Like the other mid-motors, Bosch pedal-assist machines measure the power you are delivering to the pedals (just as a stationary exercise bike does) and then adds to it in four levels or modes of assist. Bosch calls its Eco, Tour, Sport, and Turbo. Like the others, it electronically measures the torque you deliver to the pedals, pedal crank cadence (rpm), and bicycle speed, then employs software on a built-in circuit board to deliver the power-assist percentage you choose. Electrical power from the battery is then delivered through power transistors to the permanent-magnet motor, which converts it into mechanical power. Because the motor must spin rapidly to develop best power, there is internal gearing to drive the pedal crank through two reductions (in one case the ratio is 35.5 to one). All these parts are densely packaged into a cast-aluminum housing, allowing a Q-factor (width across the outer faces of the pedal crank arms) of 180mm (slightly more than 7 inches). Compare this with a typical mountain bike Q of 170mm—the difference is small.