Sunbeam: S7

Note: This is a sub-section of Sunbeam: Motorcycles.

The Sunbeam S7 and S8 are British motorcycles designed by Erling Poppe based on the BMW R75 designs that were acquired by BSA (together with the full rights to the Sunbeam brand) at the end of World War II. Built in Redditch, the engine layout was an unusual in-line 500 cc twin which drove a shaft drive to the rear wheel. The in-line engine made this technologically feasible—horizontally-opposed ("flat") twin engines on BMW motorcycles had already used shaft drives.

Three models were produced, the S7, S7 "Deluxe" and the S8. The original S7 (the "Tourer") (2,104 produced from 1946 to 1948) was expensive and did not sell well.

In 1949 the S7 was updated to become the S7 deluxe (5,554 produced) and the S8 (8,530 produced). Both had new cylinder linings, redesigned frames and increased oil capacity. The S8 was sold as a "sports" model with increased performance from higher compression pistons with a top speed of 85 mph. It also had new forks, a cast aluminium silencer and chromed wheels (with narrower tyres to replace the "Balloon" tyres which had led to uncertain handling at speed).

S9 and S10 models were planned but never made as BSA decided to concentrate on the more traditional twins. Another "sports" model was also tested but never put into production. This had a much higher compression ratio with a different OHC design but was never sold, reputedly because of the undamped front fork system which affected handling. There were also trials with a rigid version for a cheaper model but this design was also abandoned.

Some early models of the original S7 were produced in black but most in the now familiar "Mist Green". The S7 Deluxe came in either "Mist Green" or black and for export abroad BSA supplied Sunbeams in any colour.

Erling Poppe’s design was originally based on a captured BMW R75 but Sunbeam did not want the S series to look too "German", so an in-line OHC, parallel twin was designed instead of a flat twin "across the frame". Serious problems with vibration made the new Sunbeam bikes uncomfortable to ride and all production originally sent to South Africa was recalled. The excessive vibration was cured by mounting the engine on two bonded rubber engine mounts. BSA had machinery they had inherited from Lanchester Motors to produce worm gears. This created problems with the shaft drive, as the gears tended to strip under power. BMW-style bevel gears would have been superior. Sunbeam's solution to this was to reduce the power to 24 bhp, which did nothing to help post war sales.

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Notes from a contributor

The numbers are incorrect on how many 1946 to 1948 Sunbeam S7 were made. Late 1946 to 1947 419 were made. 1948 there were 1,279 were made. First bike number is #101 to #520 then #521 to #1800.

Sources of Information

• [1] Wikipedia