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Bafang M400 cutting power when wheel magnet passes the sensor [SOLVED] (feddit.uk)

submitted 1 week ago* (last edited 1 day ago) by Perspectivist@feddit.uk to c/bikewrench@lemmy.world

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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.

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[–] avidamoeba@lemmy.ca 3 points 1 week ago (1 children)

Without access to some debug interface that shows you what values come from the various components, you're stuck with hypothesizing and replacing parts, which is annoying and expensive. In that world, I'd also try replacing the torque sensor. I've seen it sold on Ali. I think they use a common part between multiple units but I'm not sure. I have their BB torque sensor and it seems it uses a spindle that looks like the ones used in their mid drives. They're not cheap though.

[–] Perspectivist@feddit.uk 3 points 1 week ago* (last edited 1 week ago) (1 children)

I’ve serviced the motor before and I’m familiar with the part in question. At this point it’s not even about the cost - I just dread spending another minute wrenching on this thing instead of riding it. I’m tempted to just take it to a shop and tell them to figure it out. What really throws me off is how this only happens in the lowest gears, even though the bike has no way of knowing what gear I’m in. I even took out the Eggrider display and put the Bafang unit back on but no difference.

[–] avidamoeba@lemmy.ca 3 points 1 week ago* (last edited 1 week ago) (1 children)

Yeah that part really is crazy. Obviously there's some mechanism but I don't know what. You've alreade checked change in torque and cadence as a cause. The only other thing that comes to mind is the force pointing backwards, experienced by the spindle from the chain pulling on it, which changes depending on which gear you're in and how tensioned the rear derailleur is. But I would think any such effect would be drowned by the forces of your legs on the cranks.

[–] Perspectivist@feddit.uk 3 points 1 day ago* (last edited 1 day ago) (1 children)

I figured it out!

I was suspecting that the previous owner may have messed with the motor settings with the Eggrider app so I was looking for the default values online so that I could put it back to factory values. By pure chance someone made a sidenote where they described my exact symptoms and then later followed up with a solution: add a second wheel magnet. Apparently on low speeds the time between pulses gets too long which confuses the motor controller and cuts out the power. That's why this wasn't happening on high speeds at all. Still doesn't explain how my other almost identical bike gets by with just one though.

[–] avidamoeba@lemmy.ca 2 points 1 day ago (1 children)

Interesting! So it wasn't torque related at all but speed control related. I presume you had to adjust the number of pulses per revolution to two? Cause adding a second magnet would cause any control loop think the wheel is spinning twice as fast.

[–] Perspectivist@feddit.uk 3 points 1 day ago (1 children)

Yeah, in the motor settings - where it's defined as using an external sensor - there’s also a setting called “Speed meter signal,” which I changed from 1 to 2. I’m about 85% sure that refers to the number of magnet passes per wheel revolution.

[–] avidamoeba@lemmy.ca 3 points 1 day ago

Beautiful. I had totally forgotten mid-drives required wheel magnets. I've been riding a hub for 5 years after some crazy mid-drives. Hubs almost always have buily-in speedos.

[–] Grass@sh.itjust.works 1 points 1 week ago (1 children)

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.

[–] Perspectivist@feddit.uk 1 points 1 day ago (1 children)

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.

[–] Grass@sh.itjust.works 2 points 1 day ago

That's worth a try. I'll see if I still have some magnets I can just tape on for now.