E-bike motor stopping momentarily during steep climbs

Question

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?

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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).

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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.

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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?

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.

Answer 1

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

Answer 2

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.

Answer 3

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.