Join Date: Nov 2003
Bikes: Leader home built hardtail, Diamondback Response
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Both will work, but nominally, the 10 tooth is a replacement for 8 speed and earlier equipment and the 11 is for 9 speed. The reason Shimano changed was to have an odd number of pully teeth engaging an even number of chain links so that on every other pass any given tooth on the pully would engage an inner chain link or an outer. This is supposed to improve pully tooth wear.
Remember when you do the replacement, that the upper pully does not get the shim washers that the lower idler pully gets. This allows the pully to have a little lateral float as the chain moves around.
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