image from http://www.americanlisted.com
And if begun. The idea of motorizing the skates was far too appealing to NOT do this. I quickly turned to the internet to see if it had been done before. Well, I should have expected that the idea was not too original. In fact, I quickly found a pretty cool model of a pair of electric skates, built by none other than my EV instructor: Charles Guan.
Charles said that it would be an interesting challenge and that clearly it could be done. That's really all the encouragement I needed to begin designing.
The first thing was coming up with a basic layout for the skates. The main two layouts were inline skates or the quadropod style skates. I decided to mix the two and have one wheel in the front and two in the back. The added wheel would provide more stability that I was sure that I would need.
Next came just about everything else. I started working out power calculations, finding a top speed and using energy equations to try to pick the right stuff. By stuff I mean motor and motor controller. While deciding on speed and torque I had to be searching for a motor that would provide a proper Kv value and an ESC (Electronic Speed Controller) that would burn out on me.
Calculations were pretty straight forward: power in from battery is just the voltage times the current and power out (at steady state) will be wind drag, rolling drag, and power needed to go up an incline. I later realized that there's also a lot of power that is dissipated in the bearings and the gear train system.
Nonetheless, I got this done and went straight into CAD mode on SolidWorks. I had a bit of experience with SW but I really improved on my skill and got to work with assemblies for the first time. After many, many hours of extrusions and such things I developed this CAD model:
This is the final CAD model from the back (a bit more interesting than the front). The motor protrudes from one of the sides in order to align the sprocket on the motor shaft to the sprocket on the wheels. I decided to go with chain drive since a smaller chain seems less likely to break than a small belt and inefficiencies of one design over another seems to be a topic of debate. Only one wheel is driven on a dead shaft so leaning could get you off of the driven wheel if control was ever lost.
It was a bit of a challenge laying everything out in such a small space. The batteries consumed like 60% of the space and basically set the dimensions of the skates. Most of my work went into creating the stand off and wheel assembly. The Stand offs under the plate held the shaft that would have two wheels, one sprocket, and 3 spacers on it. The bottom side was connected to the top through the 1/8" 6061 Aluminum plate with steel threaded rod. The top stand offs where drilled all the way through to allow the threaded rod through. The top plate would rest on the stand offs and would be secured with nuts on the threaded rod. Thanks to the accuracy of the lathes I used all the stand offs were exactly the same size.
From there I had to drill, cut, and bend 2 1'x3' 6061 Aluminum plates. That actually also took a good amount of time since I had to make sure everything was perfectly aligned. Probably one of the most important things I learned was about taps and drill taps and the correlations between. Very basic thing, but something that NEEDS to be known to build stuff. Never again will I do this by hand! From now on I will always be water jetting because this took way too long. And I wish someone had told me about the brittleness of 6061 aluminum. I was aware that bending 1/8" aluminum wouldn't be easy but didn't know that bending 6061 could cause some definite fracture of the piece. The main frame manage to get through it since there was more wiggle room and I planned for a large bend radius, but most of the smaller pieces didn't survive. That little crunch sound the aluminum would make when it had just cracked was mortifying.
One of my favorite things was chopping the plastic skates in half. I only needed the wheels and the boot section of the skates. Something I really wanted to do was make it so that the electric skates only had to be strapped on to your shoes so there be no need to swap shoes for skates. The foot would be secured with nylon straps that had Velcro on them. The ankle would be supported with the boot and another nylon strap. Sadly the nylon straps for the boot kept breaking. In the interface between the Velcro and nylon the straps structural integrity was not quite good enough. The Velcro kept coming off to my dismay so I was forced to use the old laces to guarantee ankle support. Ankle support is key to safe use of skates!!!
Once the components were ready it didn't take long to put everything together. Some people had some pretty involved circuitry but not I. Battery connects to the ESC with a switch thrown in there along with battery charging terminals. The ESC is then connected to the motor and the HobbyKing Rx. The Rx simply connects to the trigger (Tx).
And now comes the cool part: running the motors. It was pretty awesome to see these things go after the weeks spent on them but I wasn't quite ready to ride them.
I was still missing the nylon straps and I didn't have a second switch. So I actually didn't get to ride them for like 3 weeks as I waited for the magical switch that somehow got lost. But it did come in! And it twas awesome.
It was kind of hard to kick of as you can see. The additional 6" didn't make it too easy to keep my balance. Fortunately I had set the ESC to low power so I didn't fall on my arse (this time). During the ride I noticed that I had to tighten the chains. They kept bouncing onto the frame. I slotted the holes for the motor mount to adjust the tension, it seemed easier than devising a tensioning system.
During one of the runs I lost my balance and I cracked one of the skate boots. For some reason one of them was a lot more brittle than the other. I though I was screwed and I new that glue wouldn't be able to help me out here. Charles jokingly introduced the idea of carbon fiber and epoxy. I took some time to look it up and said that I might be able to get the results I wanted by using a t-shirt instead of carbon fiber. Charles called this "Ghetto Fiber." The process I got from YouTube videos and just simplified it a bit. I put down a layer of epoxy and when it gets sticky (takes like 30-45 minutes) apply the cloth and smooth it out. Over time apply subsequent layers of epoxy to the cloth to make it nice and strong. You can start sanding it a bit after a few layers and apply another layer to smooth it out a bit if you really want to make it look nice. Surprisingly this process worked really well and now the booth is really freaking strong.
Everything turned out great and my skates are fully functional. They reach about 4mph on the pavement but have a good amount of torque to them The motors are brush-less and have no sensors attached and can start from a standstill (which usually doesn't happen, instead you stall your motors) so I think I have some leeway to play with my torque and speed. I could change out the bigger sprocket for another small one to have a 1:1 ratio and theoretically go 4 times faster.... But that might not be a good idea. After an occasion where I pressed on the trigger a bit to hard from a standstill and fell straight back I deemed my skates SkullSkates: if you're not careful they might just crack your skull. So yea, be gentle with the trigger.
This was the completion of my first big project and I'm glad that it was successful! I didn't just learn about the basics of design and manufacturing but also about budgeting, shopping for parts, and evaluating. This class taught me how to pursue future projects in a smart manner.
In the future I would love to design another pair of skates. Something not so tall (smaller battery packs) and more compact. The skates aren't too user friendly and are a bit intimidating to others. I can't imagine why? Hub motors will probably be the way to go. And adding some kind of "suspension system" so the skates (and rider) don't vibrate so much on non paved roads. Maybe I could even design my own ESC to save more room.