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.
Wikimedia
Wikipedia: Wheel
An encyclopedia article from Wikipedia on the wheel discusses how wheels operate and travel on surfaces, and gives information on the invention and use of the wheel in ancient times. Links are also provided for additional information.
TeachEngineering
Teach Engineering: Wheeling It In!
In an open-ended design activity, students use everyday materials (milk cartons, water bottles, pencils, straws, candy) to build a small-scale transportation device. They incorporate the use of a wheel and axle, and lever into their...
Science Struck
Science Struck: Simple Machine: All About the Wheel and Axle
Describes some history of the wheel and axle, how it works, the formula for calculating its mechanical advantage, and some applications of the wheel and axle.
Sophia Learning
Sophia: Simple Machines: Wheel and Axel: Lesson 1
This lesson will explain how to calculate the mechanical advantage of a wheel and axel. It is 1 of 2 in the series titled "Simple Machines: Wheel and Axel."
Sophia Learning
Sophia: Simple Machines: Wheel and Axel: Lesson 2
This lesson will explain how to calculate the mechanical advantage of a wheel and axel. It is 2 of 2 in the series titled "Simple Machines: Wheel and Axel."
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.