After my wheel building project that initially seemed to have gone sour, I’ve made some incremental progress on a bicycle I’ve been rebuilding.
It started off as a mountain bike in rough shape that my brother gave me. I cleaned and lubed the whole bike, upgraded the brakes, added fenders, added road tires, and swapped out the cranks. Sadly, none of this resolved a drive train issue where it seems to skip on the smallest rear cogs while under any sort of load.
A little creative thinking later, and I took a three-speed internal geared hub and built it up to a modern 26″ aluminum rim. Immediately upon attaching it to the bicycle I realized I was going to have a few technical issues to contend with.
The first problem was that the derailleur chain was the wrong size. I was very dismayed by this issue since the chain that went with the hub is destroyed by a mass of rust and not worth attempting to recover. A little research revealed that there was a pretty good chance the issue was the chain width – internal gear hubs and single speed bikes use a 1/8″ wide chain which is slightly wider and less flexible.
A quick visit to the local bike shop was very helpful since they had such a chain for $9.99.
I cold-set the rear triangle to the narrower width of the internal hub and sized the chain. Ideally the chainline wants to be to the innermost gear on a 3-gear crank, however the gear ratio that produces leaves little to be desired. Coasting works fine, however back pedaling quickly will throw the chain so it needs to be tensioned. The reason for this is the rear dropouts are vertical, meaning the wheel cannot be pulled back into position to tighten the chain.
To solve this, I filed an axle sized groove into the dropouts so the chain can be tensioned. Unexpectedly this raised another problem – since the chain is going to the middle cog, the chainline is not perfectly straight. Once the chain is tensioned it starts to bind on the cogs since it is less flexible than the narrower chain.
That’s pretty much where I took a break from it so I can devise the best method of resolving this. Somehow I need to reduce the angle of the chainline while maintaining an adequate gear ratio. I may have to re-examine the angles on the rear triangle and do another round of cold-setting the metal.