1949 BSA B31 350cc-OHV motorcycle left side

The BSA B31 is a

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, cruising ...

that was produced by

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 tool, hand, po ...

. The BSA B31, introduced in 1945, was the first new model introduced by the company after the

Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

. Based on pre-war designs, it used a

single cylinder A single-cylinder engine, sometimes called a thumper, is a piston engine with one cylinder. This engine is often used for motorcycles, motor scooters, go-karts, all-terrain vehicles, radio-controlled vehicles, portable tools and garden machinery ...

four stroke engine A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either directi ...

that displaced . Initially, it had a rigid frame and

telescopic forks A telescope is an instrument designed for the observation of remote objects. Telescope(s) also may refer to: Music * The Telescopes, a British psychedelic band * ''Telescope'' (album), by Circle, 2007 * ''The Telescope'' (album), by Her Space H ...

, the first use of such on a BSA. It developed about , adequate for the roads of the day and enough to deliver a top speed of around .There was a competition kit available for the original rigid model B31 and B33. It was immediately popular and was soon joined by a version, the BSA B33 and competition equivalents, the BSA B32 and BSA B34; the side-car versions, with stiffer suspension and different final demultiplication, were known as BB31 and BB33. Plunger rear suspension was offered later, with a

swingarm 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 ...

rear suspension frame available from 1954. The model continued in production until 1959, by which time the traditional Lucas magdyno had been replaced by an

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.Go ...

and coil ignition. The B series expanded through its life to include the famous

BSA Gold Star The BSA Gold Star is a motorcycle made by BSA from 1938 to 1963. They were 350 cc and 500 cc single-cylinder four-stroke production motorcycles known for being among the fastest bikes of the 1950s. Being hand built and with many opt ...

s, and the bottom half of all engines has much in common with the M series side valve models. The M33, designed for

sidecar A sidecar is a one-wheeled device attached to the side of a motorcycle, scooter, or bicycle, making the whole a three-wheeled vehicle. A motorcycle with a sidecar is sometimes called a ''combination'', an ''outfit'', a ''rig'' or a ''hack''. ...

work, combined the strong M series frame and the better-performing B33 500 cc

overhead valve engine An overhead valve (OHV) engine, sometimes called a ''pushrod engine'', is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with earlier flathead engines, where the valves were located bel ...

See also