Why Don’t Bicycles Fall Over?

Aug 5, 2016

It's all about bikes in this episode of But Why? Why bicycles can stay up when you're riding them, but fall over when stopped. Olympian Lea Davison tells how to get started when riding, and we learn how a bike chain moves a wheel.

Four-year-old Ryland from Underhill, VT says he want to learn about why bicycles don't fall over when they are going straight or leaning around the curve but they do when they are stopped?

For an answer, we turned to Andy Ruina, a professor of engineering at Cornell University.

"The reason it's a good question is because some things you think should fall over, and some things you think shouldn't fall over. If you look at a glass on a table and no one is shaking the table, you think, that shouldn't fall over.

"Why bicycles don't fall over when they are going straight or leaning around the curve but they do when they are stopped?" — Ryland, 4, Underhill, Vt.

"What are the things you think would fall over? The things that you think would fall over are the things that aren't held up in a very broad or very wide way at the bottom. Like if you take a broom or a mop and take the head of it and put it up and take the handle and put it on the floor, and you try to balance it, that falls over.  If you take tall things and you just support them on a point or support them on a narrow line they fall over. A bicycle is really that kind of thing.

"Imagine that someone is riding a bicycle and you are right behind them. It looks like the bicycle is balanced on a point because the two wheels are one in front of the other. There's nothing to keep the bike from falling over sideways. That's the way brooms are and needles and pins and the things that you think fall over. There's nothing to keep them from falling over sideways.

"Why do they fall over sideways? Because if they just tipped a little one way or the other — and they're always a little one way or another — and gravity is pushing down and the ground is tipping up, and those two pushes are not lined up with each other, and it makes it turn over. If it's leaning to the right, gravity pushes it more to the right, and the ground force is on the left and it pushes it over. That's why things fall over, because if they tip a little bit there's nothing to hold them up from falling over. The gravity force pulls them over.

Bike pedals are fixed to a sprocket. When you pedal you are turning the sprocket. The teeth are connected to a chain which spins a sprocket on the back wheel of the bike.
Credit Melody Bodette / VPR

"Now the thing that's easiest to understand is a person standing up. A person standing up, they have big feet. So you say they shouldn't fall over because their feet are big and that keeps them from tipping over.

"What about a person on stilts? If somebody is standing on stilts, they basically can't stand still because if they did, they'd fall over like broom or a needle or any tall narrow thing.

"What people do when they are on stilts, is if they start to take a fall, they take a step. They move the support point over to the side to keep them from falling. That's what happens on a bicycle too.

"If you look at a bicycle from behind, someone is riding the bicycle. Imagine they start to fall, and the bicycle is leaning to the right, what they have to do is move the wheels to the right to keep them from falling over. They want to take a step with the wheels, but you can't take a step on a bicycle. What happens is the person on the bicycle turns the handlebars to the right and because the bicycle is moving forward, when they steer the bike to the right it moves the wheels to the right. So it moves where the wheels are touching the ground back under the person and they can balance.

"If a bicycle is not moving forward, you steer to the right and it doesn't move to the right at all, you just turn the wheel and you sit there in place. What's the thing about a bicycle moving is that when a bicycle is moving it makes it so that when you steer the bicycle, you turn the handlebars it moves the tires sideways. You think of steering the bicycle as causing it to go around corners, but really the main thing steering does is it balances the bicycle because on a moving bicycle you steer it and it moves sideways.

"If the bicycle can't steer it can't stay up and if it's not moving, it can't stay up either."

— Andy Ruina, professor of engineering, Cornell University

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

Also in this episode, we learn the mechanics of a bike and about how to get started from Olympic mountain biker Lea Davison.