Rick’s Fixes

Classic Bike (UK) - - CLASSIC WORKSHOP -

Your prob­lems solved

My mate Matthias Neu­mann in Ger­many asks how he can tell if the 80mph Smiths Chrono­met­ric speedo he’s been of­fered is the right ra­tio for his girder-fork BSA M21.

In pre-war days, there were fewer ra­tios than came along sub­se­quently. Back then, most speedos were driven by a right-an­gle box on the front brake plate, en­gaged with a toothed ring on the hub, and man­u­fac­tur­ers sup­plied the cor­rect sized ring to cor­rect the ra­tio.

Even when the sealed speedo drive boxes came along, most bikes used 3.25 x 19 tyres, so the ra­tio was pretty con­sis­tent. It was only later, with greater va­ri­ety in tyre sizes and some ma­chines driv­ing off the gear­box, that it be­came com­pli­cated. A code on the dial iden­ti­fied the cor­rect clock and lat­terly a fig­ure like ‘1440’ was added, rep­re­sent­ing ca­ble rev­o­lu­tions per mile at 60mph. The Army M20 is sim­i­lar to Matthias’s bike and that seems to have used an S433 speedo with 1450 ca­ble revs per minute.

But the good news is that all chronos from the D-shaped 55mph Ban­tam up to the im­pos­ing 150mph 6in Vin­cent clock have broadly the same in­ter­nals. The speed range is set by the weight of the in­ter­nal bal­ance wheel; small ad­just­ments are made by adding clip-on shims, so any 80mph speedo should work and if there is a slight dis­crep­ancy, it can be ad­justed by a spe­cial­ist.

LEFT: Bal­ance wheel al­ters speedo range


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