The inside black portion of this wheel is made from molded polycarbonate plastic. The outer tread is TPU (Thermoplastic PolyUrethane). This robust but lightweight wheel provides good traction on carpet and soft tiles. Stealth wheels are the perfect drive wheel for smaller (<50 lb.) robots, or for intakes and other auxiliary robot mechanisms. These wheels are available in a variety of bores and durometers. 3 in. and 4 in. Stealth Wheels feature a 6 bolt pattern on a 1.875 in. bolt circle for mounting sprockets, hubs, and gears. Nub Bore Stealth Wheels feature a 4 bolt pattern on a 16 mm bolt circle found in most FIRST Tech Challenge building systems. The lower the durometer the softer the wheel is, this makes it wear away faster, but also makes it more grippy.