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Speedo reading goes crazy when motor over 400 watts

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    Speedo reading goes crazy when motor over 400 watts

    This for a Bafang BBS02 with 750C display. I posted that under the Bafang troubleshooting section without response, maybe somebody here knows what to do...

    I have two bikes with Luna BBS02 kits and the nifty 750C display (which has a few bugs I can live with like losing time or no battery percentage). But what really annoys me is the speedo reading on one of the two kits goes haywire whenever the applied motor power exceeds approx. 400 watts (be it by increasing PAS or using the throttle). Under that threshold the speed reading is correct, above it the speed reading jumps rapidly to a steady 42-44mph (similar effect when changing to km/hr). I first thought it might be the 750C display but maybe it is the speed sensor itself or the BBS02. Otherwise the motor kit works just fine and the too high speed reading does not make the controller cut power. It almost looks like EMI electrical noise affecting the sensor but an attempt to improve that by connecting the negative battery/controller wire to the bike frame did not make a difference.

    The other bike with exactly the same configuration does not have this effect.

    Any idea what to try? Maybe just get a new speed sensor? Or did anybody have something similar happen?
    Last edited by JerryBurks; 08-09-2017, 06:37 PM.

    #2
    I'd try swapping sensors and see if the problem follows? I never have much luck with anything more than 5mm air gap for the wheel magnet to sensor. Centered careful, of course.

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      #3
      I have never had an issue with the speed sensor on my BBS02, but on one of my other bikes with a standard cycle computer, I added a couple of additional small round earth magnets to the supplied one on the wheel. I assumed this gave a stronger pulse to the sensor. It also allows one to "fine tune" a bit the distance between the sensor and magnets by adding more or less earth magnets. Because the attractive force between two magnets is so strong, there is no need to cement these together as they stay in place.

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        #4
        I had a similar issue at first. Playing with the distance from the speed sensor to the magnet, and it's alignment (the ring near the sensor tip @ magnet sweep arc) got me going. I think, as hinted above, that EMI can overpower the signal, if signal is barely readable in the first place.

        About 4-5 mm with stock magnet is where I am. Too close or too far was intermittent.

        There's a screw on the sensor mount to adjust this.

        Fabrication is fun! Build something today. Show someone. Let them help. Inspire and share. Spread the desire.

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          #5
          Thanks guys...I will keep experimenting, maybe first swap the speed sensors of the 2 bikes and see if the problems goes to the other bike.

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            #6
            Sorry for the necro, but my wife’s bike has this exact problem. Was a solution ever found?

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              #7
              Originally posted by Zuul View Post
              Sorry for the necro, but my wife’s bike has this exact problem. Was a solution ever found?
              Every time I change my wheels around, I get speedo glitches until I play with magnet positions. Again, yesterday. Fro me, it just starts losing pulses. So speed will be half actual, for example.

              The strength of that wheel speed signal is variable, it changes with magnet position. If you are getting errors above a power level, maybe at that level the interfering electrical noise is overpowering an unusually weak speedo signal. You can have the magnet ALMOST right, and get speed readings sometimes. If you try playing with the magnet position, you may find a stronger signal. I've got my about 1/8 from the center of the sensor, now.

              Best bet is to try that, I think. Maybe check for bad wiring, too.

              One of my bikes has a ripped off speedo sensor, just wires, for the Battman speedo. That one WILL read speeds, crazy high ones, sometimes. It has no speedo signal but the electrical noise is picked up and shows me going 80 mph sometimes. So I think there's something to the interference theory.

              Good luck. Let us know if you beat it.
              Fabrication is fun! Build something today. Show someone. Let them help. Inspire and share. Spread the desire.

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