Bosch Performance motor "surging"


I recently purchased a Trek Super Commuter 8s with the Bosch Performance motor. I live in a very hilly area and I've been riding it every day. No problems at all, very natural feel. Today I tried a much stepper hill and ended up in the lowest gear on the cassette and in Turbo on the motor. I currently have 125 miles on the bike. At first it was fine but it started surging. I wasn't able to maintain a steady cadence, and if I paused in pedaling, the motor would continue to pull. Then, when I would begin pedaling again, the motor would lag initially before surging again. Once I cleared the hill, it continued this behavior in a number of gear and motor combinations. I immediately took it to my dealer and they said they put it through a diagnostic test. It showed no errors. It feels, to me, like an issue with the pedal sensor.

Any suggestions?


Active Member
This may, or may not apply to your situation, but it's worth mentioning. We all have a dominant hand, and leg, which is stronger than the other. You may unconsciously be applying more force with your strong leg which will cause a surge.

rich c

Well-Known Member
Not sure Bosch uses a traditional pedal sensor. It will be inside the motor case and I would be surprised that if it has a problem, it didn't show up on the diagnostic. I climb two hills on my usual route. Both require lowest gear and turbo setting. Ride them about 5 days a week. Never had that problem.


The other thing is, following that hill, I returned to a portion of my normal ride and the problem continued through that.

When I stopped to push a crosswalk button, the bike pulled forward a couple of extra feet.

On every ride, in every situation before, it felt like the motor responded to me. In other words, I supplied input and the motor responded. Now it feels as if it is trying to get ahead of me. Several times, the bike acted as if self propelled.


I've ridden 60 miles since I picked up the bike following the shop trip. Only thing I can figure is that either the bike experienced a (hopefully) one-time error or the combination of the very steep hill and my lack of strength created a situation where the bike was confused. Either way, it's working fine now. Both Bosch and Trek were very helpful and patient.

Love the bike.