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Why don't bicycles fall over?

Melody Bodette
Vermont Public

It's all about bikes in this episode of But Why? How come bicycles stay up when you're riding, but fall over once you stop? We turn to Andy Ruina, professor of engineering at Cornell University, for the scientific answer. We also learn how a bike chain works and Olympic mountain biker Lea Davison tells how to get started when riding.

Download our learning guides: PDF | Google Slide | Transcript

  • Tall narrow things fall over when they are upright because there is nothing to stabilize and keep them from falling sideways when they start to tip. And they start to tip because gravity is always pushing them down toward the ground. So basically, gravity pulls them over.
  • Imagine a person with feet. We don’t fall over when we stand because our feet are relatively big and help us stay upright. So how does a person balance on stilts? They can’t stand still! They move every time they start to fall to one side or another, allowing them to get the bottom part of their body back underneath the tipping top part.
  • A bike is tall and skinny - and the way it balances is like that person on stilts. The person on the bicycle is always steering from one side to the other to stop from falling over.
  • Imagine someone riding a bicycle. The bike starts to lean to the right a little bit, so the rider starts to steer to the right, just a little bit, to match the tilt, and they don’t fall over. When they steer the bike to the right, it moves the wheels to the right, which essentially moves the wheels underneath the tilted person, and they’re balanced again.
  • So steering a bike isn’t just about going around corners; it’s also crucial for balancing.
  • Scientists have done experiments. If you tie up or lock the steering on a bike and try to ride it, you will fall over. The same thing works on riderless bikes too! A riderless bike can stay up and balance for quite a while. But a riderless bike with fixed or locked steering will fall over fast.
  • Why can a moving bike steer itself and stay up even when a person is not on it? Ruina has written a scientific paper on this, but ultimately, he and his colleagues think it's so complicated no one will ever understand it. Watch a video here.
  • Once you slow down and the bike begins to stop, you can’t balance anymore by moving the wheels underneath the direction the bike and rider are tilting, and the bike will fall over with the force of gravity.

Try it! 

Want to try a balance experiment to illustrate the principles of how a bike stands up (without having to fall off your bike)? Go grab a broom and take it out to an open area where you won’t hit anything if it falls over. Place the end of the broom handle in the palm of your hand, with the bristles of the broom straight up in the air. Now try to balance the broom. You might have to move your hand around to keep it balanced (and maybe even your whole body). If the broom starts to tilt over to the right, move your hand to the right and see if you can get the broom to stay up. If the broom starts to fall to the left, you move the palm of your hand to the left, underneath the broom. See if you can get the broom to stay upright by constantly shifting your hand underneath it to keep the balance just right.

Jane Lindholm is the host, executive producer and creator of But Why: A Podcast For Curious Kids. In addition to her work on our international kids show, she produces special projects for Vermont Public. Until March 2021, she was host and editor of the award-winning Vermont Public program Vermont Edition.
Melody is the Contributing Editor for But Why: A Podcast For Curious Kids and the co-author of two But Why books with Jane Lindholm.

But Why is a project of Vermont Public.

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