Sunbeam S7 and S8

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Sunbeam S8
Sunbeam S8 1951.jpg
Manufacturer Sunbeam Cycles Limited (Subsidiary of BSA)
Production 1946–1956
Engine 487 cc parallel twin OHC four stroke
Power 24 bhp (18 kW) @ 6,000 rpm
Wheelbase 57-inch (1,400 mm)

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.[1] Built in Redditch, the engine layout was an unusual in-line 500 cc twin which 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.

Models[edit]

1950 Sunbeam S7
A mildly customized Sunbeam S7 motorcycle sits along the edge of a driveway.  It shows evidence of normal wear and tear, but appears to be in very good condition.  It's all black, with a vaguely egg-plant shaped fuel tank that features rubber pads for the rider's knees, and white-wall tires.
A mildly customised Sunbeam S7 motorcycle.

Three models were produced, the S7, S7 "de luxe" 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 de luxe (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 (137 km/h). It also had new forks, a cast aluminum 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.[2] 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 undampened front fork system which affected handling. There were also trials with a rigid version for a cheaper model but this design was also abandoned.[3]

Mist Green S7 with characteristic balloon tyres

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

Design problems[edit]

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.[2]

The inline engine was inherently suitable for shaft drive, and BMW-style bevel gears would have been ideal. However, having inherited some worm-gear machinery from Lanchester Motors, BSA opted to specify worm drive rather than bevel gears. Worm gears were entirely unsuitable, since the direction of transmission (input shaft vs output shaft) is not reversible when using large reduction ratios. Inevitably, this created problems with the shaft drive, as the gears tended to strip under power.[4] Sunbeam's "solution" to this was to reduce the power to 24 bhp (18 kW), which did nothing to help post war sales.[1]

Owners clubs[edit]

The Sunbeam Motor Cycle Club is one of the longest established in the UK and was founded in July 1924 at the London showrooms of John Marston Ltd in Holborn Viaduct who made the Sunbeam motorcycles in their factory at Wolverhampton.[5] The Sunbeam Motor Cycle Club is, however, a sporting motorcycle club that organises trials and vintage bike rallies and runs and the "owners section" was just a small part of it. When Sunbeam production ended BSA sold the remaining stock of parts to Stewart Engineering. Bob and Chines Stewart were longtime fans of the Sunbeam and for a time members of the Sunbeam Motorcycle Club. In 1963 they, along with other enthusiasts, broke away from the Sunbeam Motorcycle Club and formed the Sunbeam Owners Fellowship (SOF) to support owners of an S7 or S8 with any problems. A good number of Sunbeams motorcycles survive in perfect working order and many owners have been fellows of the SOF since its inception, having owned their Sunbeam since bought new or second hand in the 1960s.

A web-based owners club has been created.[6]

References[edit]

  1. ^ a b Kemp, Andrew (2001). Classic British Bikes. Bookmart Ltd. pp. 124–125. ISBN 1-86147-058-4. 
  2. ^ a b "Shaft Drive Sunbeam Motorcycle History". Retrieved 2008-06-03. 
  3. ^ "A Brief History Of Sunbeams". Stewart Engineering. Retrieved 2008-06-02. 
  4. ^ Stewart Engineering's Guide to the Subeam S&7 & S8
  5. ^ "Sunbeam Motorcycle Club". Retrieved 2011-12-28. 
  6. ^ "Sunbeam Owners Fellowship". Retrieved 2011-03-19. 

External links[edit]