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Sunbeam Model 90

Make Model.	Sunbeam Model 90
Year	1928 - 33
Engine	Single cylinder, four stroke, SOHV
Capacity	493 cc / 30.1 cu in
Bore x Stroke	80 x 98 mm
Compression Ratio	7.0:1 (Optional 7.5 or 9.0:1)
Cooling System	Air cooled
Exhaust	Twin
Lubrication	Wet sump
Starting	Kick
Ttansmission	Three-speed with handshift meganism, medium ratio (BT) or close ratio (CT)
Final Drive	Chain
Gear Ratios	BT: 1st 10.3 / 2nd 6.9 / 3rd 4.6:1 CT: 1st: 8.4 / 2nd 6.2 / 3rd 4.6:1
Frame	Single cradle, steel
Front Suspension	Druid-pattern, girder forks
Rear Suspension	Rigid, spring loaded saddle
Front Brakes	Drum
Rear Brakes	Drum
Wheels	Steel, laced wire spokes
Tyres, Front and Rear	3.25 x 26" Dunlop Racing option: 2.75 x 27"
Dimensions	Length: 2083 mm / 82" Width: 889 mm / 35"
Wheelbase	1359 mm / 53.5"
Saddle Height	711 mm / 28"
Dry Weight	143 kg / 315 lbs Racing: 141.5 / 312 lbd
Fuel Capacity	11.4 L / 2.5 Imp gal / 3.0 US gal
Oil Capacity	2.8 L / 5.0 Imp pt / 6.0 US pt
Top Speed	145 km/h / 90 mph

The best years of Sunbeam were in their early history. They built up a tremendous reputation for quality and finish and this was to lead eventually to the downfall of the firm. By 1928 Sunbeam had become part of the ICI combine but did not add very much to group profits so in came the accountants and much of the tradition had to go. More parts were bought in and slowly some of the quality began to evaporate.

Sunbeam had tried to develop an OHC racer but that proved to be unsuccessful: the camshaft Sunbeam that appeared in 1925 was seriously lacking in torque and power because of problems with valve timing.

The much more direct ohc mechanism necessitated a totally different valve timing, of which there was very little knowledge at the time. Factories like Velocette and Norton decided to carry on experimenting and finally got the knack, but Sunbeam returned to the pushrods.

The factory turned to advice to Harry Weslake, who discovered the importance of the right shape of port and the optimum direction of gas flow. On this then new technology Harry Weslake built a career as an engine consultant- the splendid XK Jaguar engine was one result, the excellent reputation of the Model 90 another.

The engine was internally much different from the standard machines, with highly polished I-section conrod, 7.5 : 1 piston running in a special machined barrel. The head had enormous port finning and hairpin valve springs.

The model 90 was very successful in racing for a number of years; in 1929 Charlie Dodson won the Isle of Man TT with an average speed of 73.5 mph.
At Monza the Senior TT was won with a Model 90 that lapped the circuit at 94 mph.

The last mode/ 90 was made in 1933 and was superceded by the .model 95