timdog Posted February 15, 2020 Report Share Posted February 15, 2020 (edited) I have a c15 which has standard road gearbox, I’ve lowered gearing including primary drive to make first ideal for sections but obviously it makes fourth rather slow and the rear sprocket is large, chain has jumped off couple of times. I’m not very up on gearbox internals so hoping someone can help. If I replace 1st and 2nd with these 29 tooth 40-3176, + layshaft 2nd gear 26 tooth 40-3132 and mainshaft or 2nd, 24 tooth 40-3130 will it make significant difference. Does it lower first gear internally so I could reduce size of rear sprocket. Thanks in advance for any help Edited February 15, 2020 by timdog Missed part out Quote Link to comment Share on other sites More sharing options...
timdog Posted February 15, 2020 Author Report Share Posted February 15, 2020 I’ve read some older posts on gearing but admit I’m still unsure. I haven’t had the gear cluster out yet to check sizes but the opportunity for these apparently close ratio parts came up. It’s a distributor engine if that makes difference. Quote Link to comment Share on other sites More sharing options...
markparrish Posted February 15, 2020 Report Share Posted February 15, 2020 Hi - I am a step behind you so I'd be very interested to know what size sprockets you are running internally and externally at the moment. I am having similar thoughts about my c15 as I'd like trials speeds but with the possibility of riding on the road without holding up the traffic! I have been told that non-standard internals are hard to come by. It is geared up for the road/greenlaning at the moment - I estimate it does about 30 in top gear (with a huge rear sprocket 72 teeth!). 1st is still a bit too fast though. Quote Link to comment Share on other sites More sharing options...
b40rt Posted February 15, 2020 Report Share Posted February 15, 2020 FWIW, I have a high 4 gear in my 2 stroke pre65. It's obviously a bonus on fast tracks and road work, but on moors etc your restricted to 3rd much of the time. Still a trade off. Quote Link to comment Share on other sites More sharing options...
timdog Posted February 15, 2020 Author Report Share Posted February 15, 2020 Top speed not a priority it’s more being able to reduce the rear sprocket size and possibly making second a bit more useable. Think may just try and source a full trials cluster. Currently running 18t engine 14t gearbox and 64t rear sprocket. First is great, chugs away slowly down to tickover, second ok for hill climbs. Looking to put belt drive clutch in so seems ideal time to have gearbox done too. REH forks on since the photo now just need to ride it better! 2 Quote Link to comment Share on other sites More sharing options...
section swept Posted February 15, 2020 Report Share Posted February 15, 2020 (edited) Can’t see if you are using a chain tensioner. The bike looks really good but the chain coming off the sprocket could be an alignment issue, allow by the standard presented in the photo you have probably covered that. Do you have sufficient clearance between the rear tyre and drive chain, its possible with tyre flex for the tyre to force the chain out of line and jump the sprocket. Your final drive ratio of 4.57:1 seems a bit low.....high engine revs not much speed. Running a smaller gearbox output sprocket may create chain inertia problems. I’d see if there is any data on a genuine BSA C15’T’ available. Here are some C15 specs. C15 road first 16.27 second 10.74 third 7.80 fourth (top) 5.98 gearbox sprocket 17 rear wheel 52 C15T trials first 28.53 second 22.05 third 14.67 fourth (top) 9.00 Quite a difference a difference with top producing an estimated 40-50 mph. Bear in mind the C15T had a larger inlet valve and different camshaft, there was an optional higher compression piston available giving a 10:1 ration. The BSA B40 WD spec bike had similar low gearing i.e. first and second virtually useless for road work but good for slow progress with a Squadie aboard furiously footing away through mud and crap. But third and fourth producing useful forward progress. I cannot claim the specifications as from my knowedge, rather 5 minutes on Google and all was revealed?Certainly using the ratios you have indicated will lower the internal gear ratio considerably, which will need you to decrease the rear wheel sprocket size and also the gearbox sprocket. Good luck? Edited February 15, 2020 by section swept 1 Quote Link to comment Share on other sites More sharing options...
timdog Posted February 15, 2020 Author Report Share Posted February 15, 2020 Hi section swept thanks for the above. Chain tensioner, alignment and clearance all fine. Recently rebuilt into Faber frame from a modified c15 frame. Chain has only come off on steep rocky gulley, still annoying costing 5’s which I can get myself without the aid of the bike! ongoing project as always but that’s half the fun. I have a fair bit of info to sift through Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.