Hub Motors on Everything: ChibiKart

The year of the tiny, chibi, and cute projects continues!

So here’s the backstory. I’m sitting on top of a veritable mountain   pile small cardoard box full of 100mm-class hub motor parts that I commissioned on a whim last fall to test out the workflow on, of being even lazier than I am already and hiring my machining work out to shady Chinese backwater CNC machine shops (because those exist). I discovered that Chinese job shops seem to be every bit as legitimate as American ones, and will definitely hold the tolerances you are not used to holding for your own parts made with your own drawings. And even though the pricing is inexpensive for such machined parts, I am still just hiding several hundred dollars of shiny precision machined electric motor parts in a box.

Not very productive – I’d rather that money be either put into builds or making me more of itself. That doesn’t mean I’m going to start selling motors left and right just yet – I have none of the other support parts at the moment, including important aspects of a wheelmotor such as the wheel – right now, I’m still just carving the center out of 100mm skate wheels. Add to that the amount of actual completion detail that needs to go into a motor such as proper windings and termination, rotor magnets (those things are getting expensive) and possibly even sensor boards and Hall Effect sensors (which would need to be designed, sent for fabrication, and assembled), all of which comprises several hours of manual work, and the case is still not very strong for my immediate entering into the dubious market of small personal transportation implements. Yet. Product development takes real time commitment, something which I have yet to convince myself is worth pursuing.

But to counter that, I am more inclined now to take custom commissions than I was before, like for your Air Trek skates or electric suitcase or nanosegway or whatever. The main reason is that I lied just a little above – I do have a potential source of stators for both the 100mm-class “skate motor”, and the 125mm wheel Razor scooter sized motor. The stator, as I’ve explained before, is pretty much the hardest thing to get custom made for you if you are buying in volumes with less than like six figure units, and it’s harder than I thought to just “buy a few” stock/premade ones. However, this part investigation is still forthcoming – the other reason is that I’ve found several “islands of stability”, or at least islands of reliable part numbers, in my everlasting quest to catalog the stator sizes of all extant copier motors. Yes, that document exists!  Use it for your own hub motor building endeavours.

I’ve literally spent hours sitting on eBay hunting for new part numbers to buy and catalog. Tearing motors apart to make other motors is not a mass production method, but for my one-offs, it’s an acceptable compromise. This means that I also have an asston of random copier motors hanging around, several of which have stators which fit the original 100mm skatemotor design, and I would be totally unsurprised to learn that all my random eBay binging has resulted in more sunk money.

Poetic waxing aside, the combination of too many motor parts and too many random stators means that I need to build something to use them. I’ve pretty much made a pledge to not build any vehicle that is not propelled by hub motors from here on, since doing otherwise would mean I put off development of the motors to something reasonably resembling a product even more, while my surplus parts continue to build up.


Here it is!

…well that’s not very exciting.

This thing is already going to be comically small – that frame rectangle is only 30″ long by 18″ wide – with even more comically small wheels. Coincidentally, 30 x 18 is the exact dimension of the front half of tinykart. Part of this build is also fueled by go-kart envy. Ever since the venerable LOLrioKart was officially decommissioned, I’ve not had a four wheeled rideable object. Two tracks is kind of okay, but not really quite the same. But for me to just build a copycat kart is not very enlightening. Thus, Chibikart.

Four wheel drive, maybe four wheel steering (those corner pods are symmetric in case I pursue it), and really really small.

…and possibly with only slightly more horsepower than the Razerblades, since I’ll literally be using the same kind of motor.

Alright, maybe now there is a better sense of scale. That seat is not a couch or bus seat or something, but a riding lawn mower seat from Surplus Center. Yep, this contraption will be on the same length scale as Amy’s profoundly awesome SAM, but definitely a bit longer.

The seating position right now is “legs out” with a front bumper or other structure out beyond the 30 x 18 and the foot pedals, and a steering linkage vertical immediately next to the front 80/20 bar. During this session, I discovered how pleasant CADing with 80/20 extrusion was. After I figured out, of course, that there were only two narrow perpendicular faces on each side to constrain to.

The pedals are generic Chinese electric kart pedals, sold around the Internet using some form of the name “Simple Hall Foot Pedal” or similar. I’ve yet to receive the ones I ordered, but luckily TNCScooters had reasonable dimensions on their drawings (interpolation, estimation, and “screen calipering” aided in completion of the CAD model too). One of them will have a linkage extension to actuate cable brakes – the actual mechanical braking method is yet to be designed, and might just end up being scooter fenders with rigged cable linkages. Hey these are scooter wheels.

I’ve made a few changes here. First, the pedals have been moved inboard. I’ve decided on a significantly more “cab-over” driving position, again similar to SAM (but not QUITE that much…). The steering linkage will be planar and located below the main frame rails. Overall ground clearance is slightly under 1.5 inches – the wheels are not mounted on center in the 80/20, more like 3/8″ under, but the linkage will take out some of that.

Through careful ergonomic studies involving sitting on a block of foam  on the floor of MITERS, I added the foot brace bar that crosses in front of the pedals. In practice I’d put most of my weight on that and be able to tap the pedals instead of hovering over them while trying to hold your leg steady.

Most of the front end mechanicals are done now. I added a reverse cowcatcher/bumper/whatever it is – either way, makes the thing look a little less hilarious. There’s some mechanical design (read: lengths of 80/20 to copy and paste) still to come such as the seat mount proper and battery/controller/electronics mounts, but I will hold off until the parts arrive.

What’s going to be powering this thing? The orange battery is an A123 special that will remain generically orange and prism-shaped for the time being, since I’m fairly certain it’s Not Supposed to Exist Yet. Bottom line, the vehicle will have a 32 volt (10s LiFe) electrical system, a fairly chunky battery.

Powering the motors will be four of my most favorite alignments of Chinese manufacturing probability, 350W class Jasontrollers! I’m taking a major risk by going all sensorless with this thing, but the Jasontrollers have proven themselves in being able to start high torque hub motor vehicles. I’m hoping that with four of them there’s never going to be a “twitching equilibrium” moment. The downside is no regenerative braking (and no, the E-STOP wire doesn’t count – tried that already on Melonscooter, almost died), but someone has the ability to fix that if only he’d write a startup routine, right?

Chibikart will be a test to see if the skate hub motors can push any reasonable power. Combined with the Jasontrollers, they will hopefully form a reasonable system which in my mind is more product-able than just discrete parts alone – because what the heck are you going to do with just a motor can or a raw unwound stator?

If it turns out they don’t – well, who knows, maybe I’ll just stuff them back in the RazErBlades somehow, or build an updated version of them – after all, they needed more power.

Hub Motors on Everything, Part I: The RazErBlades Contingency Plan

Reason #1 to not engineer things at 5 in the morning: You think that putting hub motors on inline skates is actually a good idea.

Reason #2 to not engineer things at 5 in the morning: You forget how many magnets each hub motor needs, and like a total dumbass, only order half the number you need.

Well, guess who is guilty on both counts. Late in June, I put in a reorder of the custom arc-segment magnets that I got for the first two skatemotors from SuperduperfabulousMagnetGeorge. Each motor takes 7 “north” magnets and 7 “south” magnets, where the designations just describe which pole is on the inside face of the arc. So, I ordered 16 magnets in total, 8N and 8S, so I have 2 spares in case I break something.

wait, what?

If you’re keeping track, the left RazErBlade has 2 motors. That means I only ordered enough magnets to make 3 wheel drive skates. By the time I discovered this minor oversight, it was already two weeks ago, so I hurriedly put in an appended order. The custom magnet service has a minimum turnaround time of 3 weeks, and there was (at that point) 3 weeks left until Otakon. Now there is one, and I’ve been informed that my appended order will ship next Thursday.

You know, when I leave for the con. Clearly, this was not going to work at all.

And so I deployed the backup, pain-in-the-ass-but-it-would-get-them-moving plan, and dropped some more dimes on a set of rectangular magnets for the left side motors.

Using GoBrushless’ excellent rotor magnet placement calculator, I discovered that SMG’s stock 20mm x 5mm x 2mm magnets were a good fit for the can if I doubled them up side-by-side. They would require some spacing games, but I was used to playing that with RazEr anyway.

And here they are! I got the shipment notice 90 minutes after I entered my order – that’s essentially on par with McFaster-Carr. Due to the miracle of express shipping, they were in my mailbox the day after.

I printed out the generated magnet placer graphic to use as an epoxying guide. Step one is to put in the “keystone” magnets, the first 14 of alternating poles. Trying to jiggle too many magnets next to eachother, I have found, always results in unsatisfactory placement and a dent or two in the workbench from my forehead.

After the first 14 magnets set in each motor, I crammed their complements in next to them. Putting 2 magnets of the same pole orientation next to eachother means they tend to force themselves apart. To combat this, I wedged little plastic spacers into the horizontal gaps as I placed each new magnet.

I made the spacers using a handy-dandy sheet metal notcher tool and some strips of thin unknown plastic.

new rimz

Usage reports from friends who actually are good at skating have told me that the 72-78A durometer scooter wheels are too soft to perform most skating maneuvers effectively, such as sliding or otherwise breaking traction.  So I wasn’t totally crazy when I thought the ‘blades handled like bricks – they actually do!

Solution: Hop online and find some harder compound wheels. I decide to upgrade one step and go to 85A wheels. Finding 100mm wheels was actually pretty difficult, since the vast majority of inlines use smaller wheels such as 72 or 80mm. Then came the issue of filtering those 100mm wheels to find the ones which can be hollowed out to 2.5 inches on the inner diameter, which was a requirement not met by most.

I finally located these K2 wheels on and had a pack rush-shipped (By this point, I think express shipping has almost matched the cost of parts for this project).

These wheels have a glossy, blank white tread and a black plastic core. Very plain, yet functional, and I was impressed by the quality and finish.

No matter, they’re going on the lathe NOW. I made a quick mandrel to grip them by their bores, since the urethane was actually too slippery to grip with the outer diameter chuck jaws. A flying pass with a boring bar severed the spokes from the outer part of the rim.

Well, mostly. The bar broke through at a place that was not the outer diameter of cut, so now I have these spoke stubs to contend with. When the shops with bigger machines open again, I’ll just knock those out by virtue of gripping the wheel’s OD in a bigger lathe.

final preps

This weekend (and extending into next week, likely, due to laziness) I plan to re-engineer the Skatroller to allow for manual activation of the DEC modules’ electric braking. My spare force-sensing resistor will be hidden under the original wrist-forward trigger point such that it will detect two possible states – willful activation of braking and the palms-open-oh-shit-i-am-about-to-die faceplant mitigation position.

Which, mind you, may possibly be mutually coupled.

I’m also going to switch the analog op-amp circuit to an Arduino Nano based solution, because it’s much easier to throw some if() statements at the two FSRs than try to play the AND/OR/MAYBE game with logic gates and linear components.

Did I just advocate the use of software? Doom.

Past that, I’m going to refine the power system of the Skatroller to use a single lithium polymer cell with a Lilypad boost converter unit. This ought to net me much more efficiency and subsequently battery life, as well as avoid stressing out the XBee by running non-spec voltages.

Non-straightjacketed Agito, theoretically coming to an Otakon near you. Because I'm totally going to be able to stay upright while 95% blind, without the use of my arms, and with motors attached to my legs. Yeah.