September 3, 2014

Carbon Fiber Scooter

You may be wondering what's been happening with that extremely shiny hub motor I built ages ago.  Well, the vehicle being built for it is nearly complete, but I've been bad at documenting things this summer, so I haven't gotten around to making a post about it.  Until now.  Get ready for a long one.

Since the motor was intended to serve as the drive for some sort of reasonable and practical vehicle, it will be stuck on a kick-scooter shaped object.

Aluminum U channel and rectangular tubing are great ways to make scooter frames with integrated batteries.  Unfortunately, MITERS, did not have any of either of these types of stock in appropriate dimensions for efficiently packing batteries.  However there was some carbon fiber cloth, epoxy, and foam from the same stash I used in my bicycle building adventures.

I quickly laid up a rectangular carbon fiber tube:

I tried out something new for layup.  Instead of vacuum bagging the foam mold, I bent an aluminum sheet metal form, and clamped the carbon-wrapped foam mold into it.

The surface finish on the bottom and sides was excellent directly out of the mold, but not great on top.  It'll all get covered in more carbon anyways though.  The pink foam was melted out with solvents.

To hold the hub motor, I machined some aluminum forks that extend from the carbon fiber tube.  They were thoroughly machined away for weight reduction.

The side in contact with the carbon fiber was wrapped in fiberglass to prevent galvanic corrosion, and they were epoxied in place.  They were additionally screwed in place, using small aluminum plates to distribute the pressure across the carbon fiber, like a big washer.

I also added a rear fender using a segment of a conveniently sized carbon fiber tube I found at a lab cleanout over a year ago.

I machined my own fork from some thick walled aluminum tubing.

The fork is two parts.  The bottom half was thick walled tubing with 1/4" walls and 1" I.D, while the actual steerer tube is 1" O.D tubing.  The two halves of the fork were crammed together with a very tight press fit.  Maybe I'll bother with adding a fastener to prevent the press fit from slipping, but probably I won't.

The head tube assembly for this scooter is by far the most strangely shaped object I've ever tried carbon-fibering.  I started out by cutting a section of carbon fiber tubing from a broken bicycle frame.  I bored out the ends, and epoxied in bearing races for a bicycle headset.  I then cut up a carbon fiber hockey stick handle, and tacked the pieces to the head tube.

The form was vacuum bagged over with move carbon fiber, and then epoxied to the rectangular tube of the scooter frame.  A new and more powerful vacuum pump at MITERS has greatly improved the quality of my vacuum bagging.

Hey, that kind of looks like a scooter

The entire assembly was vacuum bagged with even more carbon fiber over everything, with a few extra layers around the joint holding the head tube.

This was probably the best layup I've done, and took only minimal sanding to get smooth, despite the awkward geometry of the head tube joint.

I made a long split collar to attach the steering column to the steerer tube, and used some bicycle seatpost clamps to hold it all together.  The quick release at the top allows the entire handlebar assembly to be removed for compact storage.

Mike TIGed me some sweet aluminum handlebars:

As far as scooter internals go, the chassis was designed to just barely fit a 12S 2P pack of A123 26650 cells.  For motor controller, I am using one of Shane's FF1.1 controllers.  For the fancy field oriented control algorithm to properly commutate a motor, it needs to know some of the motor's characteristics.

Phase resistance of .6 ohms was measured with a four-wire measurement.  That's kind of high, and it may be worth rewinding to pack in a bit more copper.

Measuring the back EMF took a couple tries to get right, because either I was using the MITERS scope wrong, or the scope was being weird.  I switched to a fancier scope and got it sorted out though.  This was done by scoping two leads, and spinning up the motor by friction driving it with a drill:

Kind of funny looking back EMF.  Fairly trapezoidal but with something 5x the electrical frequency added in.

Looking at the amplitude and frequency of the back EMF gave me a torque constant of .31 N-m/A, or 30.6 RPM/V.  I was shooting for 30 RPM/V when I wound the motor, so this worked out surprisingly well.  This gives a no-load top speed of  ~18 mph with a 12S battery pack.

There's not too much left to make this thing rideable.  I built a battery pack for it, but I did not assemble it quite flat enough and it had to be forced into the carbon fiber tube.  I'd like the battery and electronics to be easily removeable, so I need to reassemble the pack to make it a millimeter or so thinner.

Finally, the motor controller needs to be reprogrammed again with the correct motor parameters.  When I did this the first time around, my torque constant was off by a factor of 3 somehow, so the motor didn't commutate particularly well.

In other news, we built a roller coaster, and it was great.  Documentation to follow.

Also, there is now a virtual python-scripted version of the robot arm that draws virtual squares.  The virtual robot arm also can do some virtual gradient descent tuning on its virtual controller, which is pretty cool.  Once I improve that a bit, the real robot arm will have a real controller that doesn't suck.


  1. I love this swagtron scooter. Everyone has question how far it go and how long the battery is?.

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