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  1. #1
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    Shortening crank length and affect on gears

    With all other components remaining the same in the drive system, if I change to shorter cranks how does that affect my gearing? Im wondering if there will be a noticeable difference in the effort it takes to turn a particular gear with say a change from 175 down to 170mm cranks. My current bike has a lower BB than I am use to and I hit the cranks on the ground more than I would like.

  2. #2
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    You can go to Sheldon Browns web-site and get the technical data. (Sheldon Brown's Bicycle Gear Calculator)
    I could feel the difference from 180's to 175's. The longer crank gives more leverage but you can't spin it as fast (rpm wise) as a shorter crank. There was a noticeable difference in strikes with the longer crank as well. The longer crank felt like having a slightly smaller ring up front.

  3. #3
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    Shortening crank length and affect on gears

    Quote Originally Posted by Pivot4 View Post
    With all other components remaining the same in the drive system, if I change to shorter cranks how does that affect my gearing? Im wondering if there will be a noticeable difference in the effort it takes to turn a particular gear with say a change from 175 down to 170mm cranks. My current bike has a lower BB than I am use to and I hit the cranks on the ground more than I would like.
    Crank length changes the feel of the drivetrain no matter the gearing. Longer cranks do not have the same affect/feel as a lower gear ratio, nor are shorter cranks the same as a higher ratio.

    I do not take the crank length into consideration when making gearing choices, and I have more trouble spinning a 165mm crank quickly than I do 185mm cranks.
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  4. #4
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    Should I expect a big enough difference to the point that I should look at getting a different cassette to get the same range of gearing that I am accustomed to? Or do I understand correctly that the two are almost independent of each other? gearing and crank length.

  5. #5
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    Cranks act as a force multiplier so there is a relation between crank length and gearing but because there is also an ergonomic factor here they're not normally a factor in considering gear ratios. Going from a 175 to 170mm crank will raise gearing by about 2% which is nothing really noticeable so consider other stuff first...

    Pedal strikes are usually the result of incorrectly set up or matched suspension meaning that your bottom bracket is too low so if you have suspension look at that first and if that's not the problem then look at your crank arm lengths. I would advise that you keep within normal parameters for your height/inside leg though, nothing extreme.

  6. #6
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    Shortening crank length and affect on gears

    Quote Originally Posted by Pivot4 View Post
    Should I expect a big enough difference to the point that I should look at getting a different cassette to get the same range of gearing that I am accustomed to? Or do I understand correctly that the two are almost independent of each other? gearing and crank length.
    They are not independent. Shorter cranks do reduce the force applied, and that can be compensated for with a lower gear ratio. But with different crank lengths you are always applying the force in a (slightly) different manner regardless of the gear ratio used. You may or may not notice a difference going to 170 from 175. I do, and am not comfortable with cranks under 180mm for any length of time (and prefer 182-185). I have one bike with 175s and another with 165s. Both feel like I am trying to walk with my shoestrings tied together, or running up steps that are just a little too short.
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  7. #7
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    Thanks for putting in that last sentence! I now have a better grasp on the issue.

  8. #8
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    As has been said, all kinds of factors apply here. I'm 5'7" and have a 30' inseam. I started with mountain biking with 175's in the early 90's and I always felt like a 170 might fit better, but everybody I talked to told me it wouldn't make much of a difference. I started by putting 172.5 cranks on my road bike and I only felt a slight difference. Later, I went to 170's on my mountain bike. I feel more balanced on both the downhills and the climbs with the 170's. I don't think I changed the Q-factor much with the new cranks either. It feels like I lost a little torque for the steep technical rock crawls, but the shorter crank has resulted in far less pedal strikes on my rocky trails. My Turner Flux doesn't have a crazy low bottom bracket either. Maybe I am just more careful because I care about my new cranks? Who knows? The point is that you will probably feel a difference, but you won't know how much until you make the change. It's one of those things that you need to try to know. It is not so severe of a change that I feel like I need to change the gearing on my cassette though.

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