Minimal 250W RC chain drive for road bike


1 mW
Jul 3, 2019
With extra time on my hands thanks to not commuting, I figured now was a good time to make a start on the conversion idea that's been kicking around in my head for a long time.

The brief: a minimal weight, RC motor based, UK road legal electric assist for my aluminium road bike. I've long wanted to build a proper road legal electric vehicle, and it'll be a challenge building something that's lighter and less bulky than the average hub motor or mid drive. I also don't want any permanent modifications so I can still ride the bike unpowered, meaning it's got to be something that can be removed (so no hub motors/mid drives).

The concept: a gearbox mounted on the seat tube holding a Turnigy 125kv orange can motor with a primary belt reduction, then a jackshaft to a chain sprocket driving a #25 gear on the left side of the rear wheel. The combined gear reduction is 14.4:1 which gives me about 1kph/volt. The jackshaft also lets me use a salvaged 12mm sprag bearing as a freewheel so the motor won't be driven by the wheel. A small Hobbyking LiPo will provide power - a 6S 5000mAh fits in a regular bottle cage and contains ~120Wh which means 30 minutes of constant assist.


The control will be minimal, inspired by the Kepler friction drive idea of a single button. I'm currently leaning towards a single on/off button and a small potentiometer knob to set current, an RGB LED for status and possibly a 1" OLED screen, all driven by an Arduino Nano. That will also handle the input from a pedal sensor (most likely reed switch/hall sensor with a magnet on the crank) so it won't power the motor if it isn't detecting pedal rotation. The ERPM limit in the VESC can handle the "no assist above 25kph" and "250W max" so by my reading of the rules it should be road legal with all of those in place.


Progress so far:
I've prototyped the electricals with a switch standing in for the pedal rotation sensor. I'm now working on the motor mount/gearbox bit, and have a 3d printed prototype with the motor and bearings mounted and a jackshaft machined.



The next problem is figuring out how to mount it to the bike frame. I intended to use the bottle cage mounting bosses on the seat tube, but they are too close to the BB so the cranks would hit the gearbox, and are tilted slightly to the side so it doesn't mount straight. I'll probably end up bolting a piece of 3mm flat bar onto those and clamping it higher up the seat tube, then mount the gearbox to that.

I imagine the #25 chainline will give me some headaches, but part of the reason I'm doing this is to give myself a challenge and test out some techniques before I commit a lot of money to a proper motorbike conversion. Total investment is pretty small: <£40 for the VESC, about £90 for the motor and belt drive bits, and I already had the battery but it would be <£50. The #25 sprockets and bearings were salvaged so the only other expense is threaded rod for the jackshaft and 3d printer filament.

I'm writing up the progress on my blog and will try to crosspost on this thread - I checked the rules and mentioning personal blogs doesn't seem to be verboten so you can find it here. Mods, if I'm committing blogspam crimes feel free to delete it!
Update: I've finished the 3d printed gearbox to hold the motor and primary belt reduction, and bolted that to some flat bar to mount it on the seat tube bottle cage mounts.


I also mounted a 66T sprocket on the rear wheel with a 3d printed adapter zip tied to the spokes (definitely temporary, I'll look into rag joint sprocket adaptors or hub clamps later) and got a #25 chain run from the jackshaft to the wheel sprocket. The battery and VESC fit in a frame bag for now, though they should probably get a more rigid custom enclosure that allows ventilation. The Arduino Nano setup for integrating throttle and pedal sensor inputs was soldered up on a stripboard so it could fit in an enclosure to mount on the handlebars.



Quick video of the first spin of the back wheel before rain stopped play: