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Sunbeam Motorcycles

Sunbeam 1931 Models

The model range from 1930 was severely pared down for 1931. Sunbeam was attempting to weather the Great Depression in the world's economy. The catalogue emphasised 'Sunbeam value' with lower cost machines (the Lion and new Model 10) replacing longstanding designs. It even provided details of a Sunbeam hire purchase agreement through the factory or recognised dealers.

Other than the 596cc 'Model 7' flat-tank machine (unaltered since 1928 and still available to order) the side-valve range was reduced to one machine: the cut-price 'Lion' introduced mid-season the previous year. Gone were the longstanding Marston side-valves: the 347cc 'Model 1' and 'Model 2' plus the 'Model 5' and 'Model 6' versions of the 492cc Longstroke.

The 347cc OHV 'Model 8' was also lost. However, it was replaced by a new introduction to the range; a John Greenwood and George Stephenson designed 344cc OHV machine named the 'Model 10'. This machine had a distinctive crankcase, holding a wet sump - a design only previously seen in the 1925 OHC racing machines. Its design did not allow for the use of Sunbeam's trademark enclosed rear chaincase. Roy Bacon also refers to Sunbeam offering a tuned version of the 'Model 10' for the mid-season in July 1931.

Mid-season, in July 1931, a 599cc version of the side-valve Lion was also added to the range. Designated the 'Model 7A', it introduced a side-valve engine with a detachable head and 'boxed-in' valve springs which became the standard side-valve design for 1932 and thereafter.

The 493cc OHV Model 9 in its standard and 'Model 90' sporting varieties continued to be available but it was now provided with the chrome tank and Webb-style forks of the newer models. 'Model 9' was available with both single and twin-port cylinder options; 'Model 90' only as a twin-port. The 'Model 90' was offered in both road racing and 'fast touring' trim.

Cycle parts

Druid forks are replaced by centrally sprung Webb-style front forks, produced in the Sunbeamland factory.

Chrome-plated, welded petrol tanks were introduced, finished in chrome with black panels. These did no prove popular and they reverted to the earlier colour scheme for the following season.

1931 Model List:

Model 10

Engine
Cylinder capacity: 344cc
Bore x stroke: 74mm x 80mm
Compression ratio: 6 to 1

Gearbox
Three-speed - medium ratio
Gear ratios: 5.5, 9.1 and 15.2
(options: 5.15, 8.6 and 14.3 or 5.8, 9.7 and 16.5)

Tyres
Dunlop 25in x 3.00in (wired on)
Pressure: front 17 lb; rear 23 lb

Cycle
Weight: 231 lb
Overall length: 79in
Wheelbase: 50.5in
Width across handlebars: 32in
Saddle: Terry (Lycett optional)
Saddle height: 28in (minimum)
Petrol capacity: 2 gallons
Oil capacity: 2 pints

Frame numbers: R1001 up
Engine numbers: SS1001 up

Lion 'Longstroke'

Engine
Cylinder capacity: 492cc
Bore x stroke: 77mm x 105.5mm
Compression ratio: 5 to 1

Gearbox
Three-speed - wide ratio (AT prefix)
Gear ratios: 4.9, 8.2 and 13.7
(side car: 5.25, 8.7 and 14.6 or 5.6, 9.3 and 15.5)

Tyres
Dunlop 26in x 3.25in (wired on) - 3.50in optional
Pressure: front 16 lb; rear 19 lb

Cycle
Weight: 300 lb
Overall length: 82in
Wheelbase: 53.5in
Width across handlebars: 32in
Saddle: Lycett (Terry optional)
Saddle height: 28in (minimum)
Petrol capacity: 2.5 gallons
Oil capacity: 4 pints

Frame numbers: B9001 up
Engine numbers: J5001 up


Model 9

Cylinder capacity: 493cc
Bore x stroke: 80mm x 98mm
Compression ratio: 6.5 to 1

Gearbox
Three-speed - medium ratio (BT prefix)
Gear ratios: 4.6, 6.9 and 10.3
(optional: 4.3, 6.5 and 9.7 or 4.9, 7.4 and 11.0)
(side car: 5.25, 7.9 and 11.8 or 5.6, 8.4 and 12.6)

Tyres
Dunlop 26in x 3.25in (wired on) - 3.50in optional
Pressure: front 16 lb; rear 19 lb

Cycle
Weight: single-port 308 lb; twin-port 319 lb
Overall length: 82in
Wheelbase: 53.5in
Width across handlebars: 32in
Saddle: Terry (Lycett optional)
Saddle height: 28in (minimum)
Petrol capacity: 2.5 gallons
Oil capacity: 4 pints

Frame numbers: D9001 up
Engine numbers: L5001 up (LL for twin-port)


Model 90

Engine
Cylinder capacity: 493cc
Bore x stroke: 80mm x 98mm
Compression ratio: 7 to 1 (optional 7.5 or 9 to 1)

Gearbox
Three-speed - medium ratio (BT) or close ratio (CT)
Gear ratios BT 'box: 4.6, 6.9 and 10.3
(optional: 4.3, 6.5 and 9.7 or 4.9, 7.4 and 11.0)
(side car: 5.25, 7.9 and 11.8 or 5.6, 8.4 and 12.6)
Gear ratios CT 'box: 4.6, 6.2 and 8.4
(optional: 4.3, 5.8 and 7.9 or 4.9, 6.6 and 9.0)
(side car: 5.25, 7.1 and 9.6 or 5.6, 7.6 and 10.2)
Racing option: foot gear change / no kickstarter

Tyres
Front: Dunlop 26in x 3.25 (wired on)
Front - racing option: 27in x 2.75in (wired on)
Rear: Dunlop 26in x 3.25 (wired on)
Pressure: front 16 lb (27in tyre - 23 lb); rear 19 lb

Cycle
Weight: racing 312 lb; 'fast touring' 315 lb
Overall length: 82in
Wheelbase: 53.5in
Width across handlebars: 35in
Saddle: Terry (Lycett optional)
Saddle height: 28in (minimum)
Petrol capacity: 2.5 gallons
Oil capacity: 5 pints

Frame numbers: E9000 up
Engine numbers: NN5001 up


Model 7 (flat-tank)

Model 7A

Introduced mid-season (July 1931) the earliest Model 7A inserts the new detachable-head engine design into the cycle parts of the 1930-31 Lion, including its three-speed gearbox.

The new engine design was to become standard for both 500cc and 600cc side-valves from 1932 whilst cycle parts and gearbox were both subject to change.

Engine
Cylinder capacity: 598cc
Bore x stroke: 85mm x 105.5mm
Compression ratio: 5 to 1

Gearbox
Three-speed - wide ratio (AT prefix)
Gear ratios: 4.9, 8.2 and 13.7
(side car: 5.25, 8.7 and 14.6 or 5.6, 9.3 and 15.5)

Tyres
Dunlop 26in x 3.25in (wired on) - 3.50in optional

Cycle
Overall length: 82in
Wheelbase: 53.5in
Width across handlebars: 32in
Saddle: Lycett (Terry optional)
Saddle height: 28in (minimum)
Petrol capacity: 2.5 gallons
Oil capacity: 4 pints

Source: sunbeamsidevalve.com


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