multimodal commuter
Join Date: Nov 2006
Location: NJ, NYC, LI
Posts: 19,808
Bikes: 1940s Fothergill, 1959 Allegro Special, 1963? Claud Butler Olympic Sprint, Lambert 'Clubman', 1974 Fuji "the Ace", 1976 Holdsworth 650b conversion rando bike, 1983 Trek 720 tourer, 1984 Counterpoint Opus II, 1993 Basso Gap, 2010 Downtube 8h, and...
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The Shimano 105 triple that I have, and no doubt many similar cranks by Shimano and others, have two big chainrings attached at the five 130 BCD holes, and a small chainring attached at a smaller set of holes drilled and tapped into the spider. The older 110 BCD MTB cranks have a similar arrangement. On my 105 triple the rings are 30, 42, 52; and I'm pretty sure you can find a smaller ring than the 30.
That said, I have a hard time imagining you'd ever need such a small ring. I know, what I can or can't imagine is not really relevant to the question, but I had to say it anyway.