This post is about the wheels on the Mars Curiosity Rover. If the wheels are this complex - imagine the complexity of all the technology involved in this amazing robot!
The following information below comes from an article here by Emily Lakdawalla :
Below is a photo of a flight wheel -- that is, one of the six that's sitting on Mars today. Each wheel tire was machined from a single block of aluminum. It is 50 centimeters in diameter and 40 centimeters wide. It has grousers (treads) that protude 7.5 millimeters from the wheel skin. Grousers are spaced 15 degrees apart. Unlike Spirit and Opportunity, the grousers are not straight; they have chevron features designed to prevent sideways slip.
The skin of the wheel is 0.75 millimeters thick -- the absolute thinnest that could be machined. The grousers provide structural strength; the skin is for floating the rover atop loose sand. There is a slight crown to the wheel to make it more robust to the moment of touchdown on Mars (remember that the rover landed on its wheels, with no lander underneath them). There is a vertical rim on each edge of the wheel, again for structural strength. There is another, double rim located about one-third of the way into the tire, the structural stiffener to which the wheel flexures (spokes) are attached. One section of the tire has a set of odometry marker holes drilled into it, which provide a way for the rover's navigational software to measure its driving progress across soft surfaces by photographing the tracks. For fun, these holes spell out "J P L" in Morse code.