There are two different solutions here, a geared solution and a belted solution

The gear solution worked well for me originally but turns out my motor wasn't fully concentric so the gears would fail to mesh and lose encoder steps causing issues, it works fine if you don't have any concentric issues.

There is also a belt solution, I preferred this one but its a bit more work
First you will need to file entire the lip of the hoverboard shell so it is flush (see pic with red line, I no longer have this setup so I can't take pics) otherwise the belt will collide

You are going to want to get a closed loop GT2 Belt, what you need is a 
348mm GT2-6mm belt (I personally used 170T/340mm belt but that felt a bit too short)
The tooth count doesn't matter at all

To tension the belt you'll want to raise the encoder and tighten it down (I'd prefer if there was an idler on a spring or something)

PPR Calculation: Encoder cpr * ( big tooth / little tooth) * 4
eg: for belt with 1000 PPR = 1000 * (120 / 20) * 4 == 24000

The part that needs improving is the “belted encoder holder for hoverboard shell”
I originally  made a bracket out of my old ender 5 stepper holder but that cracked from the tension, so I designed this part but I never got to test it, it may need reinforcement but that's for you to decide.

Orientate on the flattest side down, print with 4 or more walls for strength, 20% infill is fine

You can ask questions but I've moved over to a MIGE setup now and got rid of the hoverboard so I don't know how much help I'll be

Changelog:
Edit 02/07/2025: Rewritten Description, re-organized project, added belt solution

Edit 09/01/2024: Reuploaded encoder_gear as step, you can import them directly to your slicer (mine is orcaslicer)

Edit 08/01/2024: Reuploaded encoder_holders step, you can import them directly to your slicer (mine is orcaslicer)

Edit 07/01/2024: Added a new encoder holder, if your motor mount plate's bolts don't sit flush with it (like in the photo) print "encoder_holder_with_bolt_cut_outs" instead

Special thanks to chrisbeeves and the ffbeast discord https://discord.gg/CejjtpBg for providing help and the initial motor gear files

Credits for files taken:

https://www.printables.com/model/568306-120-tooth-gt2-pulley-for-65-hoverboard-motor

https://www.thingiverse.com/thing:16627

Category: Video Games