Gearing on MY 58 SR.
I was told by SRM and my local BSA specialist, that A10SR use 19T gearbox sprocket for Side Car use and 21T for standard, but for using the Sidecar the recommendation was to keep the gearbox at standard ie 21T and use a smaller engine sprocket ie 16-18T ( where 21T sprocket is the standard for solo ).
So MY gearing is. ( now running without a side car ) 23T engine, 43 clutch = 1,870 ratio
Gearbox 21, rear 42 = 2,00 ratio
Over all ration in total = 1,870 x 2,00 = 3,74
Using 19” rear wheel/rim giving a theoretical max speed of 98MPH @ 6500 rpm.
(Giving it a 19T gearbox sprocket, result in a max speed of 89MHP)
“Test of the day” was carried out on “special” prepared motorcycles.
(ie the tuning department might have selected very good units, with extra high compression etc.)
Calculating the matter, I use the spreadsheet developed by Steve Nott
www.stewenott.comSo basically I recommend the 23T engine sprocket. To adapt the gearing to modern roads..