Electric Mountainboard Build Part 4: Drivetrain and Layout

So the board finally arrived, which means I can start work on the hardware components of the build. First off, I would like to comment on how properly massive the mountainboard is compared to my regular longboards. The longboard you see above next to it is 40" long, and is by no means small even in longboard standards. The mountainboard positively towers over it in all dimensions, with a track nearly half a foot wider and far taller. The longboard could easily roll underneath the mountainboard, which gives you an idea of how large it is. Bindings are definitely unfamiliar to me, and the large track and tires give a very distinct feel to the board unlike that of a regular RKP or TKP board.

In this log, I will cover work done to the drive train, as well as mounting solutions explored for the boxes.

For the previous part, click here!

Wheel Sprockets

The wheel sprockets are one of the more difficult portions of the build, as it's crucial to get it precise enough to spin correctly and fit on the wheel. It also required the most fabrication compared to the other parts. Thus, work started on it long before the board arrived. 

The first step was to figure out where the holes should be drilled to mount the sprocket onto the wheel. The Rockstar pattern uses five screws to hold the rim halves together, and those will be used to hold the sprocket on as well. To do that, I had to copy the hole pattern over onto the arms of the sprocket.

Rockstar hub

This was made very easy by using CAD to create a jig to let me easily center punch the areas without painstaking measuring. Using technical drawings off MBS's website, I designed up a pretty simple jig. It matches the size of the sprockets, so it clips onto each arm and is automatically centered. It also has separate hole pieces on each arm, since from testing, I found that the center punch sometimes would crack the plastic from the force, and it was cheaper and faster to replace a single insert than an entire jig.

Jig in use

After the holes were punched, I double checked my work by measuring the distances between them and making sure they matched the specifications. To be double sure, I rotated the jig a few times and punched a few more times, just in case one arm was slightly longer than the other due to printer tolerances. It turned out to be just fine. At this point, I waited until the board to arrive so I could visually double check the positioning against the actual hub, just in case there were tolerance differences in the actual build. That turned out to be just fine, and it was time to do the drilling.

Drilling of the mount holes

Drilling the holes was just as nerve wracking as the center punching, if not more so because it was not reversible. If the center punches were off, I could simply flip it over and try again. Not so on the drilling. I made absolutely sure the holes lined up with the bit before going at it. The screw size was M4, but I measured the actual width, and it ended up being around 3.9mm or so. 5/32" was the closest size up from that, so I used that to drill the holes. The part was clamped down, and WD-40 was used as a cutting oil since I didn't have any lying around. It worked reasonably well, but it definitely wasn't as effective on the steel compared to aluminum. Still, it was far better than nothing, and kept the bit cool.

Then came the fitment test of the sprocket to the hangar. In CAD, there was just enough clearance to go over the hangar, but in reality, it was actually just too narrow, which meant the inside had to be reamed out slightly. Thankfully, I had assumed I needed to do that step anyways to increase clearance, and I had the tools necessary to widen it. I used two pieces of scrap wood to build a sort of clamp that let me clamp down the sprocket from both sides of the drill press, and raise it from the bed so I don't cut into it. I then went through with a step drill bit, which took quite a bit of work. This is one of those instances where WD-40 was just a little too thin for the job, although quite a bit did vaporize, which meant it was doing its job keeping it cool. Even at the slowest speed, it sometimes did stall the drill press, so I had to be careful getting it to cut without biting. This was definitely the sketchiest part so far.

Reaming setup

The alignment of the sprockets on the hub was just about perfect, and the bolts all went in straight through and not at an angle. However, the stock wheel bolts aren't long enough once the spacers are involved, so I ordered some longer bolts and 10mm spacers. They should arrive shortly, and all that's left is to get the chain on, and the drivetrain itself should be complete.

Motor Mounts

Thankfully, the motor mounts were much more plug and play. I did have some trouble figuring out how the clamps actually clamp on, as it seemed like it didn't quite fit perfectly on one edge. I was able to cinch it down, and it holds, but two sides don't contact the hangar. We'll see if that causes any issues down the line, but it holds, and is quite strong. The clamps were placed just before the brake hole points on the truck, which left just about enough clearance for the motor sprocket to be as close to the mounting plate as possible. This worked out to 24mm from the hangar end to the motor mount surface. The actual mount bolts onto the clamps, and I chose to angle it as close to the deck angle as possible, pointing up a little bit to keep it away from the ground. Motors were then attached, but not tightened as it still needs to slide in and out to tension the chain. Once they are tensioned, then I will loctite them and tighten them in for good. I chose to route the phase cables upwards, as it didn't really work out any other direction due to cable length. With the motors mounted, it's really starting to look like an electric board!

Motor mounts and motors on

Battery Box

There wasn't much I could do about the battery box while the batteries haven't arrived yet, but one thing I did get around to doing was shortening the pressure release valve screw. This way it doesn't protrude into the box and potentially puncture the batteries. To do this, I marked off where it sits flush against the case, and cut it with a dremel cutoff wheel. I then smoothed it out with a file. It ended up being ever so slightly proud from the case, but that's not a huge deal compared to what it was before. There's no retaining clip either, but that shouldn't be a big deal either since I will not be touching this valve at all. 

Screw before

Cut down screw

Layout

Now that the board is here, I am now trying to figure out the orientation of the enclosures and where the inputs should go. The big one is how the rear ESC enclosures gets positioned. Surprisingly, it fits perfectly on the tail, and it doesn't stick out on either side as I had anticipated. However, it's closer to the binding than I expected, which causes some issues with wire routing. The best orientation to deal with the pigtails off the ESC was to mount the ESC backwards, such that the battery leads face rearwards and the motor leads facing forwards. This lets me use the extra cable to turn the inputs around, which lets me keep all the cables inside the box. It's also slightly off center to allow for room for the power switch, which will be mounted probably on the side. The ESC will eventually be velcroed in place once the position is finalized. It's entirely possible I may need to add vents to the lid to improve cooling, at the cost of water resistance. That is the one downside of a sealed enclosure, and since the heat sink is facing the bottom, it's difficult to build outwards facing heatsink fins either. 

ESC placement

The battery box is laid out similarly. Thanks to the width of the board, I am able to mount it flush with the handle facing forward. This keeps the battery from being placed off center, which plays a bigger difference here compared to the ESC due to the weight. This does mean I would need a longer battery lead though, which means I need to make my own extension cable rather than soldering together two pigtailed XT90s connectors by the ends. 

Box Mounting

Finding a way to mount the boxes onto the board is turning into an unexpected challenge. I am going to cover both boxes in separate paragraphs, since they both have different challenges with their placement.

ESC Box

The ESC box is the easier of the two to mount. It mounts to the flat tail, which means simple velcro will do. However, the trouble comes from the truck hardware, whose heads stick out of the deck. It appears to be raised enough that I can't simply use the thickness of Velcro to have a good hold. This leads to a couple of possible solutions.

One solution is to drill holes in the box to mount the box straight onto the board with the truck hardware. This is the strongest solution, but it runs into a few problems. First, the truck bolts aren't long enough to accompany the added thickness the box will bring. Second, drilling holes into the box will cut into its waterproofing, as it adds another possible entry point for water to come in. Lastly and probably most importantly, it simply moves the head problem inside, and I still get that issue when trying to secure the ESC itself inside the box. I would need to design and print a spacer to work inside, and the temperatures inside are higher than what I am comfortable with for PLA filament.

The second solution is to fabricate or print a spacer for the bottom of the box, and velcro that onto the deck. This is the simplest solution, but it also has its downsides. The biggest downside would be material, as the spacer would likely be printed PLA plastic, which doesn't do particularly well in outdoor use. I can probably make one out of wood or some other material, but I don't have the tools or materials to do as clean a job as I would like. It also doesn't have the same rock solid connection as physically bolting it onto the deck, but a case could be made for ease of service.

The third solution is to replace the hardware or countersink the holes in the deck so that the screw heads would be recessed. This would also leave me with a flat surface, but this is the most destructive method. Those screws are under a lot of tension, and I do not believe it is a good idea to thin out that area. Plus I do not have an end mill or Forstner bit to create a flat bottom in the recess.

Ultimately, I am leaning towards the second solution. It's a cleaner solution overall, and offers the least deviance from the original design. 

Battery Box

The battery box is quite difficult to mount due to the asymmetric concave of the deck, as well as deck flex. The concave means that the box will contact the edge of the deck while the middle is floating, and the asymmetric part of it means that the dip is hard to model, as the lowest point is actually closer to the toe side than the center. The deck flex means that the deck will turn from concave to convex, so if it's mounted on the center, the corners would force it up on a bump, but if it was mounted from the sides, the center of the deck would hit the box when it's at rest.

Deck cutaway courtesy of MBS, with straight line overlaid on top

I had a look in the forums to see how people mounted their own top mounted battery enclosures on this deck, but unfortunately most people seemed to skip this step in their documentation, so I was left to guess how they did it. I utilized those observations to come up with some of the solutions highlighted below. 

The simplest solution is to use four bolts and bolt it directly onto the deck, with springs or rubber shock absorbers in between to take up the gap and provide vibration reduction. However, this is a destructive process, and requires holes to be drilled in both the deck as well as the box, potentially compromising water resistance. Plus I'd run into the issue with bolt heads sticking out again, and while there is foam inside the box, I'd prefer there to be a smooth bottom for the battery. 

The second solution is similar to the one for the ESC box, and that is to fashion some sort of spacer in between that matches the concave. However, it would be fairly difficult to get it exactly the same, granted the velcro should smooth out any differences. Plus, the direct mount is harsher on vibrations compared to the one with rubber standoffs. 

The third solution is to use belts to cinch it onto the deck. This gives it some give when flexing, while I don't need to worry about the concave at all since it's not held on by adhesion. However, removal will be slightly more involved compared to Velcro, as the lid would now be inoperable. Plus the strap has to pass on the underside of the board, opening it up to getting caught on rocks and stuff and tearing or shifting the entire thing. 

This one is still up for debate. I still would like the battery box to be easily removable in the event of an emergency, as well as for facilitating a battery transfer to another board in the future. The closest solution that gets me to that goal is the second one, where I have velcro not only between the spacer and the deck, but another one between the spacer and the box. This sandwich should allow me to transplant it onto a flat board if necessary and absorb more vibration, but it's another potential failure point in the connection. 

Next Steps

The next steps will be finalizing the drivetrain by fixing the sprockets onto the rear wheels, and sizing the chains. I'm also planning to start work on drilling the access holes in the ESC enclosure. I will also start looking for a suitable spacer material for the enclosures, and possibly start fabricating those as well. It's definitely starting to take shape!

For the next part, click here!