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Join Date: Aug 2010
Location: Madison, WI
Posts: 11,177
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
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I have had bad knees for the past half century. Often on longer rides I have to add a patellar band to limit knee pain part way through the ride. I have 170mm cranks on a vintage bike, have 175mm on the rest of my bikes. After I do a long ride, I find that walking up a set of stairs seems to be easier on my knees after riding my vintage bike with the shorter crank, but there could be other factors, this is not a scientific test. A 5mm crank arm difference is roughly a 3 percent difference in torque on the crank.
As noted above by several others, cadence would likely rise on shorter cranks and you might find that you are favoring a slightly lower gear since your torque on the crank is reduced slightly, if you are doing comparisons on a virtual training machine with adjustable crank arms, you might be able to measure these differences.