Electric Longboard - GoBoard

Over the thanksgiving weekend I sat down for a few hours and cranked out a rough electric longboard. And then I ordered $100 of parts. So maybe this will happen. We'll see. Everything was designed around a very simple deck and standard trucks and wheels from the interwebs. The only custom bits will be the motor mounts, whatever connects the pulleys to the wheels, and the battery enclosure. I may look around for a simple charge controller so I don't have to take the thing apart every time I want to charge it, but let's leave that for version 2.

From the side - modeled with a flat board.

The board was designed around a motor and batteries I already had. I don't totally remember but a top speed target of 20 seemed pretty reasonable and realistically I'll go about 15 anyway. Safety first?

Closeup of pulleys/tensioner. 7.5" belts is what SW came up with. 5:1 ratio 

Obligatory 'the top comes off' picture. Battery assembly will be 3D printed.

Hooray for efficient-yet-dangerous transportation

Electric Surfboard (extreme sports in slow motion)

After my last post, the surfboard got a brief water test in the Charles and there was evidence of a very slow leak in the tail. To remedy this, the board got two more hot coats to (hopefully) seal things off. (Spoiler: it worked)

Fins and fin mounts (compatible with commercially available fins) were hastily 3D printed from ABS. Fins were reinforced with aluminum rods but the fins were actually very strong without them. The weakest point of the fin is by far the point where it is held by the mount. The fin mounts were attached to the board with epoxy/microballoon slurry, where balloons were added until the mixture had the consistency of yogurt.

Fin mount

The aluminum bits are where the screws contact the fin and push inward/up to hold it in place.

The lab's resident Hawaiian and surfing expert noted that these fins are WAY too thick.  Next came the 'waterproof' battery. I built a 22Ah 13.2V pack and submerged it in epoxy. When you do this, you prevent the cells from being able to vent, which is bad. I tried to remedy this by attaching pieces of foam to the pack to allow for some displacement but the foam just soaked up the epoxy and by then it was too late. The pack is completely sealed and will not be happy if it ever needs to vent.

Attempt at foam with kitchen sponges. This did not work.

After the epoxy had set fully, I put on a plastic cover and cut a section out of a smoothie bottle to allow for sealing off the battery while still allowing access to power and balance leads. This was done in a rush. Adrian, don't do this again. Bad. It still worked fine in the water, but we must remember that the Charles is freshwater. No sea legs for this setup.

Sealed up

Another precaution I took was a 'deadman' switch. This is so if I fall off the board, it doesn't keep going. There are controls on the motor itself, but they don't require the user to give continuous input and it will stay on if it isn't turned off. The monstrosity below is a springy 3D printed frame with aluminum contact pads that come together when pressed to complete the circuit. When released, the circuit is opened and the motor will turn off. Umm. The glove makes it waterproof. Yeah.

Next was the motor mount, which involved very very drippy microballoon epoxy and an aluminum plate with holes for 80/20 to be attached. It looks like crap and I know it.

It's a surfboard?

The motor was attached next and damn was it heavy. Not pictured: the battery on the nose of the board for counterweight.

Sometime in October, a few friends and I headed out to test. It wasn't a sunny day but the weather wasn't bad. I had a wetsuit so I stayed pretty warm throughout the trip. Huge thanks to Tommy, Calvin, Dan, Rodrigo, and Andrew for helping out!

Everything was tied down with twine, especially the motor and GoPro.

By the way, the battery is in my backpack along with a whole bunch of foam for buoyancy in case it falls off.

It floats!

It floats... but lower

Here goes...

I went about 1/3 of the way across before turning back.

There is footage of the trip but it's 30 minutes long and I haven't edited it yet. Overall, the trip was a success. The motor didn't push me nearly fast enough to stand up. This was partially due to the motor's power being fairly lacking (it is the smallest of its class) and also because when I was just sitting or laying on the board, I was mostly underwater, providing more resistance and making it more difficult to gain speed. I topped out at about 5 mph, which was still quite a lot of fun. Also I haven't grown any extra limbs since being in the river, so I've got that going for me, which is nice.

Summer Project Progress

With my first year at BU finished, I am now free to start as many projects as I think I can complete in the next four months. I have two new projects completely established and started, two tentative ventures having to do with watercraft, and of course that pesky hub motor scooter I've been 'working on' for the past ...10 months or so.

One of the established projects will remain off this blog until a certain friend has a birthday. The other project is a gas-to-electric conversion of a minibike which was kindly donated to me by Charles.

Stripped of all the fancy stuff.

The stator is Delta-wound, which means that if you uncoiled all the teeth and didn't remove any of the electrical connections, you'd have a triangle. My first hub motor is wound in a Wye configuration where all three phases meet in the middle. Delta-wound stators tend to spin faster while producing less torque per amp. If you had two stators of identical dimensions, number of wraps, voltage, etc., but one was wound Delta and the other was wound Wye, the Delta would produce less torque and higher rpm by a factor of √3.

Standard procedure for brushless motors is to open them up to look for stray windings, bad connections, or misaligned magnets. Boy, did I find some misaligned magnets. A quarter of the forty magnets in the can had come off and had clumped together in a few locations. A jig for positioning the magnets was lasercut from .25" acrylic. I wanted it to be clear so I could see both ends of the magnets. The acrylic worked very well, and after the epoxy between the magnets and the can had set for about two hours, I added some epoxy between the magnets to prevent slippage side to side.

N = 51... ?

Magnet meeting

Magnets removed, old epoxy removed, can surface sanded to better accept new epoxy.

Jig for positioning magnets.

Epoxy added to the gaps between magnets to help prevent them from sliding. Looks good! 

After the magnet epoxy was allowed to cure for 24 hours, I brought it to the Edgerton Center shop to try to use a lathe to gently lower the can onto the stator. Credit for this idea goes to Ed Moriarty, and the hands you see in the pictures are those of Mark Belanger, who helped me figure out how to do this and offered to do the procedure due to my damaged left wrist. Mark came up with the idea of turning the end of a shaft to a point that would fit into the tapped hole at the front of the shaft connected to the can. The shaft was held in the chuck on the right, and the can was firmly held in the chuck on the left. The stator assembly was placed over the shaft and was advanced so the two shafts met and could not move. The stator was carefully moved towards the can and was 'sucked' into position while maintaing the proper alignment thanks to the lathe setup. The motor was then fully pushed into its bearings with the tailstock.

Note: lathe was not spinning during these photos. 

It looks the same as it did before, but here's that picture again.