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Fitting hubmotor in imperfect dropout spacing

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    Fitting hubmotor in imperfect dropout spacing

    Question was posed to Luna Support this week regarding a hub not fitting within dropouts.
    Ideally the dropouts should be measured to prevent this but sometimes the hub is a nonstandard size or some other reason for needing a workaround.

    Here is the pic sent in, about a 10mm difference:

    Click image for larger version  Name:	i726^cimgpsh_orig.png Views:	1 Size:	4.60 MB ID:	63994

    We have two approaches here for getting this to fit. Either spreading the dropouts, or filing down the axle so the flat portion extends further inbound.

    Spreading

    This is certainly possible on steel frames, and doable on modern aluminum frames as well to a certain extent. The only thing it is impossible on is a front suspension fork.

    Please see this link for a discussion on spreading dropouts.

    The techniques range but the basic idea is the same, to make the dropouts farther away from eachother.

    Filing the axle

    Click image for larger version  Name:	image.png Views:	1 Size:	1.00 MB ID:	63996

    This would be a bit more tricky as you would want to carefully file back enough of the rounded portion of the axle so that it is flat like the portion that is further outbound.
    • Take care not to damage the threads while doing so
    • Be mindful not to make this flat portion thinner than the existing flat portion, it needs to be a tight fit within the dropouts
    • Do not file so far inbound that you run out of thread on your nut when tightening

    If you do find that you are close to running out of thread when filing back a good portion, use a washer or two and you should be fine.


    Ideally in this situation you would use a combination of these two approaches. File back enough of the axle that you reach a length you are comfortable spreading the dropouts to, then proceed with spreading. It would also be advisable to use a torque arm on both sides if using this approach just in case the extended flat portion was uneven so the motor does not potentially spin out of the dropouts.
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