Sunbeam S7/S8

This was to be the jewel in the crown of post­ war British motorcycling, and it had the full weight of the giant BSA Group behind it. During the war years, BSA acquired two things; the Sunbeam name, and a captured German BM\XI R75 outfit. As the war was coming to a conclusion, BSA recognised that they needed a flagship model, and set about creating one under the Sunbeam banner , which had always been associated with ‘gentlemen’s conveyances’.

Erling Poppe, essentially a car man, was the project’s chief designer, and the resulting machine adopted much from automotive technology of the day. It borrowed even more from the BMW – frame and running gear layout, dynamo, brakes, wheels and tyre sizes, and the concept of a shaft-drive twin with the gearbox behind the engine. Unfortunately the one vital German piece that was not purloined turned out to be the Sunbeam’s downfall.

Specifications

Engine

Horsepower

transmission

Frame

Brakes

top Speed

Years in Production

air-cooled 487cc OHC tandem twin

26 bhp @ 5800 rpm

4-speed

tubular twin loop

drum / drum

80 mph

1949 – 1956

BSA might also have pinched BMW’s flat-twin engine, but that would have been a little too obvious . Instead , Poppe designed a new powerplant, still a twin with the crankshaft fore­-and-aft , but now with cylinders in line astern rather than opposed. Astonishingly, this was the only new overhead cam motorcycle engine to go into production in post-war Britain until the (even more ill-fated) Hesketh in 1981.

Poppe also designed a new four-speed gearbox with bevels allowing the kick-start to be sited much less awkwardly than the Bee-eM’s . Unfortunately, this meant ditching the proven crown-and-pinion final drive in favour of an underslung worm gear. Apparently the choice was made because the BSA Group , which also produced Daimler and Lanchester cars, was familiar with making this type of drive!

BSA leaked details of the new machine liberally, as well as presenting one to Field Marshal ‘Monty’ Montgomery of Alamein. Such was the sense of anticipation, and so great the curious crowd, that at the 1948 Motor Cycle Show the Sunbeam stand collapsed under the weight!

Yet this excitement  was  sadly  misplaced.  Early prototypes of what was actually a fairly free­ breathing and potent engine had already shown that final drive worm gears lasted no  more  than  5000 miles . Rather than address  the  real  problem, Sunbeam designed an entirely new cylinder head to reduce power. T he resulting 23.6 bhp , allied to a dry weight of 435lb’s, left the  S7  seriously under­motivated.

There was more. An  early  batch of S7s  rushed out to South Africa had to be recalled when vibration proved too severe. Later examples had rubber-mounted engines (and a corrosion-prone flexible exhaust section) – and the handling, on those fat 4.75 x 16 tyres, was distinctly odd (and gave the S7 the look, according to a contemporary report, of ‘a motor trapped between two doughnuts’) .

By Rikita – Own work, CC BY-SA 4.0

Billed as ‘The world’s most magnificent motor cycle’, the S7 went on sale in 1947 at the very high price of £222. This ‘magnificent’ creation was overweight, slow, fairly thirsty, looked weird, didn’t handle and soon attracted a reputation for unreliability. Only around 2000 S7s were made before 1949 when the heavily reworked ‘sports’ SS and S7 De Luxe were introduced.

Both received numerous engine changes (notably a bigger oil capacity), and the S8 adopted BSA A10 wheels and forks but retained plunger rear suspension. The S8, was about 5mph quicker and 30lb lighter than its predecessor. Ridden prudently, it proved reliable, but pedestrian – that flawed final drive was more dependable, but it still soaked up power. Arguably it is the ‘dignity’ of its performance (and the snootiness of Sunbeam dealers), as much as anything aristocratic in its design, that gave the Sunbeam the aura of gentleman’s carriage. Still, the ‘Beam began to sell, and by December 1952,  10,000 had been built. From  then  on sales steadily declined , and the range was dropped when BSA and Triumph merged in late 1956 .

The attempt to produce the first post­ war superbike had failed.

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