Step 3: Motor Mount
So, this is basically the most tedius part of the build if you are looking to design a motor mount out of ABS that can withstand large braking forces. The files can be found on my Thingiverse. It has been designed in such a way that you can remove it when you want to, but stays exactly where it is during riding. I wanted to test the limits of 3D-printed components and after 150 km (as of March 2017), I can say that it is holding up very well. Most builds use metal and weld the mount directly onto the trucks, which is fine too.
First, insert the 2 x M8 Nylock Nuts into the slots at the axle and screw in the 2 x M8 Set Screws from outside. This will help the mount grip onto the axle. Next, slide the motor into the mount and insert the 4 x M5 Washers into the mount. Next, use 1 x M5 x 16 mm Machine Screw on the axle side to hold the motor in place. The idler system will be using the other three holes in the next step. Now you can attach the pulley wheel to the motor to test out the fit. The BLDC motor I am using has a 10 mm shaft, so make sure the pulley also has the same bore size. Make sure there is no contact between the pulley wheel and the mount. Do not permanently mount the pulley yet, we still need to adjust it later.
Your Own Design
You can design your own mount by first drawing your trucks in CAD, this is quite a tricky process but after a few tries you will get it, don't give up too quickly. Next, you'll need to measure and record all the dimensions on your motor and trucks. Then, start drawing a primitive shape that will hold the components together. Once you feel quite confident, you can print your first prototype either on your own or using a service like 3DHubs and try to iterate from there. Constantly take notes on what to improve and incorporate it into your next design. On Fusion360 it took me 45 Versions and 6 printed prototypes to get to my end goal, you might require less tries. I also used Parametric Modelling to help me change the dimensions easily to get the perfect fit. Do not be discouraged, it is well worth it ;).