Sunday, September 19, 2010

Pneu (New) Scooter

And the winner is...

...the skinny pneumatic wheel.

After careful consideration, this is the wheel of choice for my new kick scooter project, beating out three other options. It addresses one of the most significant problems with BWD: the flat, hard, urethane wheel treads. A pneumatic tire will afford much better ride quality, not to mention vibration and shock absorption for the frame and electronics. And at 6", it will be able to clear larger cracks in the sidewalk or the road. But, it's still closer to the kick-scooter look and feel than large electric scooters with 8-12" wheels.

Pneu Scooter, as it will now be called, is my new attempt to play with the engineering trade-offs of electric scooters. Size, weight, range, power, noise, ride comfort, and durability are just some of the interdependent knobs you can turn. I wish I could make a cool visualization to show how I'm (blindly?) feeling my way around this design space, but I can't, so here's 1,000 words instead:

Starting with size. I should be very clear that I'm making a kick scooter. A kick scooter is like a Razor scooter or a Xootr, not like the kind you sit on. Going a step further, I'm not interested in a bulky/wide electric kick scooter such as...well, pretty much every commercially available one. Stealthy electric kick scooters are all the rage these days, enabled by lithium-ion batteries and cleverly-designed in-hub motors. The problem is, 125mm hard urethane wheels or the equivalent are no match for the 200x50 (mm) pneumatic tires on most commercial electric kick scooters. At 150x30 (mm) these wheels are a good compromise.

This is what happens when you mix SolidWorks with MSPaint.

IMO, it totally looks like a normal kick scooter with these wheels. Maybe not the little kid's Razor scooter...but it could easily pass for one of the slightly larger models. The key difference is that, as depicted in an artist's rendition above, it can ride over sidewalk cracks without the freight train effect thanks to the pneumatic tires. Can it hop curbs, clear speed bumps, jump potholes, or ride over railroad crossings...probably not. But hopefully it will have just enough shock absorption to protect the important stuff from the normal scooter environs of asphalt + sidewalk.

Unlike BWD, Pneu Scooter will only have rear wheel drive. BWD had an "excessive" amount of torque anyway, and having one hub motor will cut down on weight and complexity. Speaking of the hub motor, the reason you don't see it in the above render is because it's on the other side of the wheel. (I tend to ride right-footed, so I lean left.) Here's a close-up render from the other angle:

The hardest part of using these wheels is adapting BWD's hub motor design to mount directly to the wheel rim. Ultimately, there's no way to make this wheel/motor combo as stealthy as, say, Project RazEr. It's going to stick out, the only visible clue that it's not a regular kick scooter. But it is still more compact and more quiet than a belt drive. In fact, with sinusoidal commutation it could be nearly silent. Here's a closer look at the motor construction, as planned:

The stator is made of leftover BWD laminations from Proto Laminations. These are custom ~83mm laminations that, no offense other-mini-hub-motor-makers around here, kick ass compared to copier motors. They're optimized for torque. To complement these, the rotor will have 1/4" thick NdFeB magnets, just like BWD. These allow an extra large air gap without too much performance loss. For the rotor can, I'm trying something new. I don't have any more Proto rotor laminations, but I also really dislike the idea of relying on adhesives to hold magnets in place. So, I sent BWD's rotor file to Big Blue Saw's low-taper waterjet service, to be made from 1/4" cold-rolled steel plates. 

The rotor sees less time-varying flux, since it's dominated by the permanent magnets, and so solid steel will suffice as far as eddy currents are concerned. But, unlike a plain cylindrical can, this will have indents for aligning 14 magnets, which will make assembly a lot easier. This trick on BWD probably saved a full day's worth of spacing and gluing magnets. This rotor and stator are shorter than BWD's, which will trade a bit of torque for speed, all other things being equal. Just something to keep in mind when choosing the number of turns for the winding.

The most important part on the whole scooter is this one:

It's responsible for basically all of the alignment of the rotor and stator, and is my attempt to mitigate the risks of using the plastic wheel hub itself as part of the motor's structural loop. This adapter, made from aluminum, has features on both sides which interface with the outer surface of the wheel rim and the outer surface of the rotor can, hopefully making them concentric. Seven countersunk 4-40 screws hold the adapter to the rim, while seven through-bolts into tapped holes in this part hold the rotor to the adapter. This is the "scary" part of the build, but hopefully the extra-large air gap and outside bearing will help out here, too.

The shape of the wheel lends itself to this design mostly because the concavity in the hub matches up with where the motor windings will be. This will allow plenty of room for termination and wire exit on the wheel side of the stator. Here's a cross-section showing the space inside the motor:

If anyone says "three bearings" in the comments I will hit them with Alex Slocum's book.

The exact winding (configuration, wire gauge, and termination) is TBD. I will be targeting a torque constant roughly equal to BWD's front motor (0.20 Nm/A or Kv = 47rpm/V). But, it will have a denser winding, utilizing all twelve teeth instead of six, so it should be able to pull more Amps. I would be happy with 30A-40A peak and 15A continuous at up to 33V. Having holes in the wheel hub raises some interesting cooling possibilities, though I will probably want to seal those for weatherproofing instead and just live with lower continuous current.

To supply the power and range, the deck is essentially a battery holder made out of McMaster PN 1630T351 aluminum U-channel:

It'll have exactly the same batttery pack as BWD, a 4.4Ah LiFePO4 battery at 33V. So, 2xDeWalt. Unlike BWD's front-loaded pack, this one will be bottom-loaded. This means real structure at the front of the deck where the Razor handlebar will attach, the bane of BWD's existence. Fool me once...

Lastly, the controller will be the 3ph HD, mounted upside down to the semi-infinite heatsink / deck. Sadly, it will probably not be playing music while moving, since it requires a host computer to stream the MIDI file. But, maybe in a small room as a demo. Otherwise, it'll make a good load test for the HD under normal operating conditions at 500-1,000W input.

That's the plan. Time to press play.


  1. Hey Shane, that is so nice! After reading it it made me tough about something. I let my entire class ride my scooter and one of my friend is a motorcycle type of guy and he tends to lean a lot when he turns. When he was outside in the middle of the road the mini kick stand was rubbing the concrete while he was turning. I also had the same issue when I was riding it. Not all the time but you so of forget about that when you start riding it, you have so much fun until you here this crazy metal to concrete sound. On you cut out view of the motor, it seems that you have 22º of tilting on the right side. So yeah, it's small but like you said you lean left!! I wish the best luck with that build. Like always, keep up the great work it's fascinating!

  2. Shane, your site is handsomely designed and appeals even to the viewer who is untutored in mechanical engineering. Your design seems destined to have broad appeal.

  3. Kick scooters are a fun way for to get around and to express your creativity. This can be used instead of walking to the convenience and comfort.

  4. Lol comment spam...I let it live sometimes because I feel bad for it.