Q On my electric assist bike the motor keeps cutting out and back again every time I turn the pedals. Am I doing something wrong, or is this a fault?
A It doesn’t sound as if this is either. The bike is designed so that the motor only works when you are pedalling, and the way it does this is to have a device that can tell when the pedal cranks are going round.
This device is really quite simple, and it consists of a disc with tiny magnets all round the edge, and an electric pickup fixed to the bike. On some bikes this is all inside the bike frame, but it is often mounted outside between the cranks and the bike frame.
As it is designed to fit any bike it is not a tight fit but just a ‘push fit’. Unfortunately this means that because it can be easily pushed into place, it can easily be knocked out of line.
In my picture, the black disc with metal dots around it is the movable item, and the little black box with a red dot and a wire coming out the top is the sensor.
Of course, not all bikes will be the same, and on some the disc is the other side hidden behind the chain wheel. This makes it harder to get dislodged, but also means it’s harder to push back.
Now, what has probably happened with your problem is that the disc has somehow got bumped and is tilting with one edge nearer the bike and the other tilting up a bit. This means that every time you turn the pedals, first the low edge passes close to the sensor, then the top edge is too far away for the sensor to detect it, and the motor gets a stop/start signal.
That’s taken longer to explain than it does to fix it.
Just push the disc gently down so that it is the same distance from the bike all the way round, and get on and ride away!
It happened to me, so I know.