi (Italy) showed its first unit-construction side-valve 600 cc V-twin. In 1923, Rover introduced a 250 cc unit-construction model, followed by a 350 cc in 1924, but production ended in 1925. 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. In 1923, BMW released its own unit construction shaft drive flat twin of 498 cc. BMW has never built a motorcycle with a separate gearbox. From 1924, FN single-cylinder engines changed from semi unit construction (as seen in the last semi-unit single, the 1922 FN 285TT, in its last year of sale in 1924,) to unit construction engines (as seen in the new-for-1924 M.60). In 1928, BSA made their first and only two-stroke, a 175 cc unit construction bike, for only one season, otherwise four-stroke twins became unit construction in 1962. The 1930 Triumph 175 cc Model 'X' two-stroke, two-speed is their first "all-unit construction" two-stroke single-cylinder engine. From 1932, New Imperial was known for pioneering innovations in unit construction on motorcycles . They made the Unit Minor 150 and Unit Super 250 in this manner and by 1938 all of their machines were unit construction. In 1938, Francis-Barnett offered a 125 cc unit-construction Snipe. In 1946, the Series B Vincent employed unit construction and used the engine-gearbox as a stressed member of the frame. The 1947 Sunbeam S7, an advanced overhead-cam, longitudinal twin, unit construction motorcycle, designed by Erling Poppe, used shaft drive. In 1957 the Royal Enfield Clipper was replaced by the unit-construction Crusader. In 1957 the first unit construction twin cylinder motorcycle made by Triumph, the 350 cc (21 ci) 'Twenty One' 3TA, designed by Edward Turner and Wickes, was introduced for the 21st Anniversary of Triumph Engineering Co. Ltd. Unfortunately it also had the first "bathtub" rear enclosure, which proved a sales failure. The 1958 Ariel Leader used unit construction.
Triumph
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