2026-01-09

Last modified: 2026-01-09 23:14:52

Car

I've ordered the reversing gearbox, but I'm really struggling to work out what size sprockets I'm going to want. I can't tell exactly what ratio is inside the reversing gearbox and at any rate don't know what ratios the engine will have.

Starting from the beginning, the engine tops out at 9500 rpm.

Then I think we have overall ratio in the engine (covering reduction to the clutch and reduction in gearbox) of 3.412:1.

Then a 19-tooth output sprocket on the engine gearbox and 10-tooth input sprocket on the reversing gearbox. Then maybe 3:1 inside the reversing gearbox if the specs are to be believed. Then 12-tooth output sprocket from reversing gearbox to the final sprocket on the axle (say 22 teeth). So that is overall 9.877:1 reduction, which gets us about 1000 rpm at the wheel at full revs in top gear. If the wheel is 560mm outer diameter then that is 63mph.

A 22-tooth sprocket at the axle sounds small.

Did I make an error in my calculations?

We have a 19-tooth sprocket at the engine gearbox driving a 10-tooth sprocket at the reversing gearbox, which is an increase in speed. Is an increase desirable because it reduces the torque in the reversing gearbox? Or just not desirable and we should swap those sprockets? Or perhaps the 3:1 ratio on the reversing box is wrong, it certainly looks more like 1:1 from the disassembly video.

If it's 1:1, and we swap the 10-tooth and 12-tooth sprockets on the reversing gearbox, then we end up wanting a 45-tooth sprocket on the rear wheel to get 64mph top speed, which sounds more reasonable.

The ZS190 has a "428" sprocket on it as standard, and the reversing gearbox comes with "420". They both have 1/2" pitch so it is possible that you could get a chain to link them but would be better to make them compatible if convenient. Bot says 428 chain will fit 420 sprockets as 420 is narrower than 428.

I think the plan is wait until the reversing gearbox arrives, measure its ratio, then work out roughly what size sprockets to use, then I'll be confident that my method of attaching the final sprocket to the rear axle is going to be suitable. For example a large-diameter carrier is no use if I need a small-diameter sprocket.

Can I make my own sprockets? It doesn't look too hard, do they need to be hardened? Motocross sprockets are aluminium so presumably no.