Join Date: May 2004
Location: Wilmington, DE
Bikes: 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i (RIP), 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
Mentioned: 2 Post(s)
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Quoted: 295 Post(s)
Crank arm length really only affects your pedal stroke. A long crank means your leg has to flex more to rotate the cranks in a full circle. If you are using too long of cranks, your knees will be too flexed at the top of your stroke and your knees will hurt similarly to how they'd feel with your saddle too low. If you are using cranks that are too short, you are losing potential leverage to power the bike (Torque = force x distance: same force applied to a longer crank arm gives you more torque). The frame couldn't care less though. As reverborama notes, the only other thing somewhat affected by crank arm length is your cornering clearance.
If you Google "crank arm length" you'll likely find some articles talking about how to determine what crank lengths are most appropriate for your body. The measurement is based off your inseam.