- 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 boxer twin BMW models.
Prior to unit construction, the engine and gearbox had their own separate casings and were connected by a primary chain drive running in an oil bath chaincase. The new system used a similar chain drive and both had 3 separate oil reservoirs for engine, gearbox and primary drive.
Triumph and BSA were already using cast alloy chaincases and started converting to unit construction in the 1950s. Velocette, Matchless/AJS and Norton motorcycles continued to be pre-unit (the former machines with pressed-steel primary cases) until the end of production in the 1960s and 1970s respectively.
Advantages and disadvantages
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The advantages of unit construction are:
- the combined unit contributes to the stiffness of the entire machine, which either handles better or can have a lighter frame.
- it is shorter - amongst other things, this means the primary drive is shorter and less troublesome.
- engine and transmission are now much more accurately aligned to each other in the frame, improving primary drive life.
- better lubricated duplex (and even triplex) primary chains deliver smoother power.
- the engine is cleaner in appearance and fashionably modern, by post-war standards.
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. However, this is quieter and the tensioner does not wear greatly. The change to unit construction marked the end of choosing a gearbox from another manufacturer (e.g. a close-ratio unit for racing) and sending worn gear-box units to specialist re-builders.
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. Only in the 1960s did the familiar horizontally-split clam-shell cases arrive in Japanese motorcycles and become common, and this is not universal even today. The horizontally-split cases were the first to use the now expected single oil reservoir.
In 1921, an expanding Bianchi (Italy) showed its first unit-construction side-valve 600 cc V-twin.
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).
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 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.
Triumph Motorcycles produced its first single-cylinder unit construction model with the 149 cc Terrier launched in 1952. It was quickly followed by the more popular 196 cc Tiger Cub in 1953. 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 500 cc Triumph 5TA followed, and the 650 cc models were made unit construction in 1963. The 1963—1969 unit construction 650 cc Triumph Bonneville is now one of the most sought after models by enthusiasts.
The BSA Bantam range of two-stroke engines introduced the unit construction concept to BSA since its introduction in 1949. BSA produced their first four-stroke unit construction singles in 1959 when they introduced the C15 to replace the venerable c12 single. 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 C15 was intended as a utility "get to work" model, and served this purpose faithfully for many thousands of users. It was a simple and reasonably robust design.
Along with the C15 came the B40, the 350 cc version. This was no faster than the C15, but had a little more lugging power. A version of the B40 was also produced (in considerable quantities) for various branches of the military. These motorcycles (known as the "Ex-WD B40") were more rugged than the vanilla version (in particular, the timing-side main bearing was over- rather than under-engineered and an oil filter was fitted), slightly de-tuned and given a version of the competition frame. For these reasons, these bikes can make very good buying, and are often used as the basis for competition machines.
Several minor changes were made to the C15 in 7 years (with some variations on the theme - the "warmer" SS80 and SS90, plus competition versions).
In 1967 the model underwent some revisions and a name change to B25. The model then continued with little variation until BSA collapsed in the early 1970s.
The BSA unit single was an affordable introduction to motorcycling for many young men in the 1960s and 1970s. The simple design meant that inexperienced and under-equipped home mechanics could keep them running under most circumstances. 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.
Many BSA unit singles were built, meaning there are few 1960s motorcycles with such a large supply of readily available spares. 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 BSA design was based on the Triumph Tiger Cub, first produced in 1952. 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. By 1967 unit singles were looking slow and rattly and the "charm" of the traditional British oil-leak was wearing thin. The new breed of Japanese motorcycles arriving on the scene were fast and exotic in comparison, and the buying public can certainly not be blamed for their eventual shunning of the entire British motorcycle industry.
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- Title: The Illustrated Encyclopedia of Motorcycles, Editor: Erwin Tragatsch, Publisher: New Burlington Books, Copyright: 1979 Quarto Publishing, Edition: 1988 Revised, Page 89, ISBN 0-906286-07-7
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-  Triumph Motorcycles timeline: 1946-1962
-  Triumph Motorcycles timeline: 1963-1972: The Glory Years