Originally Posted by Sheldon Brown
There is some risk of the chain falling off in front if you don't have a front derailer of some sort. Sometimes people leave the derailer, immobilizing it by leaving off the control and setting it with the low-gear limit stop.
This is exactly the problem I had with my coversion to a single chain ring / flat bar. I ended up putting the front deraileur back on and setting it as above which solved the problem. Runs like a dream now.
Steve.