Unit construction
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- For the vehicle design where the vehicle's skin is used as a load-bearing element, see Monocoque.
Unit construction is a term used to describe motorcycle or motor car engine design where both the engine and gearbox are an integrated unit within the same casing. Engine and transmission may or may not shared a common oil supply. Prior to unit construction, the engine and gearbox had their own separate casings and oil reservoirs and were connected with additional components, usually a chain drive - see Pre-unit construction.
The advantages of unit construction are:
- fewer external oil lines to leak/fracture
- no external drive connecting engine and transmission, thus simplifying maintenance
- mounting in the frame is simpler - no need to align engine and transmission within frame
The chief disadvantage is that the engine and transmission have different lubrication requirements. In a design where the engine and transmission use a common oil supply, the oil used must take the requirements of both into account.
The chief advantage of separate construction is the ease of removing gearbox or engine separately for maintenance.
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[edit] History
[edit] New Imperial
In 1932 the New Imperial Motors Ltd motorcycle company was the pioneer of unit construction on motorcycle engines about 20 years before other manufacturers.[1]
[edit] Triumph
Triumph Motorcycles produced its first single cylinder unit construction model with the 149cc Terrier launched in 1952. It was quickly followed by the more popular 196cc Tiger Cub in 1953.[2] They made the first twin-cylinder unit construction model in 1957 with the release of the 350cc 'Twenty One' 3TA introduced for 21st Anniversary of Triumph Engineering Co. Ltd. This model was a failure in the United States because of its bathtub rear enclosure that many dealers removed to sell the bikes.[3] Several models, like the Triumph Bonneville and other large capacity twins, appeared first in a pre-unit construction design and by 1963, when unit construction was a common motorcycle engine design, these models appeared as unit construction models.[4]
[edit] BSA
[edit] Unit singles
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 C10 and C11 singles. The unit construction (in contrast to the separate engine and gearbox of the C100/C11) 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 350cc 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 seventies.
The BSA unit single was an affordable introduction to motorcycling for many young men in the Sixties and Seventies. 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 Sixties 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.
[edit] Reference
- ^ [1] IanChadwick New Imperial (Retrieved 13 July 2006)
- ^ [2] Triumph Tiger Cubs & Terriers History (retrieved 1 August 2006)
- ^ [3] Triumph Motorcycles timeline: 1946-1962
- ^ [4] Triumph Motorcycles timeline: 1963-1972: The Glory Years
