After we wrote an article about the Universal Japanese Motorcycle, an all purpose motorcycle that will perform almost any task. The discussion started with some emails which leaded to the question, was there a Universal British Motorcycle during the golden motorcycle age in the '50s? The answer is very likely, yes there were several.
One, however, represents like few others, the reliable British overhead-valve, medium-weight, single cylinder motorcycle of the 1950s. The 348cc BSA B31, although a post World War 2 model has its roots at the beginning of the war. The 1940 BSA catalog featured a 348cc Silver Sports B29. Only a handful of BSA B29's were built, and most of them went immediately into army service. The army then opted for side-valve models for the rest of the war. Production of the BSA B31 proper began in 1945. The BSA B31 lasted until the end of the '50s.
The BSA B31 is often overshadowed by its more glamorous cousin the Gold Star, but then sportsbikes tend to steal the limelight. In 1946 the trials model B32 followed the B31. In 1949 BSA produced the third version, a clubman type racer known as the B32 Special. This grew into the BSA B32GS Gold Star. The BSA B31 pictured is a 1954 model, and was imported from India, where it had suffered a very hard existence. In Thailand it has been meticulously restored, and now stands as a fine example of a Universal British Motorcycle. We want to thank Army Col. Thuwatham for his cooperation. The day we took the pictures the BSA B31 had been running within the last hour or so, and it was a hot day, so one swing of the kick starter was enough to bring the motorcycle to life. On the road, the long lazy stroke, and unhurried exhaust note were a delight. The four-speed gear-box was as smooth as on a modern motorcycle, and the suspension felt firm, but very comfortable. The steering geometry is similar to that found on the BSA Gold Star clubman-racing models so this isn't surprising. I didn't ride the B31 very far, or very fast – the motorcycle was not legally registered, and I do not wear army green, but the short spin convinced me of the motorcycle's well earned reputation as an all-purpose motorcycle.
My only criticism, and this applies to all machines from this era, is of the brakes. Approaching a corner or intersection the rider simply has to get used to braking much sooner, because it will take much longer to wipe off speed, or stop. Of course another aspect of slowing or stopping is engine braking, and 4-stroke singles tend to have this in abundance. The BSA B31 has plenty of engine braking. Factory claims say that from 50km/h the motorcycle can be stopped in 10 meters. A road tester in a 1956 motorcycle magazine said; 'There was no significant lessening of braking power when the BSA was ridden for a period of two hours in torrential rain.' The magazine went on to say that the BSA B31 is; 'A most economical single, comfortable, fast for long-distance touring, and handy in heavy traffic; excellent steering and brakes.'
The 348cc engine (71x88mm) has a compression ratio of 6.5 to 1. The motorcycle weighs in at 180 kilograms. Top speed is 116km/h (not tested). The carburetor is an Amal Monobloc. The battery is 6-volt. Transmission is four-speed. At 50km/h the engine should be loping along at 2200rpm. One of the most interesting statistics associated with the motorcycle is fuel consumption. For he traditionalists I'll quote in miles per gallon. At 30mph, 110mpg, at 40mph, 105mpg, 50mph, 90mpg, and at 60mph 68mpg... It's one of those equations managed to translate into metric.
Quoted horsepower for the BSA B31 was a modest 18 horsepower, but this had the virtue, as one pundit put it, of leaving the working parts do their job unhurriedly and without fuss or frenzy. The consequence of this is that the machine seemed to have the ability to go on forever, with little attention needed other than the addition of petrol and oil.