Dagu Wild Thumper Wheel 120x60mm can be used as replacement parts or for custom robots that need to traverse complex, rugged terrain. The black tires are made of soft, spiked rubber for increased traction, and the plastic hubs are chrome (silver) in color. The included adapters let you easily mount the wheels to motors with 4mm output shafts. These wheels are sold in pairs: you get two wheels, two 4mm adapters, and one 3mm hex wrench that matches the adapter screws.
Pololu Web Site: https://www.pololu.com/product/1557/