Sunbeam S8

 

   

Make Model.

Sunbeam S8 500

Year

1949 - 56

Production

8 530 units

Engine

Parallel twin, SOHC

Capacity

487 cc / 29.7 cu in
Bore x Stroke 70 x 63.5 mm
Compression Ratio 6.5:1
Cooling System Air/Oil cooled
Lubrication Wet sump

Starting

Kick
Electrical system 6V

Max Power

18.6 kW / 25 hp @ 6000 rpm
Clutch Single plate, dry

Transmission 

4-Speed
Final Drive Shaft
Frame Twin cradle, steel

Front Suspension

Telescopic, one way damped

Rear Suspension

Plunger, spring loaded saddle

Front Brakes

Drum, leading shoe, 203 mm / 8"

Rear Brakes

Drum, leading shoe, 203 mm / 8"
Wheel, Front 4.50 x 16"
Wheel, Rear 4.75 x 16"
Tyres, Front 4JA x 16"
Tyres, Rear 43A x 16"
Wheelbase 1400 mm / 57"
Dry Weight 187 kg / 413 lbs

Top Speed

121 km/h / 75 mph

Source

Sump Publishing
   

Designed as Britain's answer to the BMW, the Sunbeam S7 and later the S8 were odd mixtures of the inspired and the impractical, which was ultimately to condemn them to being an interesting backWater, rather than part of the mainstream of post-war motorcycling.

 

Several influences were at work, but the new machines were BSAs in all but name. Sunbeam had ceased to be a truly separate entity during the 1930s, and the trademarks now belonged to the giant BSA organisation, which reasoned that they could capitalise on Sunbeam's 'gentleman's motorcycle' image for their new tourer.

 

The design was the work of independent designer Erling Poppe, but was heavily based on the BMW R75, manufacturing rights to which had been offered to BSA as part of the war reparations. But while the double-cradle frame and telescopic fork echoed the BMW, the engine unit was a completely new design. Displacing 487cc, the engine was basically a parallel twin not unlike that offered by the BSA A7, but it was housed in alloy castings and turned around so that the crankshaft ran in line with the frame.

The first of the Sunbeam's problems arose from the choice of transmission. The intention was to use a shaft drive like the BMW, but the design adopted had a worm gear in place of the German machine's bevels. While easier to manufacture, it was inherently weak, and on prototypes the worm stripped its thread if the engine was fully used. The second problem was vibration.

This had been evident on the prototypes, but the signs were ignored until the bike went into production in 1946. When an initial batch was despatched to a police team intended to escort King George VI, it was reported that they were unridable.

The machine weighed more than 4001b - but looked heavier. Fitted with I6in 'balloon' tyres, its handling was prone to vagueness, which became aggravated as the plunger rear suspension units wore. The finish, in BSA's rather drab 'Mist Green', can hardly have helped its showroom appeal, any more than the stories of its mechanical defects, and it was a poor seller.

In 1949 BSA decided to tackle both the performance and the styling by launching the new S8 as a sports alternative to the touring S7. Lighter than its predecessor, this used cycle parts such as forks and wheels from other BSA models. It was a more popular machine than the S7, but was still no runaway success. BSA finally discontinued the Sunbeams.