Unit construction
Encyclopedia
Motorcycle
A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...
s where the engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
and gearbox components share a single casing. The term is sometimes applied to the design of automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
engines and was often loosely applied to motorcycles with rather different internal layouts such as the boxer twin BMW
BMW Motorrad
BMW Motorrad is the motorcycle brand of the German company BMW, part of its Corporate and Brand Development division. The current General Director of the unit is Hendrik von Kuenheim....
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
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.
Detailed construction
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 1970s did the familiar horizontally split clam-shell cases arrive in Japanese motorcycle and become common, this is not universal even today. The horizontally split cases were the first to use the now expected single oil reservoir.Early History
Alfred Angas Scott, founder of The Scott Motorcycle CompanyThe Scott Motorcycle Company
The Scott Motorcycle Company was owned by Scott Motors Limited, Shipley, West Yorkshire, England and was a well known producer of motorcycles and light engines for industry...
, designed a motorcycle with unit construction for the engine and gearbox. Production of the motorcycle began in 1908.
In 1911, Singer
Singer (car)
Singer was an automobile company founded in 1905 in Coventry, England. It was acquired by the Rootes Group of the United Kingdom in 1956, who continued the brand until 1970...
offered motorcycles with unit-construction 299 cc and 535 cc engines..
In 1914, ABC founder Granville Bradshaw
Granville Bradshaw
Granville Eastwood Bradshaw OBE, AFRAeS was an English engineer and inventor who designed motorcycle and aero-engines.-History:Bradshaw was born in Preston, Lancashire in 1887 as the son of William and Annie Bradshaw. His father was a jeweler and optician.Bradshaw's early work was involved with...
designed a unit-construction horizontally opposed ('flat') twin for Sopwith Aircraft, who, at the time, also made motorcycles.
In 1921, an expanding Bianchi (Italy) showed its first unit-construction side-valve 600 cc V-twin.
In 1923, Rover
Rover (car)
The Rover Company is a former British car manufacturing company founded as Starley & Sutton Co. of Coventry in 1878. After developing the template for the modern bicycle with its Rover Safety Bicycle of 1885, the company moved into the automotive industry...
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
BMW
Bayerische Motoren Werke AG is a German automobile, motorcycle and engine manufacturing company founded in 1916. It also owns and produces the Mini marque, and is the parent company of Rolls-Royce Motor Cars. BMW produces motorcycles under BMW Motorrad and Husqvarna brands...
released its own unit construction shaft drive boxer twin of 498 cc. BMW has never built a motorcycle with a separate gearbox.
From 1924, FN
FN (motorcycle)
FN was a Belgian company established in 1899 to make arms and ammunition, and from 1901 to 1967 was also a motorcycle manufacturer...
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
Birmingham Small Arms Company
This article is not about Gamo subsidiary BSA Guns Limited of Armoury Road, Small Heath, Birmingham B11 2PP or BSA Company or its successors....
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
New Imperial Motors Ltd
New Imperial was a British motorcycle manufacturer founded by Norman Downes in Birmingham, between 1887 and 1901, and became New Imperial Motors Ltd in 1912, when serious production commenced. New Imperial made innovative motorcycles that employed unit construction and sprung heel frames long...
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
Vincent Motorcycles
Vincent Motorcycles was a British manufacturer of motorcycles from 1928 to 1955. Their 1948 Black Shadow was at the time the world's fastest production motorcycle...
employed unit construction and used the engine-gearbox as a stressed member of the frame.
The 1947 Sunbeam
Sunbeam (motorcycle)
Sunbeam was a British manufacturing marque that produced bicycles and motorcycles from 1912 to 1956. Originally independent, it was ultimately owned by BSA...
S7, an advanced overhead-cam, longitudinal twin, unit construction motorcycle, designed by Erling Poppe, used shaft drive.
In 1957 the Royal Enfield
Royal Enfield
Royal Enfield was the name under which the Enfield Cycle Company made motorcycles, bicycles, lawnmowers and stationary engines. This legacy of weapons manufacture is reflected in the logo, a cannon, and their motto "Made like a gun, goes like a bullet". Use of the brand name Royal Enfield was...
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
Edward Turner
Edward Turner was a British motorcycle designer. He was born in Camberwell in the London Borough of Southwark, on the day King Edward VII was proclaimed King....
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
Ariel (vehicle)
Ariel was a bicycle, motorcycle and automobile marque manufacturer based in Bournbrook, Birmingham, England. Car production moved to Coventry in 1911. The company name was reused in 1999 for the formation of Ariel Ltd, a sports car producer.-History:...
Leader used unit construction.
Triumph
Triumph Tiger Cub
The Triumph Tiger Cub is British motorcycle made by Triumph Motorcycles at their Meriden factory. Designed by Edward Turner and launched at the Earls Court show in November 1953 the Tiger Cub competed well against the other small capacity motorcycles of the time such as Villiers...
in 1953. They made the first twin-cylinder unit construction model in 1957 with the release of the 350 cc Twenty One 3TA
Triumph Twenty One
The Triumph Twenty One is a British motorcycle made by Triumph Engineering Co Ltd in Coventry. The Model Twenty One was the first of the Triumph unit construction twin cylinder motorcycles...
(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
Triumph Bonneville
The Bonneville is a range of British motorcycles, made in three different production runs from 1959 to 1983, and 1985 to 1988, by the now-defunct Triumph Engineering in Meriden; and since 2001, by Triumph Motorcycles in Hinckley. It is named after the Bonneville Salt Flats, Utah, where Triumph and...
is now one of the most sought after models by enthusiasts.
Unit singles
BSA Bantam
The BSA Bantam is a two-stroke unit construction motorcycle that was produced by the Birmingham Small Arms Company from 1948 until 1971...
range of two-stroke engines introduced the unit construction concept to BSA since its introduction in 1949. BSA
Birmingham Small Arms Company
This article is not about Gamo subsidiary BSA Guns Limited of Armoury Road, Small Heath, Birmingham B11 2PP or BSA Company or its successors....
produced their first four-stroke unit construction singles in 1959 when they introduced the C15
BSA C15
The BSA C15 was the first four-stroke unit construction motorcycle produced by the British company BSA and manufactured between 1958 and 1967. At the time, the C15 was the largest capacity bike that a learner could ride on L-plates in the United Kingdom...
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 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
Triumph Tiger Cub
The Triumph Tiger Cub is British motorcycle made by Triumph Motorcycles at their Meriden factory. Designed by Edward Turner and launched at the Earls Court show in November 1953 the Tiger Cub competed well against the other small capacity motorcycles of the time such as Villiers...
, 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.
See also
- ABC Motorcycle
- BMW motorcyclesBMW motorcyclesBMW's motorcycle history began in 1921 when the company commenced manufacturing engines for other companies. Motorcycle manufacturing now operates under the BMW Motorrad brand...
- List of BSA motorcycles
- List of Triumph motorcycles