Bosch’s flagship mountain bike system uses a mini drive ring with internal gearing to send its power to the drivetrain. There’s some resistance in the system over 25km/h, but when you first press down on the pedals there’s an impressive surge of power, and it offers good support over a wide cadence range. Its size has an impact on the width of the cranks (the Q-factor) as well as the chainstay length of the frame, and it’s not the lightest system on the market at 4kg for the motor. On the other hand, Bosch is the most established player on the market, and its system has proven itself over many years.