BSA A7
BSA A7 Background & History
The BSA A7 was BSA’s first vertical twin, and was the first to follow Triumph’s 1938 Speed Twin, the bike that started it all, the vertical twin craze, that is. World War 2 got in the way though, however BSA tested several A7 prototypes during the war. The entire British motorcycle industry, which was substantial at the time, was clamoring to build their own vertical twins (the ones who could afford it, anyway), but BSA had the lead. BSA was by far the world’s largest producer of motorcycles, but their empire extended well past bikes. They made cars, busses, trucks, tractors, agricultural and industrial equipment, military vehicles, arms, munitions, steel and more. If anyone could trump Triumph it would likely be BSA. But nothing could happen until civilian production resumed in 1946. BSA immediately released the 500cc A7, and again the rest of the industry raced to follow. Norton, Royal Enfield, Matchless/AJS and even Ariel would all have their own vertical twins by 1949. The BSA A7 launched in 1946.
This 1952 BSA A7 has the plunger rear suspension. Swing arms would have to wait until 1954.
BSA A7 Background & History
The first vertical twin was a product of the times, and yet a game-changer. Throughout the 1920s and 1930s the British motorcycle industry, the largest in the world at the time, with a few notable exceptions, settled on the 4-stroke, air-cooled, single-cylinder as the engine architecture of choice. Nearly every manufacturer had one, or several, and they were usually their best sellers. Vincent may have built the sexy V-twin Black Shadow, but they made their money by selling lots and lots of single-cylinder Vincent Comets. Ariel is famous for their brawny Square Four, but their biggest seller was the single cylinder Red Hunter. Single cylinder bikes of this mold were widely understood by the engineers who built them, and widely accepted by the buying public. Displacement and horsepower output grew steadily through the 20s and 30s and by 1935 or so, most had generally capped out at about 500cc and 23 hp or so.
This 1954 BSA A7 was the second year for the swing arm rear suspension.
Vibration is the Enemy
The problem with big singles has always been engine vibration, enough in those days to destroy and engine in seconds. To pull make more power, the engines would have to rev faster, which led to ever more vibration. To add displacement also added weight (ie: larger pistons and rings and/or bigger crankshafts), which added to the vibration problem. Many solutions were tried, but nothing seemed to work. When Ariel bought Triumph in 1936, he put one of Ariel’s top engine designers on the task of sprucing up Triumph’s lackluster product line. Edward Turner was fresh off of designing the first Ariel Square Four and he quickly put together an engine design, the ‘vertical twin’, that seemed to do all things: 1.) Drastically improve on the engine vibration problem; 2.) Made more power and torque, and wound higher than the big singles; 3.) The new engine utilized primary drives, clutches, gearboxes, frames, etc. from their existing inventory of singles parts; and 4.) The twins used much of the same tooling and manufacturing methods as the singles, greatly simplifying production.
This 1957 BSA A7 had a flip-up rear fender to facilitate tire changes on the center stand.
This was accomplished by breaking that 500cc of displacement typical of the big singles of the day into two cylinders, each rising and falling together on a 360-degree crankshaft, but firing alternately. Where the 500cc singles only had a power stroke every other engine revolution, the vertical twins produced one power stroke every stroke, smoothing out the power delivery which greatly improved the engine vibration. Triumph Motorcycles introduced their revolutionary new 500cc vertical twin in an entirely new line of bikes: the 1938 Triumph 5T Speed Twin.
This gorgeous 1957 BSA A7 is on display at the Barber Motorsports Museum in Leeds AL.
The Speed Twin was a huge hit for Triumph, and almost the entire rest of the British motorcycle industry (those who could afford it, anyway) rushed their own vertical twins to market. However, World War 2 started in 1939 and civilian production halted until 1946. But, when it did, the business just took off, with Triumph in the lead, thanks to the Speed Twin, and it’s new hot-rod stablemate, the Triumph T100 Tiger, launched in 1939. By 1949, BSA, Norton, Matchless/AJS and Royal Enfield all had their own 500cc vertical twins. So, in 1950, Triumph bored and stroked out the 500cc to 649cc and the Triumph 650 was born in the 1950 Triumph 6T Thunderbird and the whole thing started up again. But this time with 650s, which had now eclipsed the 500 twins now flooding the market. So, once again, BSA was the first to follow Triumph with its own 650 twin, the A10 in 1950, just months behind Triumph’s release of the 6T Thunderbird, and the race was on as the rest of the bike companies punched out their 500s to make 600s, 650s, 700s, and 750s trying to keep up.
1957 BSA A7 engine, timing side. Note the iron cylinder block and head.
But that’s another story. The BSA A7 was non only BSA’s first vertical twin, it was also their entry level 500, which at the time of its launch was the largest bike in their arsenal. At that time, that would have made the A7 BSA’s premium bike. But as other versions rolled out with better performance, the BSA A7 got relegated to commuter status. In this way, the BSA A7 really followed a similar path to its mentor, the Triumph Speed Twin, which also went from premium bike to budget commuter. However, the BSA A7 remained in service from 1946 to 1963 when it was replaced by the unit-construction 500cc A50.
Just like the Triumph, the BSA A7 engine was made to drop into the frames of BSA’s existing line of singles. So, all the innovation was wrapped up in the engine. Work was begun on BSA’s vertical twin right after Triumph launched theirs in 1938. The engine came in two series, the first designed by Val Page, Herbert Parker and David Munro. Displacing 495cc, the “long stroke-engine”, as it was called, made 26 hp and was capable of 85 mph, with a bore and stroke of 62mm X 82mm. When the 650cc BSA was introduced in 1949 as a 1950 model, Bert Hopwood, on loan from Triumph, redesigned the Series 1 twin to accommodate the larger capacity. At the same time, he redesigned the BSA A7 engine, changing its bore and stroke to make it less undersquare (small bore, long stroke) to 66mm X 72.6mm for a total displacement of 497cc. Early A7s use an Amal Type 6 carburetor with a remote float. Later models used the Amal Monobloc carb. All BSA A7s used a Lucan magneto for spark and a Lucas dynamo for lights.
The first BSA A7s, from 1946 through 1954 came standard with a rigid frame (no rear suspension). However, in 1949 the plunger-type rear suspension was offered as an option. BSA introduced their swing-arm rear suspension frames in 1954 at which time both the rigid frames and those with plunger suspension were discontinued.
1957 BSA A7 engine, primary side.
BSA A7 Books
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illustratedA fascinating treatise on BSA's legendary pre-unit twins by British motorcycle expert Roy Bacon. |
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