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I noticed an interesting "contradiction" in the manual of an e-bike: on one hand it states that the frame can accomodate chainrings between 30T and 44T, but on the other hand in another section, the manufacturer advises against using another chainrings than the one sold with the bike, because the "motor is programmed to work optimally with a chainring size of 36T). This series of bike is fitted with Shimano EP6/EP8 motors. Other models of the same manufacturer (Orbea) using the same motor series are fitted with other chainrings sizes, and have similar warnings.

Does it imply that a change of chainring with an ebike requires to update some setting in the firmware of the motor? (or at least for Shimano, other brands may behave differently). And if yes, is it doable with the consumer version of the E-tube Project application (Shimano app meant to update the firmware or customize some aspects of the motor/Di2) or is it a setting that is reserved to the professional version?

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    Hmm, this is interesting. I assume that the motor has a torque sensor to calibrate its assistance. Bicycle power meters also have torque sensors, and we only need the crankarm length as the input, not chainring size. I'm going to leave this as a comment because I don't actually know the correct answer.
    – Weiwen Ng
    Feb 20, 2023 at 20:58
  • Does the manual talk about a preferred tyre sizes? Changing them would also cause the speed calculation to be out if its done on gear choice+crank-rotations, or on wheel rotations.
    – Criggie
    Feb 21, 2023 at 8:24
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    @Criggie unless the derailleur is Di2, no way to know what is the actual gear and hence infer the speed from the cadence (there is a speed sensor on the rear wheel anyway). The manual mentions tire size related to frame/mudguards clearance.
    – Rеnаud
    Feb 21, 2023 at 8:37
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    I think this cannot be answered without reverse-engineering the firmware. Everything would be just speculation, we do not know which algorithm they have implemented and how. Torque depends first of all from the gear ratio, larger chain ring with larger sprocket would give the same. But the controller may be implemented to use individual approaches for each gear, assuming its ratio, and it may figure out the current gear from speed and cadence sensors even without dedicated sensor for the gear. And we do not know if they use so sophisticated approaches or not. The speed sensor would work.
    – nightrider
    Feb 21, 2023 at 8:38
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    @Criggie (and Renaud) if we assume the smallest sprocket, the chainring could be used to work out the maximum speed - the speed sensor on the wheel might be only for display purposes and not hooked into the motor controller
    – Chris H
    Feb 21, 2023 at 12:05

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I asked in a shop: the mechanics told me that indeed, changing a chainring requires to update some setting on the motor, but they can do it.

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