These wheels are designed to press-fit securely onto the 3mm D output shafts on many of our gearmotors, including pololu's micro metal gearmotors and our plastic gearmotors with 3mm D-shafts. They also work with our 35:1 and 115:1 15.5D metal gearmotors, as well as with our selection of Solarbotics metal gear motors. The wheels have silicone tires and measure 60 mm (2.36″) in diameter. Two additional mounting holes for 4-40 screws make it possible to use the wheel with our universal mounting hubs for 3mm shafts, 4mm shafts, and 5mm shafts. The slots in the five spokes allow additional accessories to be mounted to the wheel (e.g. decorations or parts of an encoder system).