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  1. #1
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    Are 175mm Cranks too long for me?

    I have been running Truvativ Firex 170mm on my mountain bike for 5 years and I have a set of Shimano LX octalink 175mm cranks that I want to upgrade to. I am having trouble finding a set of 170mm cranks to swap with the shop. My inseam is about 31-32. I am 5 foot 8 inches tall. I weigh about 75kg. Would 175mm cranks be too long for me? I have heard people say that it will damage my knee and my legs.

    I have a set of 175 Ultegras that I run on my road bike and they feel okay but my coach said that there will be long term damage if I run them too frequently. They do feel harder to pedal but I think that's just because of the gear ratio on double chainring cranks and the fact that I am running a 11-21 cassette. Is that really true?

  2. #2
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    Reputation: Bikinfoolferlife's Avatar
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    There are recommended crank lengths for various height and inseam measurements but that still doesn't take into account all factors, including which bike you're riding and what advantage you might want. For road riding I use a 172.5, for mountain 175 and I pretty much went with the prevailing recommendations when I got my bikes as an adult, but I'm a fairly average person in size so it worked pretty well for me. For a short time I had a small mountain bike with 170s but that whole bike felt small, so I never looked back when I broke it. I did try 177.5s on a single speed for a bit (good deal on the crankset) but my knees said they weren't working. Use a cycling inseam (not your pant inseam) when using the recommendations just like you would for frame sizing.

    FWIW that same prevailing wisdom usually advises a longer crank is recommended for mountain biking vs road biking for increased leverage at lower rpms. But I'm sure there are some that would have it the other way (like your coach?). My knees get the last say in any case.

    Just google for recommended crank length bicycle or similar and you'll get lots of recommendations and calculation information.
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  3. #3
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    It sounds to me like a 175mm crank would be the perfect size for you. The benefit of longer cranks for mountain biking would be leverage.

  4. #4
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    I have a 32 inch pant inseam and 175 work for me.

    I did take two weeks to get the knees used to it, but now no problems, and I do like the extra leverage in granny.

    I am looking for a 46 tooth chainring, to go faster, not sure this has anything to do with crank size, but your feet do have to move more.

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