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A one piece crank (US style 73mm) conversion to Bafang motor

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    A one piece crank (US style 73mm) conversion to Bafang motor

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    Hi All,

    My first post today .... looks like a great forum!

    I am looking to do a build (Bafang mid drive) on a 20 year old bike.

    This bike is something I really treasure so I don't want to buy a cheap donor bike for conversion etc. If I could do a build on this bike then that would be great.

    The issue I have is that my bike currently has:

    A one piece crank (US style 73mm) - I think in the US they call these Ashtabula cranks. The internal diameter of the Bottom Bracket is likely to be 40mm+ (I haven't removed this yet but have done a quick measurement so don't have the precise measurements).

    I understand that the Bafang motors expect 33.6 - 33.9mm diameter BB so clearly I would need an adapter to reduce the BB internal diameter down to this value.

    I recently watched a YT video where someone converted a similar bike (same brand and BB to my own), to a modern style Euro BB using an adaptor. I think the adaptor was called "Bottom Bracket Adaptor Truvativ BMX American to Euro Converter". So I was wondering if I converted the BB using something like the Truvativ BMX American to Euro Converter and then used this for the Bafang whether or not this would solve my problem.

    My question is :

    Has anyone converted a bike with this type of crank and/or is this going to be a nightmare and best avoided etc ?

    Any help appreciated.
    Last edited by zaffy; 03-22-2024, 06:01 AM.

    #2
    That should be it. Only area of concern would be keeping it from rotating. Normally there is that plate that has teeth that kinda dig into the frame but with this setup they will be into that adapter. Maybe see if those screws that hold the sections together will happen to run into the ends of tubes that extend into the BB area? It would be great if maybe one tube stuck far enough in that you could notch it for that screw to go through it and keep it from rotating. IF that does't happen to work out then you may have to go with some sort of torque bracket.

    Brakes would be the area of concern with a bike like that especially if you want to go fast(ish). Experiment with several pad types and brands to find the ones that work best and use some high end cables so at least you won't loose anything in the cables.

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