Mysterious case: chain falling off large chainring, how to fix? 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.
Yeah. A modern double chainring is ramped and pinned, in order to be more easily shifted from one ring to the next. Simply put, the chainring "wants" to pass off the chain to the next cog. Without a derailleur to keep it in place, the lateral pull of the chain shifting across the rear, combined with some road jostling, can make the chain fall off the front. 

A couple solutions: 


install a singlespeed chainring without ramps or pins, with "straight" teeth.  
use an outer chainring guard and inner "dog tooth" chain keeper  
combine the two (which is the best solution).


The free and easy way to keep it on while looking at other solutions is to reinstall the front derailler and adjust the limit screws so it keeps the chain in place.

This is a pretty common concern with cyclocross. Racers often use a single-chainring setup for the sake of reliability, and either or a dual chainguard or chainguard-and-chain watcher setup for just such reasons (also because they keep picking the bike up and putting it down whilst running and jumping over stuff), so you get stuff like this: http://www.bikeman.com/CC-SRAMS300CX.html.