The BSA unit twins were a range of

unit construction : ''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 ...

motorcycle A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle steered by a handlebar. Motorcycle design varies greatly to suit a range of different purposes: long-distance travel, commuting, cruisin ...

s made by the

Birmingham Small Arms Company The Birmingham Small Arms Company Limited (BSA) 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 ...

(BSA) and aimed at the US market. A range of , and 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 . Models for the US generally had smaller petrol tanks and higher wider handlebars than the UK models, and prior to 1966 different model names were used for the two markets. The 500cc models were discontinued in 1971 when the single cylinder B50 model was introduced. The BSA Group faced worsening financial difficulties and in 1973 merged with

Norton-Villiers Norton-Villiers was a British motorcycle manufacturer formed in the 1960s following the collapse of AMC. With the general decline of the British motorcycle industry, under a British Government initiative it was later combined with the remnants ...

. As part of a rationalisation by the newly formed

Norton Villiers Triumph Norton Villiers Triumph (NVT) was a British motorcycle manufacturer, formed by the British government to continue the UK motorcycling industry, until the company's ultimate demise. Formation Triumph had been owned by the BSA Group since 1951 ...

, production of BSA motorcycles ceased.

Design

Background

Precipitated by Lucas's intent to discontinue

magneto A magneto is an electrical generator that uses permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a commutator to produce direct current. It is categorized as a form of alternator, ...

s and

dynamo "Dynamo Electric Machine" (end view, partly section, ) A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundat ...

s in favour of

alternator An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature.G ...

s, BSA's managing director and general manager, Bob Fearon, took the opportunity to update the engines of BSA's twins and give them a new look. He hoped the new models would succeed in the potentially lucrative, but competitive, US market. Working with chief development engineer Bert Perrigo, he developed the unit construction twins using "The Power Egg" design concept.

Engines

Pre-unit, known as "non-unit" before unit construction, engines had their roots in history when motorcycle manufacturers bought engines and gearboxes from separate specialists suppliers and mounted them in their own frames. By the late 1950s, most manufacturers made their own engines and gearboxes, and were therefore able to combine the engine and gearbox in an integrated casting, producing a more compact unit. Perrigo's design team based the new engine on the old A7/A10 engines. To streamline production and keep costs down, as many parts as possible were common between the engine sizes. The 500 and 650 engines shared the same stroke () and cylinder spacing, so the bottom half of the engine could be common to both sizes. (The A7 and A10 models had used different strokes and cylinder spacing, so crankshaft and crankcases were not interchangeable between them). Among the improvements in the new engines were pushrod passages in the cylinder block casting, which reduced the risk of oil leaks, and pillars holding the rocker spindles cast into the head, leading to better valve clearance control. Primary drive was by a triplex chain rather than the single row chain of the previous models and rubber inserts were added to the clutch for a smoother transmission. Although the older engines used a

roller bearing In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two concentric, grooved rings called races. The relative m ...

on the drive side of the crankshaft, the new engine used a caged ball bearing. On the timing side a

plain bearing A plain bearing, or more commonly sliding contact bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no roll ...

was retained but increased in size. This arrangement proved troublesome and the ball bearing was replaced with a roller bearing in 1966.

Cycle parts

A new frame was designed for the more compact unit construction engine. The frame followed the lines of the frame for the older models; a half-duplex cradle with a single spine. The new frame was all welded. The rear

swinging arm A swingarm, or "swinging arm" (UK), originally known as a swing fork or pivoted fork, is a single or double sided mechanical device which attaches the rear wheel of a motorcycle to its body, allowing it to pivot vertically. The main component of ...

was controlled by Girling 3 position adjustable

shock absorber A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Mos ...

s. BSA's own

forks In cutlery or kitchenware, a fork (from la, furca 'pitchfork') is a utensil, now usually made of metal, whose long handle terminates in a head that branches into several narrow and often slightly curved tines with which one can spear foods eit ...

were used, which had compression damping only. The front brake was 7" drum on the 500 model and 8" on the 650, although later 500s were fitted with the 8" brake. The new model had a

wheelbase In both road and rail vehicles, the wheelbase is the horizontal distance between the centers of the front and rear wheels. For road vehicles with more than two axles (e.g. some trucks), the wheelbase is the distance between the steering (front ...

of , 2" shorter than its predecessor.

Overview of models

Notes

Timeline

1962

500 and 650cc models launched with common cycle parts except the front brake, which was 7" on the 500 and 8" on the 650. The 500cc model was designated "Star" and the 650 "Star" in the UK and "Royal Star" in the US.

1963

The 500 model's name was changed to "Royal Star" in the US.

1964

Two new 500 models were introduced in the US. Both with twin

carburettor A carburetor (also spelled carburettor) is a device used by an internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the venturi tube in the main meteri ...

s and were designated the "Cyclone Road" and the "Cyclone Competition". The road version was fitted with a sports cam and the competition version was devoid of lights and had a higher

compression ratio The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values. A fundamental specification for such engines, it is measured two ways: the stat ...

and a race cam. Both had the 8" front brake from the 650. A new, higher performance 650 model, the "Rocket" in the UK and "Thunderbird Rocket" in the US was introduced. A twin carb version, the "Lightning Rocket" was introduced in the US. These two models replaced the pre-unit " Golden Flash" and " Super Rocket" models that were discontinued this year. A 650 version of the Cyclone Competition was also introduced in the US and called the "Spitfire Hornet". This was a replacement for the discontinued pre-unit "Spitfire Scrambler". Police equipped versions of both the 500 and 650 were also introduced.

1965

The Cyclone models were made available in the UK as was the Lightning Rocket, although called "Lightning" in the UK. The Lightning Rocket/Lightning has a higher compression ratio than the previous year's model. A limited edition "Lightning Clubman" was introduced. This featured siamese exhaust, clubman handlebars, rearset footrests and a close-ratio gearbox. It was designed for production racing. A 500cc version was also produced, the "Cyclone Clubman". Crankshaft main bearings had been unreliable, in an effort to resolve this problem roller bearing were introduced on the drive side and an improved bronze bush on the timing side.

1966

1966 saw the rationalisation of the range and also the end to calling the same model different names in different countries: * The 500cc Star (UK) and Royal Star (US) became the Royal Star in all areas, and was fitted with the 8" front brake of the rest of the range. * The Cyclone models were renamed "Wasp" * The 650 Star (UK) and Royal Star (US) models were discontinued * The Rocket (UK) and Thunderbolt Rocket (US) became the Thunderbolt * The Lightning (UK) and Lightning Rocket (US) became the Lightning * The police models were discontinued although BSA continued to supply motorcycles to police forces but to that force's individual requirements. * The US only Spitfire Hornet was renamed "Hornet" to avoid confusion with the newly introduced high performance "Spitfire" model. The high performance Spitfire model, designated Mk II, featured 10.5:1 compression ratio, a pair of Amal 1-5/32” GP2 carbs on open bell-mouths and a close-ratio gearbox. All models were upgraded to 12v electrics in this year.

1967

The Wasp models were discontinued, leaving the Royal Star as the only 500 model. The Spitfire evolved into the MK III model with the compression ratio reduced to 9:1 and the carbs changed to the new

Amal Concentric AMAL was a British engineering company servicing the motorcycle and other light-engineering motor industries between 1927 and 1993 These changes gave improved starting and tractability at low speeds. New forks with 2-way damping were introduced.

1968

The US only Hornet offroader was discontinued and a new model, the "Firebird Scrambler", was introduced in all sales areas. All models were fitted with twin leading shoe front brakes and carburettors changed to Amal Concentrics. The Spitfire evolved into the Mk IV.

1969

With the introduction of the flagship

Rocket 3 Rocket 3 may refer to: * Triumph Rocket III, a motorcycle by Triumph Motorcycles Ltd * Triumph Rocket 3, the successor of Triumph Rocket III * BSA Rocket 3/Triumph Trident The Triumph Trident and BSA Rocket 3 was a technically advanced, high- ...

750cc triple, BSA decided that there was no longer a need for the top of the range 650 twin and the Spitfire was discontinued.

1970

Apart from minor cosmetic changes, the range remained the same for 1970.

1971

With the introduction of the 500cc single cylinder B50 model, the Royal Star was discontinued. The 650s were given new cycle parts, including the "oil-bearing frame". Apart from cosmetic items, the cycle parts were shared with the

Triumph The Roman triumph ( Latin triumphus) was a celebration for a victorious military commander in ancient Rome. For later imitations, in life or in art, see Trionfo. Numerous later uses of the term, up to the present, are derived directly or indirec ...

twins. (Triumph was a subsidiary of BSA). To produce a 750cc machine for

AMA Ama or AMA may refer to: Ama Languages * Ama language (New Guinea) * Ama language (Sudan) People * Ama (Ama Kōhei), former ring name for sumo wrestler Harumafuji Kōhei * Mary Ama, a New Zealand artist * Shola Ama, a British singer * Ām ...

Class C racing, BSA lengthened the stroke of Lightning's crankshaft by 11mm to give a displacement of 751cc. 202 A70 Lightning machines were produced to meet the minimum production requirements. All were exported to the US, most to the East Coast distributor in

Baltimore Baltimore ( , locally: or ) is the List of municipalities in Maryland, most populous city in the U.S. state of Maryland, fourth most populous city in the Mid-Atlantic (United States), Mid-Atlantic, and List of United States cities by popula ...

1972

By 1972 BSA were in serious financial trouble and the Firebird Scrambler was discontinued in an attempt to reduce costs.

1973

With rival British motorcycle manufacturer,

, also in financial trouble, the

British government ga, Rialtas a Shoilse gd, Riaghaltas a Mhòrachd , image = HM Government logo.svg , image_size = 220px , image2 = Royal Coat of Arms of the United Kingdom (HM Government).svg , image_size2 = 180px , caption = Royal Arms , date_est ...

funded a rescue package that merged the two companies into a new company

(NVT). As part of the rationalisation of the products of the new company, NVT stopped production of BSA models.

Other models

E35 Fury

The 'BSA Fury was a prototype motorcycle made in 1970. Designed by Edward Turner but substantially redesigned by

Bert Hopwood Herbert Hopwood (1908 – 17 October 1996) was a British motorcycle designer. He was, at least, partly responsible for some of the most influential designs for the British motorcycle industry and worked for Ariel, Norton, BSA and Triumph. ...

and

Doug Hele Douglas Lionel Hele (13 July 19193 November 2001) was a pioneering British motorcycle engineer with Triumph and other firms: BSA, Douglas and Norton. He was born in Birmingham in 1919 and died in Hagley, Worcestershire on 2 November 2001. ...

,Bandit In The Hills (The Classic Motorcycle 11/84) the Fury never went into commercial production due to the collapse of the BSA Group. Launched in 1971, the double overhead cam twin BSA E35 was branded the ''BSA Fury''. It was essentially the same motorcycle as the

Triumph Bandit The Triumph Bandit was a British motorcycle manufactured as a prototype by Triumph in 1970. Originally designed by Edward Turner (who was already retired from Triumph) as his last project it was subsequently substantially modified at Triumph ...

and represented the BSA factory's last attempt to compete against Japanese imports. The frame used for the Fury was designed by Rob North, designer of the frames used on the racing BSA triples. A 'Street Scrambler' E35SS and road version the E35R were developed during 1971. Featuring upswept twin silencers, the SS had an optional electric starter and indicators and 26 mm Amal concentric carburettors. The Fury delivered and could reach .

T65 Thunderbolt

In order to fulfil existing export orders after production of the BSA twins had stopped, between 200 and 300

Triumph TR6 Trophy The TR6 Trophy is a motorcycle that was made by Triumph Engineering, Triumph, in Meriden, West Midlands, Meriden, from 1956 to 1973, when it was replaced by the five-speed 750-cc Triumph Tiger TR7V. During this time, it was a successful model, ...

s were rebadged as BSA T65 Thunderbolt and exported to Spain and Australia.

References

{{BSA motorcycles, state=collapsed Unit twins Motorcycles introduced in 1962 Motorcycles powered by straight-twin engines