BSA’s Model S was launched into a world of traditional ‘flat-tank’ motorcycles in 1927 – and caused a sensation. With a shapely saddle tank, fashionable sloping engine and low seat height, its looks began a trend which would last well into the 1930s. But the revolution was more than skin deep; its sporting overhead-valve engine featured Lucas’ new Magdyno offering reliable ignition and lighting, while the internal geared oil pump spelt the end of plungers, sight glasses and external pipework. The new Model S, quickly nicknamed the ‘Sloper’, set a new trend in design and manufacturers such as Ariel and Triumph rushed to follow. In truth, P&M, later Panther, had been making motorcycles with a forward-sloping cylinder since the turn of the century, but it was with BSA that the name ‘Sloper’ would be forever associated.
The Sloper was introduced as a top-of-the-range OHV sports machine but, for the 1929 season, BSA added a side-valve version to the range, the Model S Deluxe. While the sporting role of the OHV model was clear, the side-valve Sloper’s place in the range was less well-defined. BSA’s 1929 advertising brochure describes it as ‘Embodying the majority of the unique features of the 4.93 hp OHV model, this machine will appeal to those riders who prefer the simplicity of the side-valve engine.’ Taking no chances with a conservative public, BSA offered no fewer than four 5 hp machines with vertical and sloping, side- and over-head valve engines in the Model S range, plus two 5.57 hp versions of the side-valve machines designated Model H. The OHV Sloper was now offered with single- or twin-port cylinder heads and, for riders who preferred a smaller, lighter mount, there was the 3.49 hp Model L Two-Port Sloper – although at ₤54-15-0 without lights or horn, it cost only 15 shillings (75 pence) less than the 5 hp version!
As the 1920s became the ’30s, the Sloper’s bright nickel finish was replaced by the newly-fashionable chrome. For 1930, a new frame with an I-section forged steel backbone was introduced, while the forks gained better friction damping. A four speed gearbox was fitted for 1932 and the instruments moved to a tank-top panel, but by then Harold Briggs’ trend-setting design was showing its age. The side-valve Model S had been discontinued in 1931 and with memories of the Depression years fading, the public’s appetite for new models grew. By the mid-30s the trusty Slopers had been dropped from the BSA range.