On my commuter bike I decided to rid myself of the small inner chain ring (which is almost never used) and front dérailleur (now redundant). However, I have come across an unusual problem. Whilst cycling the chain is coming off of the outer chain ring and falling onto the crank arm. This usually coincides with a shift of the rear dérailleur onto a smaller sprocket, although notably not onto the highest gearing - it has happened shifting from the 5th to 6th scrocket on the 12-27 cassette for example. Furthermore, I am not exerting abnormal force on the crank, just sitting and riding as normal. This just happens out of the blue and as you can imagine is quite disconcerting/dangerous.
I have considered and checked the following so far:
- Chain length - tension is good so no obvious problems there...
- Chain dirty - it most certainly is not, it has only seen 200 miles and its lubed up a treat ;-)
- Chain ring to cassette alignment - Hollowtech II BB and Ultegra 53T 6600 crank in 68mm shell, standard setup afaict, no obvious problems there...
- Lack of front-dérailleur - I removed this since there is only one chain ring - could this be a factor that prevented me noticing the problem before (surely not...)
- Chain ring worn - the chain ring looks in pretty good shape to me
I experimented briefly, turning the crank and applying a very slight outward pull (away from frame) on the chain and it promptly fell off onto the crank. Frankly, I was surprised how such a small misalignment could cause the chain the fall off...
My question: Has anyone else encountered this problem before?
Any comments on the things I have checked (why I may be mistaken in my assessment of the problem)?
Other possible causes I have missed would be gratefully received - this is spoiling an otherwise nice bike and I really do not want to go back to using the double crank configuration if possible.