Kitmotter 0002 Design Files

Now that I’ve put a few miles on Kitmotter 0002 in the form of Johnscooter, I’m more confident in releasing the design so people can build their own versions or modify it.

Ride testing has let me measure Johnscooter’s propulsion efficiency at about 29 Wh/mi. This is a little worse than RazEr’s 25 Wh/mi, but besides having the World’s Trashiest Bearings, the fact that Johnscooter is operating almost entirely in 25A constant-current mode (due to the current-limiting nature of the motor controllers) means I’m basically full-throttling it everywhere. RazEr and RazEr Rev had a more “open loop” voltage drive which let me draw brief bursts of current to get over obstacles at a higher speed where Johnscooter would slowly crawl through the same.

The motor itself has been holding up fairly well for being made of particleboard (more properly, hardboard, which I suppose is rather plastic-y as wood goes). The new stator hub has been holding up with no issues so far, and it was the most worrisome part of the current design.

It has a few dings on it from actual use on sidewalks and bike lanes, and I never did drill those through-holes for the wheel mounting flange. So the wheel is loose on the motor a little, but when rider weigh is pressing on everything there’s nothing that feels strange.

The trashy 5/8″ bearings have loosened up substantially over time – both in seal drag and in…. radial runout tolerances. I can now grab the motor and wiggle it very slightly, and the greatest contributor to the noise spectrum when riding is those bearings. If anyone else builds this, remember that even though I have the Trashy McMaster Bearings listed in the Bill of Materials, real precision flanged 5/8″ bearings should still be seeked out if possible.

Speaking of BOMs, the files are located here and this is what the ZIP file contains:

  • Original design files in Autodesk Inventor 2012 format. Unfortunately I’m not quite so motivated as to provide generic/open source models for everything.
  • Premade DXFs of all motor mechanical parts ready to cut on your favorite 2D Rapid Proto gadget. The materials (e.g. “0250_al” meaning .25″ aluminum) are recommendations, but the rotor rings MUST BE STEEL. Otherwise, feel free to cut the 1/8″ spacer rings and 1/4″ endcaps from polycarbonate for that clear case-mod motor effect or aluminum for extra strength.
  • Basic bill of materials with most hardware and mechanical parts. The tools I used to carve out the wheel center are also included in the BOM. There’s things that are missing from the BOM because they are variable with each design, including the magnet wire, output leads/termination methods, and optional Hall Effect sensors.

There’s some fab considerations which I’d like to share for this particular design, as they might make the motor more durable:

  • Please, please make the endcaps from something which is not wood. Pleeeeeease. (There’s nothing that wrong with it mechancially, I suppose, but just don’t ride through rain).
  • The two 1/4″ discs can be made from four 1/8″ discs – this is in fact what my endcaps above are – if it would make it easier and cheaper to fabricate. For instance, I ordered a single panel of 4 discs plus the “0125” parts from Big Blue Saw.
  • The stator hub is designed to be a loose fit with a standard non-kerf-compensating laser cutter. Waterjet cutters and BBS’ laser services all offset the kerf such that parts come out “true” to dimension. Parts made using this method will be tighter-fitting into the stator and shaft as a result and might need filing or sanding.
  • My final winding is 30 turns (+/- 2) of 9-strand-parallel 28 gauge magnet wire. This is equivalent in area of copper to a single 18.5 gauge wire. Dual stranded 22 gauge wire is also workable and 22ga is probably easier to find. This results in an intrinsic Kt (V/rad/s, or volts generated per rad/s of rotational speed & Nm/A, torque per ampere of phase current) of 0.18 – the Kt will vary a little depending on exactly what drive profile is used (trapezoidal, sinusoidal, sensored vs. sensorless, poor vs. well-timed sensors…). The final line-to-line resistance was 0.31 ohms.
  • Magnet wire is cheaper on Amazon and eBay, but here’s a McMaster roll of 22 gauge.
  • Drilling the wheel flange holes through the tire is probably a good idea. Wood/sheet metal screws, with coarse threads, should be used to fasten wheel to the flange.
  • Really, really crank down the #2-56 axial tie rods (but don’t strip the threads!). #2 hardware will pretty much fail in shear instantly if they’re loose and your weight is on them, which is why I tried to use so many tie rods to compensate. The more these are tightened, the more static friction exists between the layers of the motor, so the more weight they can take from the wheel without shearing the screws.

I expect to see more examples of this construction in the near future. Remember, buildable without big machine tools!

2 thoughts on “Kitmotter 0002 Design Files”

  1. Thank you so much for making these available! I’ve been interested in making hubmotors for a while now, and this seems like a good place to start. Now off to the hackerspace to build one (or a few)!

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