New answers tagged cassette
From Sheldon Brown: Spoke Protector A plastic or sheet-metal disc that fits between the cluster and the right-side spokes of a rear wheel. This is intended to prevent the derailer or chain from getting caught in the spokes, possibly causing very extensive/expensive damage/destruction to the wheel, the derailer, and the frame. A spoke ...
Its a spoke guard. It prevents the chain from going between the spokes and cassette causing damage if you shift too far. This can only happen on badly tuned gears or old friction shifted shifters.
I can't say for sure, but you have a decent chance of being able to just adjust the body screw on the derailleur to increase its clearance for the larger cassette (see "B-screw Adjustment" on this page). As you've also noted, the chain will be tighter when in that lower rear gear, which might work but more likely than not you're going to have some ...
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