Originally Posted by
FBinNY
...If all is OK the chain shouldn't fall off when back pedaling. Obviously there's no derailleur at the bottom of the chainring to prevent it, but by the same token the chain should stay on the chainring when pedaling (forward) without touching the FD cage, or without the need for an FD at all, such as in a 1x9 setup.
Usually when the chain falls off backpedaling it's because the RD isn't vertical, so the lower loop has more angle (cross-chain) than the upper, and is a sign that the RD hanger isn't square. You can do a quick eyeball check by sighting from the top with the chains directly above each other. The lower loop should be parallel to the upper and able to hide under it nicely.
With a bit of attention any decent mechanic should be able to keep the chain from coming off when backpedaling except, maybe, in the big/big crossed combination.
If the FD and RD throws are properly adjusted you still have to deal with the offset in the chainline, and this is greater for the big/big (and small/small) combos, and unless your hub is limited in size this problem will be hard to correct. The most you can do is to correctly set your chainline, chain length, and derraileur throw set screws, but with larger (wider) hub sprockets, you will still get some lateral pressure on the chain that will tend to force the chain of the CR's when you pedal backwards. This is true for forward pedalling as well, but drivetrains are designed to pedal forward and work better that way...to see this, just try to shift while pedalling backwards... This is especially true for the modern CR designs that have asymmetric teeth...