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I have a Batavus Zonar 2020 e-bike, Yamaha motor, 10 gear cassette, 5500km of use.

Recently the motor has stopped working momentarily during very steep climbs - it just shuts off for a second then goes back on again. This is a new phenomenon. It happens even if I take the climb at lower gears than usual.

It would seem that the issue is related to how steep the climb is - I have a long climb that slowly goes steeper and steeper and I only experience these "glitches" in motor performance at the steepest part, which is also where I move the slowest. I don't change gears during the climb.

What could be causing this?


Possibly related info:
I had the bike serviced quite recently, where I first had the casette and chain replaced (as well as tires and brakes). This caused the new chain to jump. The mechanic then suggested that I also needed to change the chainring, which sounded reasonable since it was still the old original.

After changing to a new chainring as well, the brand new chain snapped during the very first climb... and they had to replace the cassette + chain again. The suspected cause was supposedly a bent cog they found in the brand new cassette. I'm sceptical if they at all knew what they were doing, but I'm no bike mechanic so who am I to tell. Quite a botched service, but it has been working fine since then, until now (300km later).


EDIT with more info:

OK so after some hints here yesterday, I tried some things:

  • I went out with a fully charged battery to a long, steep climb. I didn't really experience the motor shutting down this time but I watched the speed meter carefully during the climb. Even though I was going at a steady pace of 18km/h, the speed meter occasionally dipped down to 8km/h. I observed several such dips during the climb.

    Maybe this is an indication of something related to the speed sensor being the culprit?

  • After the climb I dismounted and removed the battery, checking both the battery and motor with my palm to see if any part was warm, but couldn't notice anything out of the ordinary - everything was as cold as the rest of the bike.

  • I did manage to reproduce the brief motor shut-down while doing a very short but steep climb (just 5 meters or so) at the lowest gear.

  • I don't really think the battery is the culprit because it lasts just as long now as when the bike was new. If the battery was getting bad, I'd expect to notice a shorter time between charging.

  • I can't access the diagnostics, far as I know. The Yamaha manual (3-14) suggests that I need to connect an external service tool through USB. I'm not a mechanic and I have no access to such.


EDIT 2

I went chasing down the speed sensor and I think I found it, fixed to the frame on the opposite side of the cassette on the back wheel. Picture below.

And... I also noticed that the little metal thing mounted on one of the spokes was tilted at a 45 degree angle. I would assume that the little thing is a magnet? And then it should be sitting straight. It might have taken a hit somehow - the picture was taken after I readjusted it. I hope it is sitting correctly now? The spoke paint also seem worn underneath it, suggesting it has moved.

The next question I suppose is if it is located at the correct spot on the spoke or if I should move it further to the right as per the picture?

enter image description here

At this picture I'm pretty sure it's turned 180° wrong - will adjust it and see if it changes anything.

EDIT 3

After correcting the magnet position, I do not notice any glitches on the speed display any longer. So far no hiccups with the motor shutting down either. The magnet getting bumped out of position might just have been the cause - fingers crossed.

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    I can't really help on the motor part, this can happen for a million reasons but one of them is the speed sensor. Regarding drivetrain wear and replacement: It is certainly not impossible but for my taste, a chainring, and running on the second/third cassette in 3 years and 5500 km is quite a handful, unless the bike is ridden in tough conditions, but imo the LBS should then give some basic tips how to extend component life, like regular lubrication and getting the worst grit off from time to time.
    – DoNuT
    Oct 3, 2023 at 17:25
  • @DoNuT Thanks for your reply. I'm on the 3rd cassette and 3rd chain, 2nd chainring. But I also commute all year round in rain and snow. I don't clean & lube the chain nearly as often as I know I should. The last chain I used for around 3000km and then it was very worn, checked with one of them link measuring sticks. So I waited too long to replace it. The mechanic recommended me to change chain every 2000km for these reasons.
    – Amarth
    Oct 3, 2023 at 17:52
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    @DoNuT Regarding the speed sensor: I'll make sure to check the speed next time I do the same climb. If there's a glitch there, I should see the speed dropping to zero even though the bike is moving, I think?
    – Amarth
    Oct 3, 2023 at 17:56
  • OK, that explains the wear and sounds totally reasonable (worn chains eat cassettes and chainrings) but if you have a chain gauge, I would go by that rather than fixed intervals, even though 2k is probably a good measure given the riding conditions. Chris answer below extends on the speed sensor but in theory, constantly riding slow in the flat should also be an issue, then. The issue occurring on long climbs could be a hint of overheating or low voltage, going into a short but steep climb straight just works? (Feel free to add this to Chris' answer below...^^)
    – DoNuT
    Oct 3, 2023 at 18:25
  • @DoNuT Going to pay closer attention next time and report back here with an edit.
    – Amarth
    Oct 3, 2023 at 19:04

3 Answers 3

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You might see the speed drop to zero if there's a speed sensor issue, but you might not. It might use different averaging for the display and the sensor.

If it has a cadence sensor (the only e-bike I've worked on did) then too low a cadence might cause it to glitch out. Using the lowest gear of all would be your best bet then.

Finally it's possible the battery voltage is dropping and causing the controller to cut out. You can test whether it's better with a very full battery (if necessary by staying the ride fully charged and turning off assistance until just before the problematic bit). I'd normally expect it to take tens of seconds to recover, maybe even minutes, and to cut out again almost immediately if you're still on the hard bit. Overheating electronics could lead to similar behaviour

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    Thanks for your reply. I now posted an edit with a few new discoveries. It would perhaps seem that the speed sensor is the most likely culprit? I don't think it has a cadence sensor or at least that's nothing I'm aware of.
    – Amarth
    Oct 4, 2023 at 17:13
  • @Amarth if it's a pedal assist model it needs some way of detecting that you're pedaling - a cadence sensor of some form. It almost certainly is pedal-assist as that's what's most common in Europe legally, but I haven't tried to read the specifications as I can't find them in English and my German is marginal for the level of detail needed.
    – Chris H
    Oct 4, 2023 at 17:34
  • @Amarth Definitely worth checking if it has slipped or something like that and worth a shot repositioning it if it looks off. Don't know where it sits on Yamaha-powered bikes but I would guess on the rear wheel and the sensor probably on the (non-drive-side?) chainstay.
    – DoNuT
    Oct 4, 2023 at 17:36
  • And if anything is overheating it's likely to be the control circuit, which is probably near the battery and/or motor, connected to both of them and the controls you use. But I'm going with sensors.
    – Chris H
    Oct 4, 2023 at 17:37
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    After correcting the magnet's angle and position on the spoke, I no longer observe any speed sensor glitches (or well, only tested during 20km so far). I would imagine that the sensor occasionally got a bad read of the magnetic field and therefore "missed" a lap. Maybe the angle of the bike during a climb added to the error.
    – Amarth
    Oct 5, 2023 at 15:35
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I started to observe this on my older bicycle (Bafang motor) after the battery got older. In my case it very obviously depends on the charging level: the less it is charged, the more likely, does not happen if nearly full. The power loss does not trigger without climb, so I do not think it could be some sensor.

In my case the problem can be mitigated by reducing speed , switching into easier gear and charging battery more often.

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  • So for you, the battery's internal resistance likely increased with age. As a result, when it's already somewhat discharged, the voltage can sag too low under heavy load causing it to cut off. Oct 4, 2023 at 20:22
  • Yes I also think so. The battery is expensive so I prefer to use it until it still more or less works.
    – nightrider
    Oct 5, 2023 at 9:13
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If your Bike has a display that can display any errors, this is where you should start your search. The momentary loss of power could be related to various things, but since you describe it happens at the steepest part, there are two things that come to mind:

  • The battery might be near it's end of life. Reason: on the steepest climb your system draws the most power, which might result in the power provided becoming too low.
  • The controller might overheat: The highest current results in the highest heating of the electronics. It's possible they overheat.

If you have a chance to do so, you could try to borrow a battery that has seen less use, and see if you get the same problem. If so, the battery is not your culprit. You might also feel the motor housing (that should double as a heat sink for your electronics) to see if it feels very hot.

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  • Apparently this model needs an external diagnostics tool which I don't have access to.
    – Amarth
    Oct 4, 2023 at 16:49

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