The BSA S-Series of motorcycles, most commonly known as the BSA Sloper, were a series of motorcycles produced by the Birmingham Small Arms Company (BSA) from 1927-1935. Launched in 1927, the 493cc overhead valve engine was slanted, and the motorcycle featured a saddle tank that enabled a low seating position, improving the centre of gravity and handling. Designated as the new S-series, whether this stood for sloper, speed or silence is unknown, but sloper became the term used by motorcyclists and hence adopted by BSA for marketing. The Sloper range remained much the same until its demise in 1935. Each model was designated with the two letters of the year produced, hence the S31 was produced in 1931.