Junior Member
Join Date: Oct 2017
Location: Ottawa, ON, Canada
Posts: 151
Bikes: 1974 PX-10E sold, 1977 Witcomb stolen, 1980 Roberts 1 speed, 1987 Cyclops 3 x 6 friction triple crank, 2010 Masi Commuter 1 speed, 2017 Ribble 525 2 x 10 with Ergos
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If the crank is really 130/74 BCD, then the OP can add a 74 mm BCD granny ring as small as 24T easily. The bolt holes are there. Ignore or remove the outer ring if high gears are not needed, and the rear derailler will probably be fine. Lower the front derailler, shorten the chain, and (probably) fit a bottom bracket cartridge about 7 mm longer. For best shifting to the 39 T, using a "middle" chainring will provide shift aids. 39T may be too small anyway, an excuse to look for a 44 or 46 tooth "middle" ring for use as the bigger of two rings. Specialities TA or Spa Cycles. Going triple (ie keeping the outer ring) will definitely require a long cage derailler and a longer chain.
I am running a Campagnolo 135/74 mm BCD triple as a 46/30 tooth double, and it's great for me at 67 and still climbing.. I have the rings for two more "downgrades" as time passes.
oldschoolbike