9mm diameter rubber nub semi-hemispherical wheels designed to be press fit onto 1-2.5mm shafts.
We discovered that in the early days of BEAM robotics, we could use high-speed, low-torque pager motors as effective robot motors. We did this by putting a few layers of heat-shrink on the output shaft and tilting the motor, so only the tip of the motor shaft (and the heat shrink) touched the ground. This was effectively, a wheel with a diameter smaller than the motor. This let us generate enough torque to actually move tiny robot frames on a smooth surface.
This technique of tilting motors has worked splendidly in our Photopopper and Herbie-the-Mousebot kits. But as we've found out, heat-shrink wears quickly, and our hack of using pinch rollers as wheels wasn't very easy for beginner builders to do. SO, we've created our own, custom nub-wheel!
We're now using this wheel in our own Herbie kits, and it even acts as a drive wheel for the SolarSpeeder (9mm diameter is bigger than the 7mm motor diameter when the motor is laid flat parallel to the ground).