I got the rear chainring mounted on the playa trike yesterday. It took all day: I wasted five hours laboriously designing and building an adapter plate that would allow me to bolt the chainring on to a set of studs welded on to the old sprocket, which in turn was bolted on to the wheel’s original studs, only to discover that this offset the wheel so far to one side that I’d have to cut away half of the swingarm to allow it to spin freely. Mounting the chainring on the other side wouldn’t work, because the motorcycle wheel hub is aluminum and thus not weldable. After that didn’t work out I was tired and frustrated so I tore it all apart, screwed the nuts on to the original studs, hacked out the middle of the chainring so it would fit over the wheel, and then just welded it on, maintenance be damned. It wobbles and it’ll be a total pain to replace, but whatever, it’s done and I can move on to the next thing.

Today’s project, assuming I can find a way to work through the amazing heat, will be to fabricate a bracket, mount the electric motor onto the swingarm, and chain it up to the largest of the rear sprockets. After that I need to rig up a motor controller. I have a little silver box with a lot of wires sticking out of it that is supposed to be the controller for the same model of scooter the motor came from, but I am dubious: it does not appear to be robust enough to survive the playa, and in any case it has too many wires for the number of functions I expect it to perform.

I’m thinking about rush-ordering a solid-state relay, pairing it up with a capacitor, a schottky diode, and a microcontroller, and calling it a day. I don’t need reverse and it’s easy enough to program in a fixed accel/decel curve as a simple low-pass filter on the input.

Now how to control it? The traditional solution is a potentiometer mounted in a twist-grip, but it occurs to me that there’s an existing input I could use instead: this machine has a set of pedals, after all. What if I measured the speed of pedal rotation and spun the electric motor proportionally? That is, you make the bike go faster by pedalling faster, and slow down by spinning them more slowly. This method of control might be wonky in the city, where you have hills to climb, but out on the dead-flat playa the only factor that matters is how quickly you want to travel.

This mechanism would also reinforce our claim that this machine is just an unusually elaborate e-bike and not an unusually small mutant vehicle, and therefore in no need of a mutant vehicle license. If you pedal it to make it move, after all, doesn’t that make it a bike?