A synchronous drive apparatus includes first and a second rotors. The rotors have multiple teeth for engaging sections of an elongate drive structure. A rotary load assembly couples to the second rotor. The elongate drive structure engages about the rotors. The first rotor drives and the second rotor is driven by the elongate drive structure. One of the rotors has a non-circular profile having at least two protruding portions alternating with receding portions. The rotary load assembly presents a periodic fluctuating load torque when driven in rotation. The angular positions of the protruding and receding portions of the non-circular profile relative to the angular position of the second rotor, and the magnitude of the eccentricity of the non-circular profile, are such that the non-circular profile applies to the second rotor an opposing fluctuating corrective torque which reduces or cancels the fluctuating load torque of the rotary load assembly.