I'm curious myself. The largest road ring I've seen is a 53T,and road BCD's only go down to 30T. MTB BCD's max out at 48T I believe.
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C'dale BBU('05 and '09)/Super Six/Hooligan8and 3,Kona Dew Deluxe,Novara Buzz/Safari,Surly Big Dummy,Marin Pt Reyes,Giant Defy 1,Schwinn DBX SuperSport,Dahon Speed Pro TT,Brompton S6L/S2E-X