This track set includes a pair of 22-tooth silicone tracks, two black drive sprockets measuring 35 mm (1.4″) in diameter, and two matching idler sprockets along with mounting hardware. To work properly with the track, the drive and idler sprockets should be set up approximately 48 mm (1.9″) apart. The drive sprockets are designed to work with 3mm D-shafts, such as those on our micro metal gearmotors and mini plastic gearmotors. These are the same tracks and sprockets we use on our Zumo chassis and robots.