The CIM motor drive was originally developed to be compact enough to fit under a longboard deck. We decided that only one bracket was sufficient to hold the motor in position. We could have fixed other side of motor, too, but they're so light that ¼" aluminum on one side was fine.
The CIM motor spins at a much higher rpm/V than the stock motor. In order to keep stock torque and top speed characteristics, we needed to gear them down- thus the need for the planetary gearboxes. The CIM motors @ 18V + the planetary + 1:1 timing belt gear ratio ~= stock motor @36V + 19:44 timing belt gear ratio in terms of rpm and torque. Note: we know that CIM motors are meant to be run on 12V and that we are running them at 18V (2 motors in series on a 36V circuit = 18V per motor). This is fine; they handle the higher voltage and rpm without any issues.
The Exkate drive wheels have a 44T gear permanently attached to them. We did the gear ratio calculations and it turned out that a 1:1 timing belt gear ratio was fine, so we bought 44T gears to attach to the planetaries. If you use a smaller gear, you'll get more torque (and thus acceleration), but I can tell you from experience that there is PLENTY of torque. A larger gear will give you a higher top speed (but less acceleration).
But why did we go with 2 motors instead of 1 bigger motor with a solid rear axle or differential? (You can buy differentials for tricycle-style bicycles that are small enough to fit.) One reason is that both would require a complete chop-and-rebuild of the rear truck in order to get them to fit in the proper place; we'd basically have to design our own truck. But besides that, there are other problems with both. A solid rear axle would mean that both rear wheels would be spinning at the same rate. This is bad for turning. When a car turns, the outer wheels have to spin faster than the inner ones. If you have a solid axle on a longboard, the outer wheels have to slip in order to make the turn. This would seriously hurt the turning radius. A differential would alleviate this problem; however, in the case of longboards, it causes a different problem. A longboard is turned by leaning in the direction you with to go, causing more force to be on the inner wheels than the outer wheels. This means that the outer wheels have less traction. With a differential, the side with less resisting force (traction) gets more torque and vice versa. This means that, in very sharp turns where the outer wheel comes off the ground, the outer wheel will get all of the power and the inner wheel will stop spinning. This is the same problem we had with 1 wheel drive, but now on either side of the board! In summary, a solid rear axle or differential were not good ideas.
So we used two motors. However, one thing we didn't foresee until it was too late was that we had created an electronic differential by wiring the motors in series. If one motor has less load than another, it will steal power from the other motor. Ideally, the two motors should be wired in parallel. However, that can't be done with CIM motors because, while 18V is fine, 36V would probably cause them to explode. The second best possible solution is to find two relatively low rpm/V (so that you don't need the planetaries), light weight, compact, 36V motors and wire them in parallel…I couldn't find any such commercial motors. The best possible solution would be to completely overhaul the power system by using two motor controllers (one for each motor), finding motors that match our needs exactly (and don't need the planetaries), and creating a new radio scheme (because the radio receiver is integrated into the current electronics module).
Note: when I say “motor”, I mean brushed motor. We decided to go with brushed motors over brushless because of their simplicity, being cheap, and ability to wire them in series or parallel. Brushless motors are more efficient and more powerful, so if anyone wants to undertake a brushless version of this project, that would be cool! (I'm building one with in-wheel brushless hub motors, check it out here: http://www.mitrocketscience.blogspot.com
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The last thing to mention is how we mounted the brackets to the trucks. A hole was drilled in the bracket plate large enough to fit the truck through. Two small two piece clamps were machined out of 2” aluminum round stock. First, we cut 1/2 inch pieces of the 2” round. Then those had a big hole drilled in the center smaller than the diameter of the trucks at the point we were going to clamp it (if these holes end up being too small, filing can fix them). Then they were notched (see pic) and holes drilled (for a #10-32 tap) perpendicular to those notches. Two clearance holes for 10-32 were drilled through the face of each (to be used to screw the bracket to). Then they were cut in half (see pics). Then the bottom sections were threaded for 10-32 and the top sections were drilled out for clearance. The end result was two, 2-piece clamps that would fit snugly onto the trucks when screwed together. After fixing them to the trucks, the two face hole locations were transferred to the brackets and drilled and tapped for 10-32. Then everything was shimmed (to get the motors straight because I can almost guarantee that the clamps won't sit perfectly straight on the trucks) and bolted (with loctite!) together. Note: a better way to do this than clamps would be to weld the 2” round disks onto the truck.