A bicycle propulsion system including a concentric rotor assembly and a chassis assembly. The concentric rotor assembly: defines an outer drive surface; defines an inner retention surface; includes a set of sprocket brackets arranged about the inner retention surface of the concentric rotor assembly and configured to engage with teeth of a bicycle sprocket; and is configured to engage around the bicycle sprocket, wherein a center axis of the outer drive surface is concentric with a rotational axis of the bicycle sprocket. The chassis assembly: is configured to secure to a stay of the bicycle; includes a retention subassembly configured to translationally constrain the concentric rotor assembly relative to the chassis assembly; includes a drive subassembly configured to engage the outer drive surface of the concentric rotor assembly; and includes a motor configured to rotate the concentric rotor assembly via the drive subassembly.