Towards the end of last year, with the release of The Force Awakens on the horizon, I started conceptualizing what a LEGO BB8 droid might look like. That project hasn’t gone very far yet, but in the process of prototyping I also started working on this (much simpler) monowheel model. You can see it in action and how it works in the video, but there are some more notes and photos below as well for those interested.
The core engine of the model is designed using a counterweight system, with one Power Functions M-motor used to propel the wheel and another one for steering. The steering motor slides the counterweight from side to side inside the wheel to tilt it, which causes it to turn. The propulsion motor just rotates the wheel around the engine core.
There are some issues with the steering system, most notably there is no means of self-centering using the M-motor, which means bringing the counterweight back to center is hard to do reliably. I tried to use a Server motor in some early prototypes, but there just wasn’t enough space inside to fit it. I do have some ideas for improvements though, so hopefully I can refine it in the future.
The wheel also has a tendency to fall over due to the erratic steering. I think that’s just a matter if increasing the size of the lip that runs around the edge of the wheel track, but I’ll experiment with some other solutions as well.
The track is built using flexible tubes (also called rigid hoses), wrapped around the sidewall discs. Each tread of the track is created using a couple of curved slopes, and slide onto the flexible tubes through some Technic bricks with Axle Holes.
Despite the performance issues, it is still pretty fun to play with, and if I iron out some of the problems I’ll put together instructions for a future version.