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BCD or bolt circle diameter defines the max and min rings - generally. If you have less than 38t on the small ring you probably have 110 bcd.
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The distance between the bolts, usually measured as the bolt circle diameter or bcd, determine what size rings you can use. The other factor is availability, i.e. who produces what chainrings to match the bcd your cranks have.
Most front derailleurs can handle a variance of 16 teeth between the outer and inner rings.
Added: FWIW, I think the smallest chainring you can get for 110 bcd cranks is 33 teeth.
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what defines which combo can or cannot be used?
Front dérailleur is the main thing. Road fronts these days are mostly designed for a maximum difference of 16T, don't really have a minimum difference, but do like the big ring to be big.
Any combination of 32/34 inner and 46/44/42 outer will work, but at the lower end of the scale of outers the tail of the dérailleur cage will be a long way from the ring diameter so upshifts will be slow.
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There are two limiting factors. The first is the PCD of the chainring bolt holes, which will define the minimum chainring size. 39t is the smallest (realistically) you can go with a 130mm PCD (standard), 34t for 110mm (compact). The second is the difference between the two chainrings in the number of teeth. This depends as much on derailleur design and geometry as anything, but a 16t maximum difference is a reasonable starting point.
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Howard
andyp
gbj_tester
Brommers
@scorch
Hi everyone,
N00b question on road doubles. So common chainring combos are 53/39, 52/38, 50/34 and 'cyclocross' 46/36. Some manufacturers would also use 48/34 (came stock on my AWOL for example). Here's the question - what defines which combo can or cannot be used? I'm thinking of using double with 34/32 inner and would like to understand if 46/44/42 can be used as an outer?
Thanks