Join Date: Aug 2005
Location: New Hampshire
Bikes: Gunnar Sport, Soma Double Cross Disc
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 22 Post(s)
It sounds like you have a compact double, as you can't fit a 34 inner ring on a standard road double. 34/50 and 36/50 are fairly common sizes for compact doubles.
The difference between a compact and a road (or racing) double crank is the "bolt-circle diameter" (BCD) of the crank. This is the diameter of the circle formed by the chainring bolts On a compact, the bolts sit in a circle 110mm in diameter. On a road double it is more like 130mm (135mm for Campy). With a 130mm BCD, you can't have an inner chainring smaller than, I think, 39 teeth, hence the need for the compact setup. I don't think there's any prohibition on running larger chainrings on a 110mm BCD crank (assuming someone makes them in the sizes you want). I suppose stiffness could be an issue with really large rings on a small BCD crank.