I’m having a really odd issue with my e‑fatbike (Bafang M400 mid‑drive). When I’m on the two largest cassette cogs (lowest gears), the motor briefly cuts power ~~once per crank revolution~~ when the wheel magnet passes the speed sensor. It’s a clean on‑off “tick,” almost like the system thinks I stopped pedaling for a split second.
I first noticed this after switching from a 38T front chainring to a 30T. At that point it only happened on the largest cog, never on the others.
I figured it might be caused by the undersized chainring, so I put the original back in and swapped the original 1x10 drivetrain for a 1x11 and went from a 36T largest cog to a 51T. But no - the issue still persists. Now it happens on the largest two cogs. Whether I’m soft‑pedaling or pedaling hard against the brakes doesn’t seem to make any difference. It still “ticks” once per revolution.
I’m out of ideas at this point. Torque sensor, maybe? I have another identical bike with a 1x12 drivetrain and an 11–50T cassette, and it doesn’t do this, so I doubt it’s a compatibility issue. Must be something sensor‑related? With the assist turned off everything runs perfectly, so it’s not mechanical.
EDIT: Upon further inspection it seem that the moment the power cuts out seems to perfectly sync with the wheel speed magnet going past the sensor on the chainstay so I'm like 95% sure that a faulty wheel speed sensor is the issue here. I have a spare part ordered so I'm not sure yet but unless there's a 2nd update to this then it solved the issue.
EDIT2: I figured it out. It wasn't the wheel sensor but related to it: I added a second spoke magnet for that sensor on the opposite side of the wheel and the problem went away. Apparently on low speeds the time between pulses got too long and the power to the motor was cut. In addition to this I also used my Eggrider app to tweak the motor settings so that it knows there's two magnets and not just one. The setting I tweaked is under "Bafang basic settings" and I changed the "Speed meter signal" from 1 to 2 to tell it that there's two magnets.
i had this on my tsdz2 as well as the toseven dm02, but the tsdz2 was seemingly random and the dm02 was every 7 revolutions. still no idea why it happens.
I solved the issue by adding a second wheel magnet for the speed sensor. Not sure if the issues you're having are related to mine but it's easy and cheap thing to try. You might need to inform the motor controller about the second magnet as well though or it'll think you're going twice as fast.
That's worth a try. I'll see if I still have some magnets I can just tape on for now.