My bike has a 52/38 Biopace pair on a 5 bolt 130 crank (Shimano 105). I want to reduce the pedal load because I'm getting older and live in hilly Wales, UK. I have bought a 42/28 Biopace pair. Unfortunately the smaller ring has a bcd of 74. I would like to know how to fix the smaller ring. Presumably I need a spider or a new crank. I am expecting to use the existing Shimano front derailleur (FD 1055) which has a 14-tooth capacity. Is this feasible ?
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I don't know of a crankset that has one 130 mm bcd chainring and one 74 mm bcd chainring. However most Shimano triples have two 130 and one 74 chainring. So you can use this and only populate the inner and middle chainring positions. A used one shouldn't cost much.
There's a couple of quirks about using only the inner and middle chainring position. You may need a longer bottom bracket to get the chain-line correct. Also, you'll need either single speed chainring bolts for the 42t ring or some spacers (the standard chainring bolts assume you have two chainrings). Your local bike shop should be able to supply you with all of these.
This will work, I've done this on several bikes.
As an alternative you could ditch the Biopace rings and get a 110 mm bcd triple. (Sugino makes a nice one for about $80 US with 26-36-46 rings). The 110 bcd will give you a wider range of chainrings you can install.