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  1. #1
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    Need help figuring out proper crank length for my 2 boys

    I have been trying to figure out what size cranks for my boys 2 mountain bikes. I keep getting told different sizes by the LBS.

    Here is the info I have:

    11 yr old boy
    57" tall
    26" mountain 13" frame

    8 yr old... Very small for his age
    45" tall with shoes on
    20" gt stomper. Stock cranks are 145mm, but his pedals contantly hits the ground.

    Any help would be great

  2. #2
    I'm SUCH a square....
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    11-y-o can roll with 170mm crankarms. My 9-y-o is good with them, he's not quite that tall.

    I am a little bewildered with the 8-y-o's bike issue; 145 cranks hit the ground? 20" bikes routinely come with 152mm, no problems.
    A bike is the only drug with no bad side effects....

  3. #3
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    The 20" mountain bike is new to my son.. On flat surfaces he is fine when turning, on trails he is having more of a problem with them hitting the ground. We are all fairly new to mountain biking so it may just be a learning curve for him. Since he was so small I was worried the crank was too long, so should I even bother getting him smaller cranks?

  4. #4
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    When I was buying the shortened cranks for my son's 24 inch bike and getting the cranks shortened on my daughter's 20 inch bike, the recommendation I got from Mark Stonich at BikeSmith Design & Fabrication is 10% of the child's height +5mm.

    So my son is on 140mm on his 24 inch bike and my daughter is on 130mm cranks on the 20 inch bike
    13 Lenz Lunchbox punkass

  5. #5
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    For some reason my last post didn't go through.

    I did the calculation as you said. I came up with 119mm cranks.. That seems very small. My sons 16" bike are 117mm if I remember correctly and those look tiny. They may fit his mountain bike as a test fit, but I am worried they are just too small. With the stock arms, he is struggling to get up small inclines.

  6. #6
    Wēk Ss
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    8yo boy on 24", 152mm
    6yo girl on 20", 145mm (or possibly 140mm). should size down, it's a bit big for her, something around 130mm would fit better.

    pedal strikes on a 20" I've not seen before, unless he's seriously leaning, counter steering, and pedaling or he's riding through chunk. you could go shorter, but 45" tall should be getting near the end of a 20" bike.

    kids grow really fast, I wouldn't spend too much time on getting an exact match. they will outgrow an exact match really soon.

  7. #7
    Clyde on a mission!
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    Quote Originally Posted by IAmHolland View Post
    kids grow really fast, I wouldn't spend too much time on getting an exact match. they will outgrow an exact match really soon.
    This. As long it's not something ridiculous where they almost hit their chin with their knees when pedaling I wouldn't worry too much.

    If your kid grounds his pedal on the inside while turning, show him a video or two on youtube about how to corner with the outside pedal down, leaning the bike while keeping the body vertical.

  8. #8
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    Quote Originally Posted by IAmHolland View Post
    kids grow really fast, I wouldn't spend too much time on getting an exact match. they will outgrow an exact match really soon.
    True, but what some of what I've read seems to show that shorter is better than too long.
    13 Lenz Lunchbox punkass

  9. #9
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    I'm basing this on BMX crank lengths being the same as MTB.

    Ideally, it's not on the total height of the child, though that measurement can be used b/c it's more readily available by a lot of people. Best is to use the child's inseam.

    From my tech page for our local race scene is this:

    Crank Arm Length Chart

    Wondering what size crank arm is right for you? Check out the chart below and maybe it will help narrow down those decisions.
    Rider Inseam to Crank Arm Length
    <15" Custom
    15" 115 mm
    16" 120 mm
    17" 125 mm
    18" 130 mm
    19" 135 mm
    20" 140 mm
    21" 145 mm
    22" 150 mm
    23" 155 mm
    24" 160 mm
    25" 162 mm
    26" 166 mm
    27" 171 mm
    28" 175 mm
    29" 177 mm
    30" 180 mm
    31" 182 mm
    32" 185 mm
    33" 185 mm
    34" + 190 mm

    I don't like some of that chart, but overall, it's pretty good, especially in the lower 2/3 of the range. I'm a 34" inseam and 170 is what I need, so I'm definitely against the upper range.
    7 yo BMXer/MTBer -modded 20" GT Stomper.
    2 yo BMXer -Haro Micro and custom 3spd Specialized.
    '97 Gary Fisher Aquila.

  10. #10
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    Excellent post HFM!

    Spot On!


  11. #11
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    Thanks for that chart. We are about 10mm too long right now, but I am going to wait until spring to see if he grows any more. With winter coming we only have a few more times we will actually ride the trails.

    We went to White Clay Creek this weekend, and overall both of my boys did great. It was our first long trail. Oldest son did great with the stock cranks and did a few parts of the skills park. Youngest son really surprised me. He did 75% of everything we did, but his hands are small so he couldn't change the gears on the uphill enough. Figure I need to address this first. Thanks everyone for their information.

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