B25 final drive ratio.

[Ftwelder]

As I inch closer to being half way done assembling my motor, I need to decide what to use for a gearbox sprocket. I have a 18T crank sprocket and a 52 on my rear wheel.  Standard SS ratios in a dedicated trials machine. My local suppler (bsaunitsingles) has suggested a 13T as a good starting point but there is also a 12T available. 

I ran out of "meaningful" progress while waiting for parts to I took the case covers to the polishing wheel. Ill never do that again but it sure looks nice.

 

 

[section swept]

Just had a quick look at Google, typing in BSA C15 Trials gearing. You should find plenty of helpful info and probably sort a good starting compromise to fine tune the overall rear wheel sprocket and gearbox sprocket size (no of teeth). A lot will depend on the internal ratios of the gearbox, you might find that with lower overall gearing the engine will work fine in first and second, but third and fourth might be a bit too low for any road work or distance between sections. Something else to bear in mind is the torque effect of a drive chain being pulled around the smaller output gearbox sprocket which can have/cause adverse wear issues. Then you may also have chain to frame/ swing arm clearance problems. Some negatives but not un solvable! You might consider going the whole hog and fitting C15 T internals (if some can be sourced) from the visible effort that you are putting in it will be worth it. Waiting to see the finished article...good luck with your bike?

[Ftwelder]

Thanks for the advice and kind words, I have a friend who had a C15T in stock form and strongly suggested I avoid the wide ratio box. Over here events are generally held on a single piece of property so not much need for a tall gear besides being rare and expensive.  I do agree about the chain line and swing arm pivot relation. I cycled the suspension with a spring in place of several links of chain (with a 16T gearbox sprocket) and at ride height had no swing arm contact and may be 10mm chain growth. Food for thought...

 

It might be nice to sub in the close ratio Victor first gear which is a taller so first and second are closer and then select the smallest gearbox sprocket that gives good chain line at ride height, use the rear sprocket to make things work.