Originally Posted by
Little Darwin
A rear derailleur pulley mechanism is designed mostly to keep tension on the chain, not to align the chain with the chain ring to allow for successful back pedaling. In fact, for the extremes (highest and lowest gears in back) the chain line frankly sucks, and since the front derailleur can only help keep the chain from derailing at the top of the chain ring, it makes sense that back pedaling could lead to dropped chains since it tries to take up the chain on the bottom of the chain ring....
I agree with the basic premise, but disagree that dropping chains when backpedaling is normal or acceptable, except maybe in the most mis-aligned combinations.
The front derailleur is used to effect shifts, but doesn't, or shouldn't, touch the chains in normal operation. So that means the chain is expected to stay on without help, and it does. Therefore, if the line is good enough up top, it's good enough underneath (
if it's the same line).
That said, chain retention will be better on top, because the normal riding load tension keeps it home, and the FD cage is there as sort of a safety in case it wants to derail. So there's not as a reliable prevent to chains falling off when back pedaling, and some will occasionally, but only occasionally. Falling off routinely means that something is wrong and can be corrected.
IMO- chains falling off more than once in a rare while is unacceptable on a derailleur bike.