These gearmotors have output shafts with a diameter of 4 mm. The Pololu universal aluminum mounting hub for 4mm shafts and hex hub can be used to mount our larger wheels or custom wheels and mechanisms to the gearmotor’s output shaft (see the picture below). These same motors shaft works directly with the 120mm-diameter Wild Thumper wheels (see below). These motors are intended for use at 12 V. In general, these kinds of motors can run at voltages above this nominal voltage, so they should comfortably operate in the 6 – 18 V range. Higher voltages could start negatively affecting the life of the motor.
The left picture below shows this wheel mounted to a universal mounting hub for 4mm shafts.