This pdf file from Chico State University presents an in depth look at LEGO Gears, Pulleys, Wheels, Tires and Center-of-Gravity. Although some images are based on the older LEGO RIS brick, all the content is applicable to the NXT. http://www.ecst.csuchico.edu/~juliano/csci224/Slides/03%20-%20Gears%20Pu... [1]