Hub Motors on Everything: Johnscooter, the Testbed for Kitmotter 0002

Tuesday, July 17th, 2012 @ 5:12 | Kitmotter!, Project Build Reports

This scooter might rank as both my fastest-built and shortest-lived vehicle. It was called into existence solely to test Kitmotter 0002 on the evening of the 14th of July, and lasted about an hour and a half. That should tell you that Kitmotter did not fare well under load testing, but it might not be for the reason you think.

The base of Johnscooter is…. well, a scooter frame left behind by someone named John in MITERS almost a year ago. Occasionally, people drop things off and swear they will come and fix it or upgrade it, but it ends up not happening because MIT eats them shortly thereafter, or they ran away to Nicaragua, or some other life event detrimental to finishing scooters occurs. John, if you want your scooter back, I’m done messing with it.

This frame is quite interesting. A little research led me to find that it was called the FX-1 EZRider, which is probably one of the more hardcore names I’ve ever heard given to something like this. They seemed to have been produced a decade ago and used nickel metal hydride batteries, and oddly enough, sold at Walmart. It’s very compact, with 6″ pneumatic wheels native (though I’ve been unable to find where John left his rear wheel), and weighs a bit south of 20 pounds.

I started tearing down the frame to extract the components inside, and I must say I am a big fan of the design. It’s not cheap and thin like a Razor scooter – there’s 5mm and 4mm aluminum all over the place. The frame comprises a single aluminum extruded tube about 3.5″ wide and 1.5″ tall, with the forks (made of 5mm bent aluminum plate) bolted through the sides. The same bolts retain all the internal electrical components. Up front, there is a solid endcap which seals the aluminum tube from the weather.

I’m also a fan of the combination fender brake. You can actuate it both with the brake lever on the handlebar or by stepping on it.

The batteries are in stick form and they slide right into the aluminum frame tube, surrounded by rubber. I was expecting these to be lithium, and therefore toast if they’ve sat for too long, but after dismantling a small portion of the heatshrink, I found that they’re actually 4.0Ah nickel metal hydride cells, of the 4/3 A size. That’s alot of battery. Each pack (there are 2) is made of 24 cells in two sticks, each made of a 3-long string of 4 cells in parallel. Then there’s two of them. That’s 230 watt hours of battery, a number that is only rivaled in this volume by using prismatic lithium polymer or LiFe packs. Seriously, I could not make an A123 pack this good.

As far as I can tell, each pack is only 14.4v nominal, and they are used in series to run at 28.8v. To my surprise, the whole pack charged right back up and peaked at 33 volts after sitting for Robot Jeebus Knows How Long.

Some times you just can’t beat a little bit of old school technology. Unfortunately, for me to actually but 230Wh of those cells is about $140 (nearest match) if I don’t mind something generic, and a cool $570 if I want Sanyo cells. Nickel is still classy, and now you understand why I consider the advent of Hobbyking batteries to be a pivotal moment for the creation of actually useful small vehicles by hobbyists.

Now, this is actually an aftermarket pack – John mentioned when he visited that he purchased this scooter from a builder in California, nicknamed Deafscooter. I’ve actually snooped around Deafscooter’s work on the EVAlbum back before I built my first EV in 2007. In fact, I am fairly certain it is this exact scooter, because the date of sale matches roughly with John’s description. Something about going full circle…

To mount Kitmotter, I decide to cheat a little and just use the axle hole that was already in the rear forks. This involved end-drilling and end-tapping the shaft. Another solution using only ‘garage tools’ is to drill out the forks’ axle holes to 5/8″, the shaft diameter, then drill and tap set screws perpendicularly. I decided to just go with easy since testing the motor was a higher priority.

I cleaned up the battery pack wiring a little (putting them permanently in series) and then stuffed the now-charged batteries right back into the tube from whence they came.

And on the Kitmotter goes! Very little engineering occurred here: Kitmotter had a 3″ width between shaft end faces, and the scooter fork natively spanned 2.6″. Solution: Mash the bent plate forks in a vise until they became unbent.

A view from the other side, showing the quite sophisticated brake mechanism – the little off center thing at the bottom actually rotates with the brake cable, giving the brake lever variable leverage as it is depressed. Did I mention I love this frame design?

For sheer simplicity and speed of setup, I plucked a spare Jasontroller off the table. Jasontrollers handle hub motors very well (almost like they were designed to do it or something…) and Kitmotter2 is indeed a hub motor – many poles, low speed, high resistance and inductance.The throttle that came with the scooter was a generic thumb lever type that was spliced right into the controller.

And here it is!

I replaced the front 6″ pneumatic wheel with a spare 125mm wheel because it otherwise caused the thing to look very strange, and ride ‘nose up’.

Ride testing of Johnscooter indoors revealed that Kitmotter’s hardboard sides seemed to work just fine. I took it through a few bunnyhops to make sure. The #2-56 studs and nuts seem to take a little time to ‘wear in’ the hardboard surface – more washers and Loctite may be necessary in the future. Even though the wheel was bored manually, it still came out reasonably on center because of the hole saw pilot bearing hack, and the wobble wasn’t noticeable in riding.

It was, however, a little underpowered because I never actually wound it to produce torque, being a demo motor and all. I could have definitely put another 2 strands of 28 gauge in parallel and lowered the resistance, or just put more turns on to attain a higher torque per phase amp. Or both. The stock Jasontroller puts out only 25 amps, limited by hardware, so it wasn’t too much.

Here’s some indoor ride testing of Johnscooter:

Next, we took Johnscooter to Ye Olde Silley Vehicule Proving Grounds, the garage (riding the whole way just to abuse the hardboard endcaps some more, just in case). Since it was comically slow, there’s no test video, but it finished our defined test interval in 120 seconds while consuming 10.9 Wh of battery, for an “action score” of 1308.

This is actually not bad at all given its non-optimized motor. For comparison, RazEr Rev finished the same test in 82 seconds while pulling 11.1Wh, in part because it generates so much more torque per Jasontroller-limited amp. Pneu Scooter offers a more fair comparison, having performed a 99 second @ 13Wh run previously (1287).

Unfortunately, Johnscooter only lasted for one run. After the first, the motor was very unhappy – the heat could clearly be felt through 1/4″ of wood, which is a pretty damn good insulator. After letting it cool for about 20 minutes, I tried a run, but only made it up 3 levels before it totally cut out. After that, the response seemed to be intermittent, which I initially blamed on using the aged battery too roughly. As some of the other guys were riding it around, the motor cut out again and this time it felt like it failed short.

Post mortem analysis once Johnscooter was back in the shop revealed…

…that Shapeways’ “White, Strong, Flexible” is definitely not “White, Strong, Flexible, and Heat Resistant Too”. Basically, it seems the nylon stator hub heated to the point of losing structural integrity, stripping its D-flat against the steel shaft keyway and causing the whole thing to shift. This pinched and shorted the wires against the shaft.

Additionally, melt lines can clearly be seen at the stator interface. This thing definitely got hot – though not hot enough to damage the magnet wire, as it was neither discolored nor smelled funny.

But it’s clear that I can no longer suggest 3d printing a stator hub as a viable solution for anything but the lightest duty motors. I’m thinking now of doing a wood (yes, more wood) laser-cut stacked hub that uses the 3-eared shape of the stator bore fully, and also grips the entire depth of the keyway. Yeah, sure, make fun of it for being more wood, but it wo…..uld have a higher temperature resistance, especially composite wood-like substances like hardboard.

I will put Johnscooter back together – it worked very nicely for what it is, and Kitmotter needs far more abuse heaping before I can confidently tell other people to copy and paste.

 

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    3 Responses to “Hub Motors on Everything: Johnscooter, the Testbed for Kitmotter 0002”

    1. David Says:

      I am not a hardware engineer but I do work in software with a lot of brilliant people. I also love to tinker with projects and while 80% of everything you talk about is over my head I love reading your entries because they are surprisingly well written. Like I said, I work with engineers and most of their writing is atrocious. You manage to make talk of battery packs detailed enough to be interesting to geeks but also short enough that the average reader doesn’t quit out of boredom. Plus, you actually can spell. Congrats.

    2. amyq Says:

      yay, making things out of wood!

    3. the chuxxor Says:

      but MDF isn’t real wood ;o;

      You should make several wooden kitmotters for your woodkart!