Pre-unit construction

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AJS racing motorcycle showing typical pre-unit construction engine and gearbox layout with mounting plates, slotted holes and screw-thread adjusters visible AJS 7R motorcycle engine.jpg
AJS racing motorcycle showing typical pre-unit construction engine and gearbox layout with mounting plates, slotted holes and screw-thread adjusters visible

Pre-unit construction, [1] also called separate construction, is a motorcycle engine architecture where the engine and gearbox are separate components with their own oil reservoirs, linked by a driving chain within a primary chaincase. Mounting plates are usually attached to the frame allowing for chain adjustment by gearbox fore-and-aft movement and via screw adjusters and elongated mounting holes. Even though Singer [2] offered an integrated engine and gearbox in a single casing in 1911, it was not until the 1950s that technical advances meant it was possible to reliably construct engines with integral gearboxes in a single unit, known as unit construction .

Another variant is semi-unit construction, where the gearbox is bolted directly to the engine. [3] Curiously, early BSA A10 and A7 parallel twins (650cc and 500 cc, respectively) were semi-unit construction, but the factory later adopted pre-unit construction. [4]

The term "pre-unit" was coined only after such engines were succeeded by "unit construction" designs. The term pre-unit particularly applies to BSA/Triumph's single- and twin-cylinder bikes after this firm adopted unit construction in the early 1960s. (By contrast Norton and Royal Enfield never advanced beyond pre-unit construction). Even after adopting unit construction BSA/Triumph still kept separate oil systems for the engine and gearbox. It was the Japanese who introduced the practice of using the same oil in motorcycle engines and gearboxes. With the exception of the Issigonis Mini family of cars, the practice in the automobile industry has also been to keep engine and gearbox oil separate. Gearbox oil needs to cope with large shearing forces, while engine oil need to be capable of acting as a coolant, so the adoption of a single lubricant for engine and gearbox is always something of a compromise.

A pre-unit motorcycle would not normally be able to use the engine as part of the frame as a stressed member, since the engine/primary-chaincase/gearbox assembly would not be sufficiently rigid. The 1969 'Isolastic'-framed Norton Commando had its pre-unit engine, gearbox, and swingarm assembly bolted together on plates fixed to the frame via shimmed rubber bushes.

Among the Japanese manufacturers, Kawasaki produced a separate construction 650 cc vertical twin, inherited as a consequence of taking over Meguro in 1964. [5] The Meguro K models were copies of the BSA A7 and BSA A10 design, but although external appearances were similar, no parts are interchangeable with the BSA forebears. [6]

History

In the early days of motorcycles, particularly in Europe, several manufacturers (such as Rudge) were diversifying from making bicycles to motorcycles. These firms would build, say, the frame and fuel tank, but most of the other elements, such as engine, gearbox, brakes, electrics, controls and instruments were bought in from outside manufacturers such as JAP, Villiers and Lucas. This situation made pre-unit construction almost inevitable. [7]

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Unit construction

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Kawasaki W series Line of motorcycles made by Kawasaki

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BSA Golden Flash

The BSA Golden Flash, commonly referred to as the Gold Flash, was a 646 cc (39.4 cu in) air-cooled parallel twin motorcycle designed by Bert Hopwood and produced by Birmingham Small Arms Company (BSA) at Small Heath, Birmingham. The Golden Flash was the first model in the BSA A10 series. It was available in black and chrome; but it was the distinctive golden paint scheme that gave The Golden Flash its name. Production continued until 1963, when it was superseded by the BSA A65 Star.

BSA A50 Royal Star

The BSA Royal Star was a Birmingham Small Arms Company (BSA) motorcycle whose new engine design paved the way for a range of successful unit construction twins. As well as giving a clean look to the engine, with the pushrod passages part of the cylinder block casting, unit construction reduced the number of places oil could leak from.

BSA A7

The BSA A7 was a 500cc motorcycle model range made by Birmingham Small Arms Company (BSA) at their factory in Armoury Road, Small Heath, Birmingham. The range was launched in 1946 using a 495 cc (30.2 cu in) long stroke engine. An improved 497 cc (30.3 cu in) version based on the BSA A10 engine was launched in 1950. The various A7 models continued in production with minor modifications until 1961/2 when they were superseded by the unit-construction A50 model.

BSA C15 British, 250 cc single cylinder motorcycle, produced by BSA in the 1960s

The BSA C15 was a 250 cc single-cylinder ohv motorcycle manufactured by the British company BSA from September 1958 until 1967, and was BSA's first four-stroke unit-construction bike. For most of that period, after the introduction of 'Learner Laws' in 1961, a 250 cc was the largest capacity solo machine that a learner could ride unaccompanied when displaying L-plates in the United Kingdom. A road-going Sports derivative was added in 1961, and off-road versions, for Trials and Scrambles, were also available in the range.

BSA A65 Star Motorcycle

The BSA A65 Star was a Birmingham Small Arms Company (BSA) motorcycle aimed at the US market for unit construction twins. As well as giving a clean look to the engine, with the pushrod passages part of the cylinder block casting, unit construction reduced the number of places oil could leak from. A range of A65 Star twins was produced between 1962 and 1972.

Meguro motorcycles

Meguro motorcycles were built by Meguro Manufacturing Co motorcycle works (目黒製作所), founded by Hobuji Murato and a high-ranking naval officer, Takaji Suzuki, in 1937. One of the first Japanese motorcycle companies, it became a partner of Kawasaki Heavy Industries Ltd, and was eventually absorbed. Named after a district of Tokyo, Meguro had its roots in Murato Iron Works, which was established in 1924. Meguro Seisakusho, which had once developed a copy of a Harley-Davidson V-twin, was established to design and build gearboxes for the nascent Japanese motorcycle industry. Abe Industries, which had once produced its own motorcycle, merged with Meguro in 1931. The brand is being revived by Kawasaki with a new K3 model to be introduced in Japan on February 1, 2021.

BSA motorcycles Former British motorcycle marque

BSA motorcycles were made by the Birmingham Small Arms Company Limited (BSA), which was a major British industrial combine, a group of businesses manufacturing military and sporting firearms; bicycles; motorcycles; cars; buses and bodies; steel; iron castings; hand, power, and machine tools; coal cleaning and handling plants; sintered metals; and hard chrome process.

BSA unit twins

The BSA unit twins were a range of unit construction twin-cylinder motorcycles made by the Birmingham Small Arms Company (BSA) and aimed at the US market. A range of 500 cc (31 cu in), 650 cc (40 cu in) and 750 cc (46 cu in) twins were produced between 1962 and 1972, but they were really developments of the older pre-unit A7/A10 model range with less weight. The engines had a reputation for vibration, but acceleration was good for the time, to a top speed of 100 miles per hour (160 km/h).

BSA A10 series Motorcycle

The BSA A10 series was a range of 646 cc (39.4 cu in) air-cooled parallel twin motorcycles designed by Bert Hopwood and produced by Birmingham Small Arms Company at Small Heath, Birmingham from 1950 to 1963. The series was succeeded by the A65 unit construction models.

Norton Model 88 Dominator

The Norton Model 88 Dominator, also originally known as the Dominator De Luxe was a 500 cc vertical twin motorcycle manufactured by the British Norton Motorcycle Company from 1952 to 1966. It was the first of Norton's motorcycles to use the featherbed frame, which established Norton's reputation of producing fine handling machines. The 88 used the Bert Hopwood designed engine that was first fitted to the Model 7 and was initially for export only. It became available on the home market in 1953. Norton were a small manufacturer at the time and without the economies of scale the model was expensive compared to other manufacturer's equivalent machines. The 88 retailed for 20% more than the contemporary Triumph Speed Twin and was dearer than the 650 cc Triumph Thunderbird.

Norton Model 99 Dominator

The Norton Model 99 Dominator was a 600 cc vertical twin motorcycle manufactured by the British Norton Motorcycle Company at their Bracebridge St, Birmingham factory from 1956 to 1962. The 99 was based on the 500 cc Model 88 Dominator with an enlarged engine. The model was superseded by the 650SS.

The Norton Manxman was a 650 cc vertical twin motorcycle manufactured by the British Norton Motorcycle Company at their Bracebridge St, Birmingham factory for export. It was first shown at the November 1960 Earls Court Motorcycle Show and listed by the American importer, Berliner, in their catalogue from 1961 to 1963. Berliner had asked for the model to be named Manxman although the twin had never been raced at the Isle of Man.

Norton Mercury

The Norton Manxman was a 646 cc (39.4 cu in) air-cooled OHV parallel twin motorcycle made by Norton-Villiers from 1968 to 1970. It was the last Norton model to use the featherbed frame. Following the collapse of AMC and the subsequent formation of Norton Villiers, the company's focus had been on the new Norton Commando. There was a large inventory of parts from previous models that would not be used on the Commando, and to use up this stock the Mercury was conceived. The Mercury was introduced in October 1968 and around 750 machines were produced, most of the production going to the US. The model was also used by the Nigerian Police.

References

  1. ↑ Note: the term "pre-unit" was coined only after unit construction was adopted.
  2. ↑ IanChadwick Brit Bikes S (Retrieved 25 November 2006)
  3. ↑ Youngblood, Ed (June 2001). Wood, Bill (ed.). "The Rise and Fall - A Century Later, Looking Back at the Indian Brand". American Motorcyclist. Pickerington OH USA: American Motorcyclist Association. 55 (6): 30. ISSN   0277-9358 . Retrieved 2015-05-09. The middleweight design featured twin cams and a semi-unit construction, with the gearbox bolted directly to the crankcase.
  4. ↑ "Classic Bikes" - April 2019 - page 138.
  5. ↑ Erwin Tragatsch, ed. (1979). The Illustrated Encyclopedia of Motorcycles (Edition: 1988 ed.). New Burlington Books. p.  207. ISBN   0-906286-07-7.
  6. ↑ "W650 History". Ianchadwick.com. Archived from the original on 2006-12-10. Retrieved 2007-01-14.
  7. ↑ Hough, Richard Alexander; Setright, L.J.K. (1966). "A history of the world's motorcycles". New York: Harper & Row. LCCN 66018583.
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