Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
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derailleur systems are designed to tun in one direction only. For them to work the guide (ie derailleur) has to be before the sprocket, otherwise the sprocket is trying to shift it in the derailleur. If you can't visualize this imagine trying to deflect or block a pass while standing beyond the receiver.
The reason your system auto shifts when backpedaling in crossed combinations or from the innermost cassette sprocket is that the unguided chain is trying to find a straight line path and so will try to change sprockets to one closer to the side it's coming from. Sometimes new systems are more forgiving, but with chain and sprocket wear they might get less so. OTOH, I've seen new drive trains that were extremely unforgiving of backpedaling while crossed mellow as the sprockets wore in.
There is no adjustment or modification that can change this because the chain is running unguided in reverse, but if you get used to how it shifts, you can drop the RD one or two shifts so it matches the sprocket the chain has selected, thus avoiding the jam.
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