PBS
The Math of Bicycles: Wheel Figure This Out (Grades 4 7)
Several good activities here. Look particularly at Wheel Figure It Out (which is the opening page) and Gearing Up. There are answers to the very good questions at the bottom of each.
Texas Instruments
Texas Instruments: Numb3 Rs: Cycloid I
Based off of the hit television show NUMB3RS, this lesson is more about student prediction and reasoning than actual formal math. In the lesson, students are to propose the shape that would be formed by a point on a bicycle tire, while...