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.
Scholastic
Scholastic: Dirtmeister's Science Reporters: Simple Machines: The Wheel and Axle
An in-depth explanation of the wheel and axle from Scholastic. Also provides several examples and a picture for understanding.
Other
University of Arkansas: Simple Machines
A thorough discussion of all the simple machines, beginning with some thoughts on mechanical advantage and conservation of energy. Very complete. Move down to the section that interests you.
Integrated Publishing
Integrated Publishing: The Wheel & Axle
A solid chapter explanation of the wheel & axle simple machine. Examples, the basic math, explanation of mechanical advantage, drawings, solved problem. Use the Next button. There are four pages in this chapter.
University of Houston
University of Houston: Simple Machines Learning Site
This resource provides information about simple machines.