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  1. #1
    Senior Member xfimpg's Avatar
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    Bike fit question after crank length change

    Hi
    If you are fitted on a 170mm crank length, but then change to a 175mm, do you lower your seat by 5mm to compensate for the difference? How's my math?
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    I just wanna ride my bike.

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    Quote Originally Posted by xfimpg View Post
    Hi
    If you are fitted on a 170mm crank length, but then change to a 175mm, do you lower your seat by 5mm to compensate for the difference? How's my math?
    The difference is +2.5 mm at TDC and at BDC, lower the saddle 2.5 mm.

    Brad

  3. #3
    Uber Goober StephenH's Avatar
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    No, the difference is +5 mm at TDC and BDC. Anyway, lowering 5 mm sounds good, or just adjust till it feels right.
    "be careful this rando stuff is addictive and dan's the 'pusher'."

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    5mm sounds good. Just making sure to keep proper extension of leg.

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    The pedal, at full bottom, is 5mm lower. To lower your saddle 2.5 will risk some strain in your legs (most likely knees); small diffs make big issues, I have found.

    How did you 'get fitted' to the bike before the crank change? The heel-on-pedal method tells you to drop 5mm. The mathematical methods would likely come in closer to 4mm.

    Drop your saddle 4.3mm. Or whatever hit-and-miss tells you WORKS.

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    The pedal is also 5mm further forward at 3:00. Move your saddle forward 4.32mm to compensate.

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    Senior Member xfimpg's Avatar
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    I agree with the general consensus of lowering 5mm and forward by 2-5 to 5mm, depending on feel. Thanks for your input guys.
    ______________________________________________

    I just wanna ride my bike.

  8. #8
    Senior Member bud16415's Avatar
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    There is always the thicker peddle method also or shoes with thicker soles. Winter socks might also compensate.

    Have you noticed much difference with the longer crank length while riding?

    5mm = .2 inches
    What's not in your legs needs to be in your gears.

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    Senior Member xfimpg's Avatar
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    Quote Originally Posted by bud16415 View Post
    There is always the thicker peddle method also or shoes with thicker soles. Winter socks might also compensate.

    Have you noticed much difference with the longer crank length while riding?

    5mm = .2 inches
    Quite negligable to be honest. I've had 170, 172.5 and 175 in the past, I found that it allows me to spin a bit more going to a 170. This is helpful because I am a masher.
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    Senior Member bud16415's Avatar
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    Quote Originally Posted by xfimpg View Post
    Quite negligable to be honest. I've had 170, 172.5 and 175 in the past, I found that it allows me to spin a bit more going to a 170. This is helpful because I am a masher.
    I’ve rode a lot of different crank lengths and like you I didn’t notice much between 170 and 175. About 3/8 of an inch total in diameter is well within my comfort zone and dropping one gear seemed to spin me up more than crank length did.
    What's not in your legs needs to be in your gears.

  11. #11
    coprolite fietsbob's Avatar
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    thicker soles on your shoes will do it.

  12. #12
    old fart
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    Done this a few years back.
    Crank length 170 -> 165
    I think I have had a specific thread on this - use the search function.
    Lower the saddle 4-5 mm.
    Fore/aft is optional, depends on cockpit aim and changes in body flexibility.

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    Quote Originally Posted by xfimpg View Post
    I agree with the general consensus of lowering 5mm and forward by 2-5 to 5mm, depending on feel. Thanks for your input guys.
    Don't bother adjusting the fore/aft of the seat - the pedals do go more forward by 5mm, but also more rearward by 5mm. The seat height is the only adjustment that is really affected, and you should just set it up so you get your proper leg extension at BDC.

  14. #14
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    OOps! I read 172.5... sorry for the mis information!

    Brad

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