You just got a new bike with 175-mm crankarms instead of the 170s on your old bike. Now your knees hurt. Are the longer cranks to blame?
It depends on what sort of knee problems you’re having, so the short answer is—maybe.
When you switch to longer crankarms, your knee must extend more at the bottom of the stroke in order to reach the pedal because it’s farther away (5 mm, in this example). But your knee also bends at a greater angle at the top of the stroke because the cranks are 5 mm “taller” than they were before.
Both of these situations add stress to the various structures of the knee. One fix that some riders try when they install longer cranks—lowering the saddle 5 mm, again using our example—takes care of the added extension at the bottom of the stroke but means their knees are bent even more at the top.
A Couple of Suggestions
- If you developed knee problems after switching to longer cranks, try re-installing your original cranks to see if the pain stops.
- Stick with the longer cranks and ride easily to let your knees adapt. Keep in mind that many riders switch back and forth from 170s on their road bikes to 175s on their mountain bikes and don’t have knee problems. Perhaps some other factor is irritating your knees. You’ll have to experiment to eliminate potential factors one-by-one.