Originally Posted by
JeffOYB
So does it really have to happen? And do you really need some kind of guard to prevent it?
Let's see, singlespeeds don't have them.
Single speeds
1. Don't have rings with truncated teeth in "downshift zones" which encourage derailment.
2. Have a straight chain line which wouldn't cause bad interactions.
And the front der only prevents derailing at the top. So something must be happening as a tight chain goes onto the ring after pulling past the top of the rear cog. But what? Is it a slight change in tension that allows slack which in time with some kind of sideways motion or bump throws the chain off?
I've had problems with un-commanded shifts to the next smaller ring when using one of the two biggest cogs with a worn (perhaps with a slightly bent tooth) chain ring in two situations
1. Worn big ring. I fixed that problem with a new crankset.
2. Worn middle ring on a triple with the same chain-line as a single-ring crank. I replaced the ring.
Presumably the same thing could happen to the outside, and obviously a 3/32" single speed ring without down-shift zones would be a better solution for a one ring bike.
I've also gotten the upper and lower derailleur pivots dirty enough that it didn't follow the chain promptly. My problems stopped at chain slap although I suppose more could have happened.