One of the best things about this forum is that it’s up and open on the web, even Google’able. It’s not really the home of DIY OneWheel stuff, but I’ll keep this thread going because it’s better out in the open than buried in some Facebook group chat somewhere. That said there have been several Facebook/Discord/Telegram groups that have been really helpful, in fact, this post is just a summary of several comments I’ve made everywhere.
So a big part of this build was finding out if this kind of drive system is really feasible for this application, and I have some thoughts. The build itself seems mechanically sound, and I love the aesthetics, but the performance is woeful (my Pint is far better). This seemed off given the specs ‘in theory’ are far better; bigger battery, more powerful motor, better ESC.
My initial thoughts were that I just didn’t have the parameters in the balance package (the thing that makes the VESC do OneWheel stuff) tuned well. I tried many well-established working tunes, and variations of them; none of them seemed to work. Logging via the mobile VESC tool showed I was pushing over 80 motor amps on flat grass with the board really struggling.
So maybe it was the motor tune. Reran the motor detection in as many different ways as possible, always coming up with close-ish numbers. Almost have silent HFI going but it struggles with motor tracking under load, but that’s not going to fix the issue anyway. Ran a very unscientific experiment with the belt drive vs the Pint. It’s pretty simple, let both run at the max speed and try to stop the wheel spinning through whatever means. In the case of the Pint I could actually slow down and probably stop the wheel. Zero chance with the belt drive, wheel speed didn’t deviate at all no matter what I threw at it. Was kind of scary actually. However, repeat at low speeds and you get the opposite results. The belt drive definitely has the power, just not where it’s needed.
Motor KV selection and gearing was chosen to give a reasonable top speed, but I believe the actual KV of the motor is a bit higher than the advertised 50kv. Following calcs based on 14 motor poles, VESC reported voltage of 49, ERPM of 18862, and a duty cycle of 95%.
pole pairs * rpm = erpm
rpm = erpm / pole pairs
= 18862 / 7
= 2695
speed = 2695/60 * (22/49) * 0.280 * pi
= 17.74 m/s
= 63.9 km/h = 39mph
rpm @ 100% duty cycle = 2695 / 0.95 = 2837
kv = 2837 / 49
= 58
I’ve ordered a 12t pulley that will half the top speed and increase the low-speed torque. The real issue is that at these low RPMs the motor seems to be operating very inefficiently. I’m fairly pessimistic that even doubling the reduction will get it out of this inefficient range of rpm/torque. It’s worth a shot for a pulley change!
Probably not surprising that when you research the motor characteristic for low-speed torque you find out it looks a lot like what is used in OneWheel hubs. Lots of motor slots, big diameter, lots of copper windings. I still think electric motors are always better off with a reduction, but I definitely no longer think drive reductions can be used to change the torque characteristics like I needed and expected here.
If the pulley doesn’t work out this project could go several ways;
- Leave it as is and move on
- Just buy a SuperFlux/CannonCore/OneWheel hub, and add a split 6s pack up front to make 18s overall. Would need new VESC and BMS.
- Design/build my own brushless motor with more favorable characteristics.