What size chainring? your edit makes me look like an idiot, and you've changed from 13 23 to 12 21 which seems more TT-centric, so you've probably found an answer for your question elsewhere.
@6pt started
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What size chainring?
your edit makes me look like an idiot, and you've changed from 13 23 to 12 21 which seems more TT-centric, so you've probably found an answer for your question elsewhere.