Just seeing if Jonboy is still around? you posted some years ago about wanting to know about the gearing for a C15 trials project. I'm wondering if you ever got what you wanted, as I'm looking to change my standard C15 gearing for a bit of green laning. I'm not looking for trials gearing i.e. 20mph flat out but a modest 40mph tops but a bit quicker out of the blocks.
Hi, re gearing for off-tarmac use of your C15.
If road speed is your main concern the first step is to choose the overall gearing to give the top speed you want. This is independent of the internals in the gearbox and depends on the crank, gearbox and rear wheel sprocket sizes you fit.
After that you have a few alternatives re gearbox internals to give 1st, 2nd and 3rd gear ratios to suit the off road capability you want. The options are limited though, so with only four gears and limited internal gear options it's not possible to have a bike which will climb up a vertical wall in 1st and also do 50mph in top.
The type of engine you have also determines the options on gearbox internals.
Distributor and F C15/B40 engines have the fullest range of gearbox options with wide, standard and close Ratio gear sets available. This also allows mixing of individual gear pairs so that semi wide gear sets can be achieved - good for trail use.
E.g. I have a B40 for trail use which gives 45mph in top gear and 1st gear low enough to cope with all off road stuff excepting the harder pukka trials sections. This has wide Ratio top gears and std 1st, 2nd and 3rd gear pairs. The wide top gears lower the ratios of the other three gears as all drive from them goes through the top gear pair.
C15/B40 G engines have fewer gearbox options as there is only one top gear pair option. 1st, 2nd and 3rd gears can be std, wide or 'Victor' which has a low 1st with close 2nd and 3rd.
To cut to the chase, if you have a G engine the best compromise I've found works which well for trail use is wide 1st and 2nd and std 3rd.
PM me for more details so we don't bore everybody stupid. I have the various gear options available if you need them.
Regards, Rupert.