Gear motors should be mounted onto the side of the hub with the protruding teeth. The output shaft will slide into the socket easily at first but will achieve a snug fit when pressed through to the other edge of the hub.
The output shafts on the Solarbotics metal gear motors will also fit these wheels, but because they are shorter they will have to be mounted onto the other side of the wheel where the fit is tighter.
The Micro Metal Gearmotor Bracket Extended is specifically designed to work with this wheel. Please note that the non-extended version of our mounting bracket will not work well with this wheel.
This product and description is from DFRobot.