After building my own long board over a Christmas holiday, I started getting the itch to motorize it. I considered several types of drive trains:
- Cogged belt drive with pulley and motor mounted to the skateboard truck
- Casting a polyurethane wheel around the bell of an outrunner motor (got a quote for $35/ea from a molding company).
- Friction drive using a geared motors with wheels on the output of the gearhead that rub on the skateboard wheels.
I decided to use the friction drive, because I had some low profile brushless hobby motors with gearheads that could be mounted under the back end of the board. From my calculations, the power and gear ratio of these motors was about right for the application. This design also allowed for a manually operated clutch, which lets you coast down hills without applying power or without the freewheeling drag of the motors and gearheads. I used SolidWorks to design the mounting pieces and clutch mechanism. There are two motors placed end to end with friction wheels on their outputs that rub against the rear skateboard wheels. These motors are mounted to the rear truck with a hinging joint positioned in such a way to cam the drive wheels into the skateboard wheels at an angle and in a direction such that they are driven into an engaged position as motor torque is applied.
After machining all the parts and putting it all together, I tested it by wedging a handy eggplant (from my kids playfood stash) between the motors and the board. This worked perfectly to keep the motors engaged for my first test run. Later, I added a foot pedal that you could step on to engage the motors. After testing that, I opted to go with a hand operated clutch using a hand brake from a bike. To power the motors, I used hobby speed controls powered by a 3 cell lithium polymer battery pack. The speed controls were driven by a simple servo driver that I made with an Arduino. The rider holds a potentiometer wired to the Arduino which is used as a throttle.