GEARING UP

Tom Halls got in touch via my website (rickparkington.co.uk) to say that his BSA Golden Flash seems ‘revvy’ and would fitting a two-teeth smaller or so rear sprocket sort it out?

Well, probably it would, but I don’t think smaller sprockets are available, Tom. On pre-unit (separate gearbox) bikes like the BSA A10, gearing is changed

by altering the size of the crankshaft sprocket as it’s the easiest one to get to. There are four sprockets in all – crankshaft and clutch, gearbox and rear wheel – and increasing the size of the crank or gearbox sprockets is the same as reducing the clutch or rear wheel. Given that the ratio between both fronts and rears is about 2:1, adding one tooth to the crank is the same as two less on the rear wheel. Standard solo gearing for an A10 is engine: 21 teeth, clutch: 43t, gearbox: 19t and rear wheel: 42t. Engine sprockets down to 18 were supplied for sidecar use and up to 23t for the Rocket Gold Star, so there’s a good selection available – bear in mind you may need a longer chain too.

The earlier plunger-framed, semi-unit construction models have the engine and box bolted together and used a duplex primary chain, for which optional sprockets are not available. Standard sizes here 27/54 (smaller chain pitch, so smaller teeth) and 19/42 although a 49t rear sprocket was listed for sidecar use.

The generous torque of the A10 made it a very popular sidecar bike, so it’s quite possible that Tom’s BSA has been geared for pulling a chair.