Sunbeam S8: the sportier 500 cc twin from Redditch

The Sunbeam S8 is a British motorcycle designed by Erling Poppe, with styling loosely based on the BMW R71, whose designs came to BSA as war reparations. Built in Redditch alongside the S7 and S7 de luxe, the S8 was the lighter, sportier member of the family, with an 85 mph top speed and 8,530 examples produced.

Sunbeam S8 at a glance

The English Wikipedia article covers both the S7 and the S8; this page concentrates on the S8.

History of the Sunbeam S8

From BMW blueprint to Redditch

The S7 and S8 were designed by Erling Poppe with styling loosely based on the BMW R71, designs that BSA acquired as war reparations; full rights to the Sunbeam brand had been acquired from AMC in 1943. The unusual engine layout was similar to that of a car: a longitudinally mounted inline vertical overhead-camshaft 500 cc twin based on an experimental 1932 BSA design, the Line-Ahead-Twin, with coil ignition and wet-sump lubrication. Through a dry clutch it drove a shaft to the rear wheel. The inline engine made the shaft drive technologically feasible, as horizontally opposed twin engines on BMW motorcycles had already used shaft drive following the system employed by the four-cylinder Nimbus since 1918.

S7, S7 de luxe and S8

Three models were produced: the S7, the S7 de luxe and the S8. All were expensive and gave only modest performance, so sales were low. The original S7 Tourer was built from 1946 to 1948, with 2,104 made. In 1949 the S7 was updated to become the S7 de luxe, of which 5,554 were built, and the S8 appeared alongside it with 8,530 built. Both received new cylinder linings, redesigned frames and increased oil capacity. The lighter S8 was sold as a sportier model with a top speed of 85 mph. It also had new BSA forks, a cast aluminium silencer and chromed wheels with narrower tyres, replacing the balloon tyres that had led to uncertain handling at speed. S9 and S10 models were planned but never made, as BSA decided to concentrate on its more traditional twins.

Vibration, worm drive and colours

Poppe’s design was originally based on a captured BMW R75, but so soon after the war BSA did not want the S series to look too German, so an inline overhead-camshaft parallel twin was designed instead of a flat twin across the frame. Serious vibration made the new Sunbeam uncomfortable to ride and the initial production sent to South Africa was recalled; the problem was cured by mounting the engine on two bonded rubber engine mounts. The inline engine suited shaft drive, and BMW-style bevel gears would have been ideal, but BSA had inherited worm-gear machinery from the Lanchester Motor Company and specified worm drive instead. The gears tended to strip under power, and it is said that Sunbeam’s solution was to reduce the power to 24 bhp. The original S7 was produced in black, the S7 de luxe in Mist Green or black and the S8 in Silver Grey or black; for export BSA would supply Sunbeams in any BSA colour.

Design and features

• Inline overhead-camshaft twin: a car-like 500 cc engine mounted lengthways.
• Shaft drive: through a dry clutch and a worm final drive.
• S8 upgrades: BSA forks, cast aluminium silencer, chromed wheels and narrower tyres.
• Sports tuning: the lighter S8 was quoted at 85 mph.

Sunbeam S8 versions

• S7 Tourer (1946–1948): the original model, 2,104 built, black.
• S7 de luxe (1949): 5,554 built, Mist Green or black.
• S8 (1949): the lighter sports model, 8,530 built, Silver Grey or black.
• S9 and S10: planned but never produced.

Sunbeam also built cars such as the Sunbeam 3-litre and the Sunbeam Alpine. More vehicles are in the Sunbeam collection.

Video: SUNBEAM S8 BEAUTIFULLY RESTORED 4 SALE EBAY ENDS 19/9/10

Video on YouTube.

Sources

• Sunbeam S7 and S8, English Wikipedia: design, models, production, problems