Old 02-21-10, 08:58 PM
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Bikes: 2016 Hong Fu FM-079-F, 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i (RIP), 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2016 Islabikes Beinn 20 (son's)

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A standard double crankset uses rings with a 130mm BCD which can accept a minimum 38 tooth chainring. In order to fit a smaller inner ring, compact cranks use a 110mm BCD.

Your cheapest option for lower gearing is either a compact crankset (which will require only a new crankset and possibly bottom bracket) or a MTB cassette and rear derailler. The latter will give you wider spacing between shifts but retain better spacing between chainrings. The former is the reverse.

Is the bike you are considering 9 or 10 speed? If 10 speed, the crank swap should be very easy as you won't need a new bottom bracket (assuming external bearings). If 9 speed, you'll likely want to swap out the Octalink bottom bracket for an external bearing BB to allow you to use the newer Shimano Hollowtech II cranks.

Converting a triple would at a minimum involve a new crankset plus front and rear deraillers. Worst case, you'd need that plus a new left shifter and bottom bracket.
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