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Cranks: $100 for a lousy 2.5mm

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Cranks: $100 for a lousy 2.5mm

Old 01-14-20, 07:53 AM
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Cranks: $100 for a lousy 2.5mm

I never paid any attention to crank length until last week, when I bought a newly affordable Stages left Ultegra crank power meter for $350. I learned that my one-year-old Trek Domain has 175mm cranks, while my nine-year-old Roubaix, now my Zwift bike, has 172.5mm cranks. A few minor knee twinges in the past year, 4000 miles, on the 175mm cranks. Im 71 inches tall with a longer-than-average inseam of 34.5 inches; by height I should have shorter cranks, by leg length longer ones.

So here is my question: should I just install my new 175mm crank-meter and relax with a couple of Dogfish Head triple IPAs, or should I exchange it for a 172.5mm (or even 170mm) crank-meter, buy a matching right-side Ultegra crank for around $100, and THEN have the beers?
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Old 01-14-20, 09:21 AM
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Height and even leg length are not the final authority on what your crank length should be. Since our muscles are strongest at full extension, what is most important is that the saddle height is correct for the crank used, and this allows a range of crank lengths to work fine. I have a mix of 165mm and 170mm cranks on bikes. Use what feels best.

Have a delicious beer now.
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Old 01-14-20, 09:24 AM
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If something on any of the bikes I ride hurts me, I try to fix it. For $100, it's worth it to not be in pain.
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Old 01-14-20, 09:42 AM
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Going by your inseam, you could profitably spin 190 cranks. I have a 30" inseam and run 170 on most of my bike and 175 on our tandem. Can't even tell the difference, which makes total sense, since 5mm is about 3/16", nothing. Your overall height has nothing to do with crank length. I'm 5'6". Put on your new cranks. Check you saddle height with the heel-on-pedal method.
Results matter
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Old 01-15-20, 09:50 AM
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No beers (forfeit!) if the o.p. dares to squander $100 for a 2.5mm change in crank length. The width of this 'I' is just about 1mm. The manufacturing tolerance of a crankset likely is around 1mm.
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Old 01-15-20, 11:02 AM
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"A few minor knee twinges" may or may not be related to the crank length, but interesting you say this occurs only on the longer cranks? I have read several things in the last few years that suggest "standard" OEM cranks might be too long for people. Here is one such article:


I did replace my wife's crank, dropping from 170 to 165 and she says it has helped her knees a lot. I'm 73-1/2" with slightly shorter legs than you, both my bikes (58/59cm frames) came with 175s, I've not had any issues.
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