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This is But Why, a podcast for curious kids from Vermont Public. I'm Jane Lindholm. On this show, you ask the questions and we find the right people to help answer them. It's summertime, at least in the Northern Hemisphere. I hope that means you're finding time to get outside and maybe ride your bike.
Later in the show, we're going to hear from someone who liked riding bikes so much she made it into a career. Leah Davison, a former Olympic mountain biker. But for most of us, cycling is just something we do for fun. It's a hobby. When you're learning to ride, the real hurdle is getting confident in figuring out how to balance on just two wheels. And the mysteries of balancing got one of you curious enough to ask this question.
Hi, I'm Ryland. I come from Underhill, Vermont. I'm four years old. I want to learn about why bicycles do not fall over when they're going straight or leaning around the curve, but they do when they're stopped. Hi, Ryland. I'm Andy Ruina, and I live in Ithaca, New York, where I teach engineering at Cornell University. So I'm a teacher. Professor Ruina says he became interested in this question at about the same age as Ryland.
In fact, I found a note that my mother wrote about me when I was four years old and I asked her or somebody, I asked, why does the top stay up when it's spinning and not when it's not spinning? Which isn't quite exactly the same question you're asking and it has a different answer than your question, but I've been wondering about things like that since I was your age.
Let's get to the answer, but I'm going to warn you, it's a little bit complicated. The reason it's a good question is because some things you sort of think should fall over and some things you sort of think shouldn't fall over. If you look at a glass on a table and nobody's shaking the table, you think that shouldn't fall over. Or you look at the table, you don't think it's going to fall over.
Or if you look at a box on the floor, you don't think it's going to fall over. Or if you look at a lamp hanging from the ceiling, you don't think it's going to fall over. So what are the things that you think would fall over? The things that you think would fall over are things that aren't really held up in a very broad or very wide way at the bottom. Like if you take a broom or a mop,
and you take the head of it and put it up and put the handle on the floor and you try to balance it, that falls over. Or if you take a needle or a pin and try to stick it up, it falls over. Or if you take a toothpick and put it on the ground or a table, it falls over. If you take tall things and you just support them on a point or you support them on a narrow line, they fall over.
A bicycle is really that kind of thing. Imagine that someone's riding a bicycle and maybe they're not riding a bicycle, maybe the bicycle is still or it's moving and you are right behind them. If you're right behind them, just straight behind them, it looks like the bicycle is balanced on a point because the two wheels are one in front of the other and there's nothing to keep the bike from falling over sideways.
So that's the way brooms are and needles and pins and the things that you think fall over is there's nothing to keep them from falling over sideways. And then why do they fall over sideways? It's because, well, if they just tipped a little one way or another, and they're always tipped a little one way or another, then gravity is pushing down and the ground is pushing up. And those two pushes are not lined up with each other and it makes a turnover. Say if it's leaning to the right,
Gravity pushes it more to the right, and the ground force, or where the ground is touching is on the left, and it pushes it over. So that's why things fall over, is because if they tip a little bit, there's nothing to hold them up from falling over. The gravity force pulls them over. Gravity is the force that makes things fall to the ground if we drop them. The thing that's really maybe the easiest to understand is, say, a person standing up. A person standing up, they have big feet.
And so you say, well, they shouldn't fall over because their feet are big and that keeps them from tipping over. Well, what about a person who's standing on stilts? You know what stilts are? Stilts are these sticks and people walk around on stilts and they're just little sticks. They're only about an inch across at the bottom. If somebody's standing on stilts, they basically can't stand still because if they did, they'd fall over like a...
broom or a needle or a toothpick or any tall narrow thing. What do people do when they're on stilts is when they start to fall they take a step. So they move the support point over the side to keep them from falling and that's what happens on a bicycle too. Though it's a little hard to see it so I'll try to give you the picture of it. If you look at a bicycle from behind
When someone's riding a bicycle, so you're looking at them from behind, and imagine they start to fall to the right, and the bicycle's leading 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. So what happens is the person on the bicycle turns the handlebar to the right,
And because the bicycle is moving forwards, 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 underneath the person and they can balance. So just like when you're on stilts and you fall to the right, you take a step and you move the place where the feet touch the ground or the stilts touch the ground under the person. When a bicycle falls to the right,
The person steers the bike to the right and moves the wheels under them to the right and then they're held up from underneath where their weight is and they don't fall over. If a bicycle's not going forwards, you steer it to the right and it doesn't move to the right at all because the bicycle just turns the wheel and the wheel just sits there in place. So 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 in corners, but really the main thing steering does is it balances the bicycle because on a moving bicycle when you steer it, it moves sideways.
Now you might wonder about that maybe it's because the wheels are spinning or maybe you're wiggling your body to keep it up and maybe it's because it's going forwards and some people will say momentum keeps the bicycle up and all those things are nonsense. And the way you can prove that they're nonsense is you can take a bicycle and you can tie up the steering with rope so you can't steer it.
Then you get on the bicycle and somebody pushes you to get you going forwards. And you know what you do? You fall down and there's nothing you can do about it.
If the bicycle can't steer, the bicycle can't stay up. And if the bicycle's not moving, it can't stay up either. Actually, he says some really talented circus performers can stay up without moving forward. But that's because they are steering or balancing with their bodies. Sometimes you see bikers stopped at traffic lights who appear to be staying upright without moving. That's called a track stand.
Professor Ruina says they're actually moving forward and back and using the sideways motion of the wheel to balance. But for the rest of us, it's the mix of going forward and steering that balances the bicycle. Want to try a balance experiment to illustrate the principles Professor Ruina was explaining? Go grab a broom. Put the end of the stick of the broom in the palm of your hand, the top of the handle, so that the bristles of the broom are straight up in the air.
Now try to balance the broom. You might have to move your hand around to keep it balanced. Maybe you should try this outside. 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, right 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.
There's another question that Professor Ruina has spent a lot of time thinking about. How can a bike balance itself, even if a person is not on it? Not when the bike is standing still. We already learned about that. But if you push a bike forward, so its wheels are spinning, but there's no one actually riding it or keeping it balanced, the bike can still move forward and stay upright. How does it do that? The simple answer is, for some complicated reason,
The bicycle has built in it various things that make it steer to the right when it's falling to the right. So it makes it so that the wheels go back under the bike when it's falling to the right. And that's so complicated that we wrote a paper that's like a story, what scientists do in their working universities. They write these things called papers, which are stories for other scientists. And basically, the story was that we think that's so complicated that nobody will ever understand it.
So we worked so hard to understand it, and we couldn't do it. We think nobody ever will understand it. So I can tell you a lot about it, but basically why exactly a bicycle all by itself turns the right amount is kind of a mystery. That was Andy Ruina, a professor at Cornell University. This is But Why, a podcast for curious kids. I'm Jane Lindholm. Today we're learning all about bikes. And now we know how a bike stays up while you're riding it.
But how does a bike actually work? What makes the wheels spin? And what's that chain for? We stopped by a local bike shop to find an expert. I'm Kevin Rose, and I manage the Trailside Center for Local Motion down on the waterfront in Burlington, Vermont. Local Motion is a group that promotes safe cycling.
Actually, one of the things that's always attracted me to the bicycle is it is so simple. It's one of the simplest and actually one of the most efficient machines known. Just in terms of the amount of energy that it takes to move something through space, there really isn't much that's more efficient than a bicycle. Bicycles...
today really don't look that much different than they did say a hundred years ago with the two wheels the frame the handlebar the pedals the the crank which connects what's known as a chain ring to the rear wheel on gears on the rear and the rider by spinning those cranks moves the bicycle forward and of course we don't want to forget the brakes when you need to come to a stop so how does it work
Let's start by picturing the bike. You're seated on a seat, hands on the handlebars, your feet are on the pedals. You're turning the pedals around. The pedals are connected to what's known as a crank, which is turning what's known as a chain ring, which is a large sprocket up front that is connected to the rear wheel with a chain, and that chain is driving the rear wheel of the bicycle.
So the rear wheel has another sprocket, or quite often it'll have a few of them, which allows you the ability to pedal up hills or fast on the flats or down hills. So it allows you to change speeds essentially. It's a front sprocket, the one that is pedaled, that's connected to a rear sprocket on the wheel and the chain connects the two and drives the whole thing around.
So when you pedal, you're turning the chainring. That's the circle thing that looks like it has teeth on it. Those teeth move the chain, which moves the back wheel. If you shift your front gears, you're moving to a different chainring, and that determines how fast you have to pedal to turn the wheel.
The rear wheel also usually has a few gears you can shift into and out of. Those are helpful when you're going up and down hills. But most of us start out on bikes with no gears. And when bikes were first built, they were even simpler. Well, back...
When the early days of bicycling, if you've ever seen pictures of bicycles with a very large wheel on them, it was sort of like the tricycles today where the pedals are attached directly to the wheel. It was known as a direct drive. And in order to go faster, you needed a bigger wheel. In other words, when you turn that wheel one revolution,
a larger wheel goes further than a smaller wheel. So if you wanted to go faster, your option was a bicycle with a bigger wheel. It wasn't until about 130 years ago that the first bicycle that connected
The sprocket up front to the wheel was introduced. It was actually called a safety bicycle. And I don't know, maybe it was because the bicycles were so high, those high wheelers, that the safety bicycle got you lower to the ground. But anyhow, the chain...
allowed you to have a larger sprocket up front and a smaller sprocket on the rear to effectively make the wheel the same size as that large wheeled bicycle. So rather than having a large wheel you had a large sprocket up front driving a small sprocket in the rear and it allowed you to go faster.
Kevin says he fell in love with bikes when he was a kid because of the freedom it gave him to explore his neighborhood. I hope all of this talk about bicycles has made you want to get out and ride. But maybe you don't know how yet or you're still learning. Everyone has to start somewhere, even the best bike riders in the world. Leah Davison is a professional mountain biker who represented the United States in two Olympic Games.
She stopped by to show me the best way to get on a bike and get going without falling.
So you have to swing your leg over the saddle. And so then you're just like, the bike is, you're standing over, you're like straddling the bike. Saddle being what many of us would call a seat. Yes, a seat, exactly. And then you want to make sure to get going so you can get going before, like, and you don't want to fall. So you bring one pedal up to what we call the power pedal position. So if your pedals were like the armrests,
the arms of a clock. One pedal would be at like the 12 o'clock, one o'clock position. And then you put your foot on it and you want to push down on that so you have momentum and speed when you're going.
Then what do you do? So then you either sit on the seat or stand up in what we call the downhill attack position. So you stand up off your saddle, you bend your elbows and knees, and you put your pedals level to the ground. So again, with the clock reference, if your pedals were...
arms of a clock, they would be at 3 o'clock and 9 o'clock. So that way your pedals don't hit any roots or rocks and you can safely go over obstacles. So you're basically standing up like you would on the ground, like your feet are sort of in the same position. Exactly, exactly. And that's the best way to go downhill or to go over obstacles. If you're climbing, you can be seated and you're pretty much just pushing down hard on the pedals.
When you're climbing, how do you want to shift gears so that you can make it the most efficient way to climb? Yeah, so before the climb you want to make sure to shift to an easier gear or maybe your easiest gear so that way you don't get halfway up the climb and you can't pedal anymore. What about protection? You should always wear a helmet. Always, always wear a helmet. Even if you're on a bike and you're pedaling at 10 feet, then you should have a helmet on.
I just love being outside. And when you mountain bike, you get to ride single track in the woods. And it's the best way to be outside. And also when you ride a bike, you get to see a lot. So you get to go further than if you run and you get to go fast. So that's why I love mountain biking.
Some people are afraid, like me, a little bit afraid of going fast downhill on a bike over roots and rocks. How do you get over that fear? I think it takes a progression. So you just slowly ease into it. So you start by, you know, going off the curb of a sidewalk or you start by going over just a little rock and then you build up to bigger rocks, bigger roots. So it takes you don't have to do it all at once. You just ease into it.
What's it like to go to the Olympics? The Olympics is everything that's cracked up to be and more. So it really is. I mean, it sounds cliche, but it is a dream come true. And
Everything about it is just such an incredible experience, from meeting different athletes in the village. You never know who's going to walk in the elevator in the Team USA housing. It could be Michael Phelps. It could be Missy Franklin, anyone. Just from having the support of your entire hometown or the state of Vermont and representing your country is really...
something that most people don't get to experience and it's an incredible thing. So many kids want to be professional athletes and maybe it's I want to be in the NBA or I want to be a professional baseball player or I want to go to the Olympics. And you kind of need that drive to make it happen. But there are also a lot of adults in our lives who are like, oh, that's a nice dream, honey, but why don't you set your sights on something that's more reasonable? But
But you've made it happen. So what's your advice to kids who, you know, at age five or eight or 10 are saying, no, this is what I think I want to do. And I want to be an athlete and I want to make it happen. Yeah. You can make it happen. You know, my advice to kids, I was that same kid with the dream of going to the Olympics. And when I was a kid, it was I thought it was going to happen with downhill ski racing. So.
It just takes a lot of hard work. You just need to work very hard. But you can set your mind to anything that you want to do. That was Leah Davison. She got to compete in the Olympics twice. And she still has a career in cycling as a coach and commentator. And she started a really cool organization with her sister called Little Bellas that teaches girls how to ride mountain bikes.
That's it for today. Thanks to all of our guests for this show. I hope you now understand a little bit more about bikes, how to ride them, and how they work. If you have a question about bugs or bikes or hugs and hikes or anything else, rhyming or not, let us know. Have an adult help you record your question on a smartphone. Tell us your name, how old you are, and where you live. And for the adults who are listening, try not to record your kids in the car. We like a nice, quiet environment if possible.
Then send the question to questions at butwhykids.org. We can't answer every question we get, but we love hearing from you and knowing what's on your mind so it can help us shape future episodes. But Why is produced at Vermont Public by Melody Beaudet and me, Jane Lindholm. Our engagement producer is Kiana Haskin. We're distributed by PRX, and Luke Reynolds wrote our theme music. We'll be back in two weeks with an all-new episode. Until then, stay curious.
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