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
Mentioned: 35 Post(s)
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This is common on almost all modern chainrings except those for single speed, or intended for the innermost position of doubles and triples. It's a shifting gate allowing the chain to move on and off the ring easier during shifts. You don't need it on the innermost sprocket because the chain is always lifting up from those, and never coming from a smaller neighbor. You'll notice similar cutouts, and/or thinning of certain teeth on each sprocket of any modern cassette.
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