Electric Assist Unicycle

In 2017-2018, my friend Jonathan and I made an electric assist unicycle. Unicycles are inherently fixed gear, so a motor can help a rider up steep hills since you cannot downshift. We were not the first to do this; we were number 2! The first people to do it solved some of the hard problems for us, but we still managed to make things very difficult for ourselves; it's kind of my specialty.

The project was completed in two halves: A misguided first attempt in which we destroyed a motor and learned almost nothing, and the second, slightly better attempt, in which we got it working (sort of)!

If you don't like reading, I made a video showing off the finished product. If you don't like my website (🙁), I made an imgur post documenting the process.

Background/The problems

Problem the First: Unicycles are not simply half a bike. Most people who are decently technically savvy can convert a classic bike to an ebike in about two hours. So what is so hard about this? The key difference is that bicycles' wheels have fixed axles and the wheels rotate around them, while unicycles rudely have entire axles that spin. This means that all gearing systems for bicycles do not work on unicycles. This is the only reason why unicycles are not ubiquitous in our society. There is not a way to have a fixed axle while simultaneously having pedals centered at the hub. So we face a duality; we need the hub to rotate so the pedals can rotate, but we need it to be fixed because a motor needs a stationary stator that does not rotate.

Problem the Second: I don't know how to build anything. Remember in the last paragraph how I said anybody can convert a bike in two hours? I am terrible at building things. I overengineer everything and it still does not work. Princeton has a fabulous machine shop with minimal oversight. The fun part about a PhD is being able to decide I've done enough work for the day and go learn fun things.


Schlumpfs already solved the duality problem. The only commercial gearing system available for unicycles is called a Schlumpf drive. As with popular internally geared bike hubs, it operates via a planetary gear, which requires a gear to be fixed, while simultaneously allowing the hub to rotate.

The first attempt, or "Why precision matters"

Following Grin Tech's build, we started with a regular ebike motor. It turns out they come waterproofed which is basically a glue meant to keep me out. Jonathan and I used four hands and 3 feet to get the stator and rotor apart. Not only are the permanent magnets on the stator holding everything together, but the axle is very tightly press fit.

We have a stator! Removing the axle was not an easy feat though. The wires are threaded through the axle, then soldered to the Hall sensor board. We cut the wires. We use a press but it’s not working. We end up using a press with an extremely long handle and a hammer. It’s finally out. First things first, let’s remove the center. For protection, we taped up the outer rim of the stator. Then, using a drill press, we cut a 45mm hole in the stator.

We eventually will weld the hollow stator support into this hole. But first we needed to do some basic prototyping. We 3d printed a thing that looked nothing like our final stator support, thereby giving us zero useful information. But we got to try out a 3d printer so that was cool.

Hey it looks like we're professional!

I apparently forgot to take pictures of the machined stator support! Okay so I needed to weld it into the giant hole in the center of the motor, but first I had to remove the paint before welding.

There we go, there's the stator support, welded in. The best part about wedling is how pretty the colors are, so satisfying!

I needed a new spindle to bolt to the side of the motor, but I needed to remove one of the flanges so it would fit through the motor. I took a nice chunk out of my thumb in the process. Unfortunately this spindle has square tapers but more modern unicycle spindles aren't quite the right shape. Sidenote, modern spindles are called ISIS hubs, which is very mildly silly.

Time to modify the side plates! One needs an 85 mm hole to go around the 85mm x 45mm bearing that goes around the stator support. Sidenote, an 85mm bearing feels super massive, and had this version of the motor ever run, I have doubts about the longevity of one.

Lastly, we need to cut a passage in the stator support for the wires to get out...probably should’ve done this before welding but hindsight’s 20/20.

It's time to BUILD it! Here are all the parts. We had to do a few small adjustments with a dremmel, but it came together.

It's built, BUT bad news...it’s rubbing and won’t rotate freely. Why?!? Even though I’m smiling here, I’m not happy, this motor won’t turn. I try many things over the next few months to try to get it to spin all the way around, but it simply won’t; the welded stator support is out of center. I could try sanding down the rare earth magnets but I’m not hopeful; I'm a bit lost on this project. Luckily, the motor I REALLY wanted to use all along was 50% off due to a slight manufacturing error, so I snagged one. It’s time to start over with this new motor.

Unicycle Motor Build 2: Electric Boogaloo

Realization! I don't need to open this motor up, as long as I can build a stator support that bolts to the disc brake holes. I gave myself a constraint that the whole thing needed to be 100mm wide because that's how wide a standard unicycle fork is. In retrospect, I should have just bought a wider unicycle frame, as we'll see later.

A few hours in the shop and we’ve created the stator support connector. I email the folks at Grin and they’re happy to supply me with the design specs for the spline on the motor siding so I can recreate it via CNC. As I’ve never used CNC, Larry, who runs the shop, agrees to CNC it for us.

We come back a week later and oh no Larry has done the reverse of the cut we wanted...unfortunate misunderstanding!

Another few hours in the shop, and we’ve created another stator support, another week later and Larry has CNC’d the splined connection for us.

We cut a slot for the wire and then cut more holes in the bearing holder part of the stator support for the screws to access the holes through. It fits wonderfully!

We put our spindle through the motor and bolt it in. It takes some spacers and we diagnose some rubbing to shave off a millimeter here or there.

We had never laced a rim before. It's a little difficult, and moreso when the bike shop gives you the wrong length spokes. It took a few tries.

We wire it up and it's done! Here I am after a terrified ride in the dark. My shoddy machining work means this thing has about 1 unplanned dismount in it before it breaks.

She sees her finest hour with Ed Pratt nearing the very end of his trip around the world. We took it up an incredibly steep hill. There will be footage here when he releases his "Ed Unicycles the USA" series.