jons
New Member
I have owned a Trek Rail 9.7 for almost one year, having put about 3500 Km on it. At about the 2500 Km mark I noticed that every once in a while the motor would cut out or stutter so I would lose one pedal stroke every few hundred strokes. It appeared to me that the speed sensor was missing a reading every so often from the speed magnet located on the rear brake rotor. I ignored the problem because it wasn't a big issue for me. At about the 3000 Km mark I decided to change the brake rotors from the manufacturer installed ones to ones with heat fins (and they work just fine, I also changed from metal brake pads to ceramic brake pads to avoid the annoying squeal)). I instantly noticed that the motor would cut out constantly, like every second or third pedal stroke. The speed on the Purion would jump up by 5 or 6 Km/hr causing the motor to shutoff whenever the true speed was over 27 Km/hr since that would lead the Purion to think I was traveling above 32 Km/hr, which is the motor cutoff speed. The speed would also read about 5 or 6 Km/hr less than the true speed. These erroneous speeds would occur in a flash, like 0.5 seconds, but would be long enough the cutoff the motor. On closer inspection of the speed sensor and speed magnet I noticed that the magnet spins "beside" the sensor and not "over" the sensor. Having changed the brake rotor caused the magnetic field to be altered (weakened and modified from the original configuration (see my first photo for original placement of the speed magnet)). Hence, the speed sensor was not sensing the magnet on every revolution of the wheel. So I repositioned the speed magnet by rotating it so that the magnet passes over the speed sensor instead of beside the speed sensor. I did this by disconnecting the trailing screw of the magnet and slightly rotating the magnet and re-securing the magnet using just the leading edge screw so that any debris would not break off the magnet (see my second photo for the new placement of the magnet). Now the magnet rotates over the speed sensor (see my third photo). After riding an additional 500 Km with the newly reposition magnet I have no problems with the motor stuttering or shutting off. I hope this post is useful to others facing a similar problem that can be solved without a trip to the LBS. Thanks for reading.
