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  1. #1
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    Help with crank length - 89, 102, or 115mm?

    I have justed started modifying a 16 inch wheel bike for my youngest son (despite not being an expert in such things)

    I would like some advice on optimum crank length. At the moment my son is 2 (3 in November) and is 88cm tall (just under 35 inches) with an inside leg length of 35cm (just under 14 inches). The bike is a fairly long term project, so, even though he can ride it already, I will be building it for when he is a bit bigger.

    As far as I have found out, my choices are 89 or 102mm cranks from Spawn Cycles, or 115mm Sinz cranks.

    The 115mm cranks are easiest to find here (UK) but I think they would be too long for him, and probably too long for the bike.

    The 89 would probably be perfect for now, but maybe too small for 6 months (or so) time? The 102mm are out of stock at Spawn at the moment. Also it isn't cheap to get them posted internationally.

    Any advice appreciated. I would like to learn about how different crank lengths would assist him on hills etc. He stands and pedals a lot (does stuff like dirt jumps etc), I'm guessing long cranks would make this more difficult?

    Also any suggestion where I can buy very short cranks.

    Thanks.

  2. #2
    Havok
    Reputation: Jordan300's Avatar
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    I bought some Sinz cranks at 130mm. My son started when he was 3 and 115mm would have been just fine. I'd go with the 115mm.

    If you're like me, as soon as he is comfortable on that 16" you'll start working on the 20". If you can snap pictures of the stuff you're working on, we love to see what you're project looks like.

  3. #3
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    Thanks for the reply. I think I will go for the 115mm. I will start taking photos soon. At the moment I just have a box filled with bike bits

  4. #4
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    Regarding optimum crank length Highpath Engineering : Crank shortening
    Luckily the guy is in the UK, but on a tour until October.

  5. #5
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    That is brilliant, thank you.

  6. #6
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    I just went ahead and ordered the 89mm from Spawn Cycles. It wasn't as expensive as I thought to get them sent surface mail and it will work out cheaper than getting some cut down or buying the Sinz ones.

    I used the Highpath Engineering site to figure out what length he needs and at the moment it would be 85mm max so I think 89mm will do.

  7. #7
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    LOL...same boat.

  8. #8
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    Well a year later he is still using the 89mm with no problems at all. As you saw I am just starting to think about the 102mm ones

    Have you done the calculations on the link? I just did it again and got 92 max length.

  9. #9
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    I didn't see the link until after she was asleep last night, I'll give it a shot today after work.

    How is that measurement different than inseam? WHY DO I HAVE TO CONVERT INCHES TO MM!!!

    EDIT: Inseam vs. leg:
    CRANK LENGTH ? Which one? Bike Fit Steve Hogg's Bike Fitting Website

    Inseam is measured from the ground to crotch. Leg length is measured from the ground to the top of the femoral head.

    est 17" x 5.48 = 93mm

    I need to measure. I think Spawn is the only one offering cranks as short as 89mm/102mm.

    I'd also need a new chainring, hub, and hub tool.

    Again please steer me if I'm missing something.

  10. #10
    Cheapo
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    So I was right about the 17" inseam, based on that, calls for a 93mm crank.

    Using Highpath Engineering, 47cm leg would call for a ~108mm crank.

    10% of her height (40.5") would call for a 103mm crank.

    Yeesh.

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