1-2 hours

1-2 hours

Axles, wheels, gears, mechanisms

Ben Finio, PhD, Science Buddies

K’NEX is a registered trademark of Connector Set Limited Partnership.


What in the world is a differential and what does it do? A differential is a mechanism that helps cars go around turns without the wheels slipping. In this activity you will build your own working model differential from a building toy like K’Nex®!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.


  • Large set of K’Nex® or similar construction toy

  • Alternatively, you could use craft materials like pencils, toothpicks, and cardboard, although this approach may be more difficult.


  1. Watch this video and follow along to build your own working model differential from K’Nex®. Note that this activity does not provide step-by-step picture instructions like the ones that come with K’Nex sets. You will need to have some experience working with K’Nex, know how to connect the pieces, and know how to build some basic parts (axles, support structures, etc). The steps from the video are outlined below, along with some pictures of the key parts. Note that the structure you build does not need to match the video exactly. This is just one way to build a model differential.

  2. Build a support structure to hold two axles. The axles should be aligned with each other, but not connected. Connect a wheel to one end of each axle. The wheel should rotate with the axle, not spin freely on the axle. This picture shows a close-up of the support and axle.

  3. Connect a spoke to the other end of each axle. The spoke should rotate with the axle, not spin freely.

  4. You can use a bar to push on both spokes at the same time, rotating both wheels. Make sure you have these basic parts working before you proceed.

  5. The differential mechanism’s gears are the most complicated part to build. This picture shows a close-up of the gears. Watch the video for step-by-step instructions on how to build this part of the mechanism.

  6. Practice using your differential. When the amount of resistance on both wheels is the same, they should spin at the same speed. If you grab one wheel to stop it, the other wheel should keep spinning.

  7. Optionally, depending on the K’Nex pieces you have available, you can add a chain drive, a driveshaft, or even a motor to your differential. This picture shows a complete model, including a chain drive and a hand crank.

What Happened?

You should find that when you build a functioning differential mechanism, both wheels will spin at the same speed if the resistance on them is the same. However, both wheels can spin independently when needed. This means the wheels can spin at different speeds—one can spin while the other one is stopped, and they can even spin in opposite directions. However, both wheels can still be driven by a single connection to the axle. In the case of the model demonstrated in the video for this project, that connection was a chain driven by a hand crank. In a real car, that connection would be to a driveshaft which is connected to the engine.

Digging Deeper

When a car goes around a turn, the wheels on the outside of the turn must travel a longer distance in the same amount of time as the wheels on the inside. This means the outer wheels need to rotate faster in order to avoid slipping with respect to the ground. A differential allows both wheels on a powered axle to spin at different speeds. This video provides an excellent explanation of how a differential works and why it is needed:

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For Further Exploration

  • Try adding a chain, driveshaft, or motor to your differential model.
  • Try building a working differential from craft materials instead of construction toys.


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