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
‘ALL CHRONOS HAVE BROADLY THE SAME INTERNALS’