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"A" and "B" Chainrings
From WikiPedalia
In Shimano road chainrings, "A" designates 42 tooth, "B" designates 39 or 41 tooth.
All 42s are "A"s; All 39s and 41s are "B"s by definition.
There are two kinds of 53s:
- "A" type that is intended to be used with a 42
- "B" type, which is intended to be used with a 39
50s and 52s are all "B" type. Other sizes have no letter designation.
If you have poor shifting skills, matching the chainrings by the letters will give very slightly smoother shifting, but if you're a reasonably skilled cyclist, it really doesn't matter.
