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The speed indication on my ebike is acting up - most of the time it's showing a correct reading, but it occasionally drops to impossibly low values like 2 km/h for a few seconds while riding, then it recovers. Sometimes the Bosch controller will also briefly indicate "Error 503", which is an issue with the speed sensor.

Searching online for this issue will bring up many videos like this one, where it's explained there is a magnet on the rear wheel and a sensor on the chainstay, much like an ordinary bike computer in traditional bikes, and that this magnet sometimes moves out of position, causing the error.

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The trouble is that on my bike, I can't find anything like that - there's no magnet on the wheel and there's nothing like that sensor on the chainstay:

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In that second picture there are two cables running into the frame - one is the brake cable and the other goes somewhere between frame dropout and the brake disc, I can't see where it ends without taking the wheel off. Here's what it looks like from the bottom (taken at the point where it exits the chainstay, looking back):

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Does this bike use some kind of different speed sensor setup?

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  • It could be integrated inside the motor and have nothing visible on the outside at all.
    – Criggie ♦
    Sep 13 at 19:30

2 Answers 2

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For some e-bikes, the magnet is part of the brake rotor assembly. The sensor is then positioned accordingly. An example of such rotor is given in the picture below (your seems a bit different, but the principle can be the same).

brake rotor with magnet

On rotors mounted with bolts (6-bolt, or the Nexus pictured in the question), it's also possible to fix a magnet using the rotor bolts, the picture below shows an example of such magnet (from Trek).

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And for reference, there's also another mounting option with Magura brakes, although not applicable in this case (from Orbea Kemen BluePaper):

enter image description here

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    So this rotor has the magnet built into it and is marketed as such, here it is if I read the model number correctly: bike.shimano.com/en-US/product/component/…. Mine is SM-RTC60, without a sensor built in, so I guess it must be somewhere else. I'll have it checked out today and I'll report back.
    – user4520
    Mar 27 at 10:09
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    @user4520 I edited the answer to provide another example of magnet to mount using the rotor bolts.
    – Renaud
    Mar 27 at 10:21
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    The magnet is actually mounted on the brake disc as you suggested initially, it was just obscured by dirt. Thanks
    – user4520
    Mar 28 at 21:04
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If the previous info doesn't help I suggest checking the controller configuration because the controller uses the magnetic sensor data to calculate and display the speed. If one of several parameters is not correct this calculation will be wrong. I prefer to use a GPS speedometer. A smartphone running a speedometer app works fine.

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    The primary use of speed sensors on e-bikes is not to display speed (as you indicate, GPS-based systems are also good - bad parameters on e-bikes are also unlikely, as it's configured in the factory), but to comply with cut-out regulations. Without working speed sensor connected to the drive unit, electric assist won't be enabled, so a GPS-based speedometer is not a acceptable substitute to a non-working speed sensor - displays are common on e-bikes, but not always present. Some minimalist brands use LEDs do indicate the charging level, and do not have display, but still have a speed sensor.
    – Renaud
    Sep 13 at 16:10

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