January 28, 20188 yr Hi. I have decided to build a c15 trials bike. I am fitting a 400.18 rear tyre which is slightly fowling the chain. Could anyone advise me the best way to get some clearance un order to fit a chain guard. Do I move engine mounts etc?. Any advice appreciated
January 28, 20188 yr Probably not what you want to hear but I had a similar issue with a C15. Under close inspection, I could see that the cast swing-arm mount was slightly twisted on the frame. I made up some mounts so I could bolt the frame to a bike carrier and used a long (6' maybe) bender bar with pin to pull it straight. It took a lot of force but worked out OK and stayed in place. The only negative was that I did hear one "tink" as it moved. I took a lot of measurements yet never really came up with a great datum or measurement method so it was mostly by eye. After straightening, I had adequate clearance for a 428 chain and small chain guard. While I was at it, I checked the flatness of the swing arm on a surface plate and one leg was higher that the other. As I remember, it was easily straightened when gripped in a vise.
January 31, 20188 yr What rear hub are you using ? Chain clearance has always been a problem with C15s, you can pull the wheel rim over a little by slackening the spokes one side and tightening on the other ( or get a wheel builder to do it ). I widened my swing arm 1 inch to give me more room to play around. Ultimately the dimension from the centre of the front sprocket to the centre line of the engine never changes, so it’s either move the engine to the left a little and alter your rear sprocket accordingly, or move the wheel rim to the right . You won’t end up with a lot of clearance either way; my chain guard virtually rubs on the tyre and chain simultaneously, so don’t expect miracles, but it works just fine Edited January 31, 20188 yr by davetom
January 31, 20188 yr I used a Bultaco Alpina rear wheel. If I remember correctly, all I had to do was make outside spacers to center it in the swing arm. I think the spacers were the same width on each side. The chain was straight and there was clearance but not very much. I have since sold the bike so have nothing to measure. I was amazed how heavy the original brake hubs were on this bike. It was a street bike but the brakes looked like they came from a small car.
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