Electric Mountainboard Build Part 5: Powertrain and Enclosures

Some more parts have come in, and I'm coming onto the final stretches of the build process. A ton of things happened in the past week thanks to the holiday break.

For the previous part, click here!

Drivetrain

Last time, we left off with the sprockets complete, but no bolts or spacers to fit them. Well the spacers and bolts have arrived, and now it is time to assemble. However, this immediately ran into an unexpected problem. The spacers are narrower than anticipated, and they actually go into the hole where the nut should be on the hub. This shortens the distance considerably, and I needed to put a second spacer on to compensate. However, with two spacers, it is just slightly too long for the 55mm bolt, but the 65mm sticks out quite a bit. Shockingly, the assorted set I got decided to completely skip 60mm, which would have been perfect. 

Lots of exposed threads

I could shorten the spacers enough to get it to fit on a 55mm bolt, but I couldn't think of a good way to reliably shorten all ten to exactly the same length so the sprocket wouldn't wobble. I could also cut the bolts by 5mm or so, but that's a destructive process. In the end, I decided to leave it, and move the motor mounts inboard 6mm each side. This leaves plenty of clearance for the long bolts, and doesn't torque the motor sprocket too much. 

Updated spacing, right before the motor sprocket was adjusted

Now that the sprocket was attached, I could actually pump the tire up and step on the board again. However, that highlighted another problem. At full tilt, the motor mounts will contact the edges of the deck. Now I understood why people trimmed their decks. This was not going to be as easy to fix. 

Contact between the deck and the mount clamp

There are two ways I could fix this: I could either do what others have done and trim the edges so that there is no contact, or get harder bushings so it doesn't lean as much. Cutting the deck tips isn't particularly difficult, but it is difficult to get it to be perfectly symmetrical. Harder shock blocks should resolve it, but requires me to buy new ones, and that's going to take quite a bit of time, and still doesn't guarantee it's going to be fixed. In the meantime, I tightened the rear shock blocks all the way, and it only just touches at full lean. This should prevent any contact during normal use, but I will continue to monitor it and make adjustments when necessary. I may go the deck cutting route if it shows itself to be a problem.

Anticipated cut lines, if it comes to it

The next step was to get the chain on. 

ESC Enclosure

I have finalized on how the rear ESC enclosure will be mounted. The ESC is mounted backwards, but the ESC itself is now mounted more centrally to allow for clearance of the XT90 connector to not hit the binding adjustment ratchets. The motor leads will enter the box via cable glands to preserve watertightness. The XT90 port uses a 3D printed panel mount, which lets me control the tolerances a little better and waterproof the entrance.

The first step after positioning everything is to drill the holes. This was made super easy using the step drill bit. The cable glands were of size PG-16, and it turns out that 7/8" is almost a perfect fit. Thus, I used the smaller step drill bit to bore it out. Similarly with the power button, that used the same bit, but only down to 5/8". The XT90 port was a little more difficult, and I didn't really have the tools to make a super clean cut. What I did instead was to use the step drill to bore out most of the internal material, and then go back with a dremel with a round tip to clean it out. Finally, I switched to the end mill bit to straighten out and clean the edges, and did a final cleaning with a file. 

All holes drilled/cut

With all the holes drilled, it was time to add the Velcro. After cleaning the box of shavings and degreasing with isopropyl, I stuck on two 3" strips of Velcro, hook side for the ESC. Same with the bottom of the box, which got 2 4" strips of loop side. I decided to skip the spacer, as I found that the Velcro was more than thick enough to allow the box to not hit the truck bolts. The board got the remaining hook side, and the ESC the remaining loop side. This design was so that the parts that come off have the softer loop side so as not to damage the Velcro or work surface when I take it off and put it on a table. The XT90 panel mount got a small layer of silicone adhesive caulk before that got attached. The excess silicone that squeezed out got wiped, before letting the Velcro adhesive and caulk cure for a day. Then the glands and ESC went in, along with the switch. I made a miscalculation and forgot to measure the widest part of the gland, so there's just a tiny bit of conflict inside when tightening them. It works if one side is flat, while the other turns, but if I were to redo it I'd place them slightly further apart.

Whoops

Once everything was in, I mounted the box onto the board and connected all the wires. The Velcro is surprisingly strong, but we'll see how it does during actual riding. 

Completed ESC assembly, with lid off

Battery Enclosure

The batteries finally arrived from USPS hell, and that made it a great time to test out fitment. Turns out it's real tight.

No space wasted

It's even just tall enough where it gets sandwiched by the lid foam, so no other foam is necessary to keep it in. I may put some thin sheathing anyways along the side, but that means less space for cables. There's less space than I expected, but thankfully it wasn't the other case, where it's too small.

I took a good hard look at the box, and it was actually more difficult to mount the XT90 with the box lengthwise across the board than the other way around, due to all the reinforcements on the walls. Thus, I decided to rotate the box mounting direction such that the carry handle faces the side. The upside is easier cable management and no chance of the edge sticking out of the board rail, but the downside is now there is less contact space for the spacer and Velcro to stick. The spacer is now 5" long instead of 7" like before, which removes about 6 square inches of usable Velcro space. 

In addition, the ridges along the bottom of the box made flush mounting very difficult. Thus, I went at it with a dremel to remove the ridges where the Velcro will be. In the meantime, I also cut off the four feet from the box, so I get a little more clearance before the board hits the box. I was a little overzealous with the hacksaw, and scraped a few parts of the remaining ridges. It's on the bottom so no one will be able to see it, but in the future I would mask the area off better with some painters tape to provide a buffer against scratches and scrapes. The face was then cleaned of sticker residue, and sanded to help the Velcro adhere later on. 

A bit messy, but no one can see the bottom

Cutting out the XT90 port was a similar process to the ESC enclosure. The best way to wire it was actually having the port in the center, as that was the only way I could fit the wiring for the batteries. The box plastic was a tougher and thicker, so it took a little longer to cut the hole. After the hole was cleaned, the panel mount went in along with the silicone. 

Finished hole for panel mount

After cleaning, 3 strips of 5" Velcro went on both the board and bottom of the box. I decided to forgo the wood spacer for now, as the board contour is actually flatter in the middle than where the feet are. 

Next Steps

The physical board is nearly complete! The final steps are connecting the two boxes together with 10AWG wire, and getting everything buttoned down and tight. I still need the chains cut as well. After that, it's on to programming the ESC.