The Pololu Zumo chassis is a small, tracked robot platform that is less than 10 cm on each side, allowing it to qualify for Mini Sumo competitions. The main body is composed of black ABS and features a compartment for four AA batteries and sockets for two micro metal gearmotors. It ships as a kit with two silicone tracks, two drive and two idler sprockets, a 1/16″ acrylic mounting plate, and mounting hardware. (Motors and batteries are not included.)
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Lot's of info at the Pololu Website