These powerful 6V gear motors with 20-25mm diameters are a good blend of size and power while operating at 6V makes them easy to run of many battery packs like 7.2V NiMh packs. Some variants are available with high precision encoders for speed control feedback from the motor.
The output shafts are a D shaft shape making it easy to attach a wheel and lock it in place with a press fit or a locking screw. Power leads can be soldered easily to the tabs at the back of the motors.
Specifications for speed and torque at given for the output shaft of the gearbox.
Rated at 6V these motors can be driven at a wide range of voltages with corresponding effects on the power output. Voltages significantly higher than 6V will shorten the life of the motor due to excessive arcing in the brushes but they are fine running off a 7.2V battery pack. Reasonable performance can be had at lower than 6V however the motor will be less powerful.