Iām having a really odd issue with my eāfatbike (Bafang M400 midādrive). When Iām 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. Itās a clean onāoff ātick,ā 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 Iām softāpedaling or pedaling hard against the brakes doesnāt seem to make any difference. It still āticksā once per revolution.
Iām out of ideas at this point. Torque sensor, maybe? I have another identical bike with a 1x12 drivetrain and an 11ā50T cassette, and it doesnāt do this, so I doubt itās a compatibility issue. Must be something sensorārelated? With the assist turned off everything runs perfectly, so itās 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 Iām like 95% sure that a faulty wheel speed sensor is the issue here. I have a spare part ordered so Iām not sure yet but unless thereās a 2nd update to this then it solved the issue.
EDIT2: I figured it out. It wasnāt 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 thereās two magnets and not just one. The setting I tweaked is under āBafang basic settingsā and I changed the āSpeed meter signalā from 1 to 2 to tell it that thereās two magnets.
Without access to some debug interface that shows you what values come from the various components, youāre stuck with hypothesizing and replacing parts, which is annoying and expensive. In that world, Iād also try replacing the torque sensor. Iāve seen it sold on Ali. I think they use a common part between multiple units but Iām 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. Theyāre not cheap though.
Iāve serviced the motor before and Iām familiar with the part in question. At this point itās not even about the cost - I just dread spending another minute wrenching on this thing instead of riding it. Iām 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 Iām in. I even took out the Eggrider display and put the Bafang unit back on but no difference.
Yeah that part really is crazy. Obviously thereās some mechanism but I donāt know what. Youāve 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 youāre 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.
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. Thatās why this wasnāt happening on high speeds at all. Still doesnāt explain how my other almost identical bike gets by with just one though.
Interesting! So it wasnāt 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.
Yeah, in the motor settings - where itās defined as using an external sensor - thereās also a setting called āSpeed meter signal,ā which I changed from 1 to 2. Iām about 85% sure that refers to the number of magnet passes per wheel revolution.
Beautiful. I had totally forgotten mid-drives required wheel magnets. Iāve been riding a hub for 5 years after some crazy mid-drives. Hubs almost always have buily-in speedos.
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 youāre having are related to mine but itās easy and cheap thing to try. You might need to inform the motor controller about the second magnet as well though or itāll think youāre going twice as fast.
Thatās worth a try. Iāll see if I still have some magnets I can just tape on for now.
