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My five years old little girl rides a BMX micro mini bike with 18 x 1" wheels. The original bike came with 130 mm crankset length, 38 tooth chainring and 14 tooth rear cog.

However, looking at her on her trainings, came to my attention the way she was pedaling. After reading a lot in the web I found out that there are shorter crankset for small rider. Due to the fact that I can´f afford $150 US dollars crankset plus shipping to my country, $70 US plus customs fee. I decided to cut and shorter a 175 mm crankset arms to 110 mm, and I did a great job.

So, due to your expertise and knowledge, I would like to ask you which chainring and cog could you recommend for my daughter.

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  • How exactly did you cut the crank?
    – Batman
    Commented Nov 22, 2016 at 5:11
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    @Batman - It's relatively easy to saw off a crank and drill/tap a new hole, especially on cheap one-piece units. (I've seen at least one example of this.) There is a question (depending on the specific crank) of whether there's enough "meat" in the location of the new hole to produce a solid connection, but a 5-year-old does not put a lot of stress on the crank.
    – Daniel R Hicks
    Commented Nov 22, 2016 at 12:18

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Gearing is a very subjective question.

Using Sheldon Brown's Gain Ratio Calculations, 36:17 on 110mm cranks will give a 1.25% lower gain ratio than 38:35 on 130mm cranks.

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