Bizarre problem with distance measurement

[HPA]

Hi all,
I have a Cube Reaction hybrid race 29 with a bizarre problem. This bike is equipped with a Bosch drive unit. Since a few months the distance is almost 10% off. I did bring him to the shop for a check-up and they replaced the cassette and the chain. My average speed at my first commute after that was suddenly at 28km/h, normally it is around 26. So I checked the distance and that changed from 21 km to 23 km. Back to the shop were they couldn't find anything. So I continued to drive with it like that. A few months later the opposite happened. My commute is now 19 km instead of 21 The speed indication seems normal. So the bike functions normal except that the distance is off. I hope that somebody here can help me out. I checked the wheel size setting, and that is ok. Does anybody know how the distance is measured? Because the displayed speed seems correct, the little magnet and the sensor must be ok. Ideas anybody?

 

[Simon Rafferty]

The same pulses are used for the speed and distance. Speed is just the number of pulses per unit time & distance is the total number of pulses.

Because your speed varies during your journey it's difficult to quantify it being exactly the same. A small indicated speed difference adds up to a much more noticeable distance difference - as that should be exactly the same every time.

I would suspect the position of the magnet / sensor is slightly off so it's not picking up a pulse on every revolution.

In practice, it's never going to be perfectly accurate. I would just treat it as a guide at best.