Sunbeam was a British manufacturing marque that produced bicycles and motorcycles from 1912 to 1956. Originally independent, it was owned by BSA from 1943. Latterly Sunbeam is perhaps most famous for its S7 model, a balloon-tyred shaft-drive motorcycle with an overhead valve in-line twin engine.
The Sunbeam S8 is a British motorcycle designed by Erling Poppe based on the BMW R75 designs that were acquired as war reparations by BSA (full rights to the Sunbeam brand had been acquired from AMC in 1943).Built in Redditch, the unusual engine layout was similar to that of a car. The engine was a longitudinally mounted inline vertical OHC 500 cc twin with coil ignition and wet sump lubrication which, though a dry clutch, drove a shaft drive to the rear wheel. The inline engine made this technologically feasible—horizontally-opposed ("flat") twin engines on BMW motorcycles had already used shaft drives. The early S7 was expensive and over engineered, which is why it is now the most sought-after and commands a premium over the S7 De Luxe and the S8, which were produced with fewer features to reduce costs, while retaining many of the innovative parts of the early Sunbeam and updating some ideas.
These two models (S7 and S8) went on virtually unchanged until actual production ceased in 1956. The opinion is that they died because they never continued to modernise the design. However, the standard finish was so high on the S7 and S8 Sunbeams - that perhaps they could do no more than halt production, if they were not to price themselves out or resort to shoddier manufacture.
Whatever the reasons - there really has never been another machine like them and they must surely be one of the `great and innovative' British motorcycles - still sought after - and still widely ridden on a regular basis over 40 years after their demise.