Most People Don't Know How Bikes Work

Most people don't know how bicycles actually work.

• [Off screen] Let's try it again. So we modified this bike to prove it. This video was sponsored by KiwiCo. More about them at the end of the show.

When you're riding a bike and you want to turn left, I think most people just imagine you turn the handlebars to the left. This is a bike to test whether that is true. And it's made by my friend Rick here. And he's got a radio controller that allows him to lock out the steering to one side. So, what he's gonna do is as I'm biking, he's gonna pick whether I can turn either to the left or to the right. So, go for it.

• [Rick] I'm giving it a left turn. It pulls the pin out, but you can see that you can still fully steer after I've pulled the pin out. I've armed it. There's where it locks.

OK. Now, that that's when your LED comes on and that just says turn that way.

• Turn left.

• Yeah. And if I try to turn right, I can't. And if I try to turn left-

• You can.

• I can. So the question is can I successfully execute this left-hand turn? Should we give it a shot? I mean, he's not gonna tell me whether it's left or right, so I have to look at the LED to know which way I can still turn.

• [Rick] You let me know when you're ready.

• Okay. (exclaims) No! That was meant to be a turn to the right but there was no chance in hell. Left. (exclaims) Right. All right. (exclaims) Right, right, right! God!

If you look closely, you can see the problem. Here, I'm trying to turn right but steering that way puts me off balance. If you could ride this bicycle, you would find it's impossible to turn left without first steering right and it's impossible to turn right without first steering left. This seems wrong. I think most people believe you turn a bike simply by pointing the handlebars in the direction you want to go.

After all, this is how you drive a car. Point the front wheels any direction you like and the car just goes that way. But the difference with a bicycle is steering doesn't just affect the direction you're headed; it also affects your balance. Imagine you want to make a right turn so you steer the handlebars to the right. What you've done is effectively steered the bike out from under you.

So now you're leaning to the left and the ground puts a force on the bike to the left so the only way not to fall is to steer the bike to the left. You have made a left turn. If you really wanted to turn right, you first have to counter-steer to the left so you can lean right into the turn. This is something anyone who rides a bike knows intuitively but not explicitly.

• Turn left!

Film someone riding a bike towards you and tell them which direction to turn and you will find that they counter-steer without even thinking about it.

• Hard left!

When you're riding a bike, it's exactly the same as what we call an inverted pendulum or balancing a broomstick on your hand. If I'm balancing it and I just start walking toward you, it will always fall away from you. If I want to walk towards you, it's easy enough to do and people inherently know how to do it. If I pull it backward, I can now start walking that way. I have to initiate the lean to turn into it.

If you want to move the pendulum somewhere, you first move the base in the opposite direction. And now the pendulum is leaning in the direction you want to go so you can move with it. And it's the same with a unicycle. In order to go forward, first, you have to pedal back. So, you're leaning forward and then you can go forward with it.

Everything you're doing on a unicycle is all about keeping that contact patch right where it needs to be relative to you. You're balancing the broomstick. It's just that on a unicycle, you do the longitudinal balance with the pedals and you do the lateral balance, the side to side, the same as you do with a bike. You essentially- sorta, small counter-steer to get that weight, to get the contact patch out, and then you can pedal and bring it under you.

Now I should point out that sometimes when the steering is locked, we just happened to be leaning in the right direction to execute the turn.

• Right, right, right, right, right! Right, right, right, right.

• [Off Screen] Oh, managed it!

Essentially, by sheer luck, we had counter-steered before that side of the handlebars locked out. Now, I can keep going.

• [Rick] Yeah, but don't turn left or you're gonna be screwed.

I can't turn left. What's interesting about this is it shows that you can still ride the bike perfectly well, right? It's just you can't turn left. The funny thing is that you couldn't initiate the turn. I mean, the wild takeaway is that steering is not just for turning the bike; steering is for balancing.

• That's exactly right.

Why is it hard to balance on a stationary bike? I think most people believe it's because the wheels aren't spinning so there's no gyroscopic effect, but that's not it. The truth is you use steering to keep the bike underneath you but steering doesn't work when you're stationary. Your balance comes not so much from how you position your body over the bike, but by how you steer the bike to keep it underneath you.

Even when going straight, you're constantly making small steering adjustments to maintain balance.

• You're moving the contact patch of the front wheel under you. You're doing exactly what you do when you balance a broomstick on your hand.

So, if the rider is responsible for steering the bike to keep it balanced, how do bikes without riders stay upright? As long as a bike is moving with sufficient speed, it can keep coasting indefinitely. I first became aware of this phenomenon through the great videos by MinutePhysics, which inspired me to make this video. You should definitely check them out.

But it turned out the ground where we went to test this effect was really bumpy, but the bike still manages to absorb all these perturbations and remain stable. So, how does it do this? I think most people believe it's the wheels spinning that creates some sort of gyroscopic effect that resists falling over, just like in this demonstration of gyroscopic precession.

The wheel stays upright even though gravity is pulling it down. But this is not why bikes are stable. Just watch what happens when we lock the handlebars completely so you can only go straight ahead.

• Locked out, locked out. Whoa!

All that is happening is the steering is locked. You just got to ride it. You don't have to turn. You just ride. Letting go.

Some people tried going really fast. (group laughs) Others experimented with extreme balancing techniques.

• He's leaning. Don't go too fast! (group laughs)

But even with the gyroscopic effect of the wheels, no one was able to keep the bike upright for more than a few seconds.

• (crowd exclaims) This is not safe for a second.

It is just as hard to balance on a bike with locked steering as it is to balance on a stationary bike.

• No, this one is impossible.

Because you can't steer the bike back under you. The real reason bicycles are stable without riders is because they're cleverly designed to steer themselves. If they start falling to one side, the handlebars turn in that direction to steer the wheels back underneath them.

At least three mechanisms are responsible for a bike's corrective steering. The first is that due to the angle of the front fork, the steering axis intersects the ground in front of where the wheel touches the ground.

So, if the bike starts leaning to the left, the force from the ground on the tire turns the wheel to the left. If the bike starts leaning right, the force from the ground pushes the wheel to the right. The front wheel of a bicycle is essentially a caster wheel, like those you find on strollers or shopping carts. Whichever way you drive them, the wheel falls in line and rolls in the same direction.

The second reason for a bike's corrective steering is that the center of mass of the handlebars and front wheel are located slightly in front of the steering axis. So, when the bike leans left, their weight pushes the front wheel to the left. If the bike leans right, their weight steers to the right.

And the third mechanism is a gyroscopic effect, but it doesn't keep the bike upright directly; it just helps steer. If you have a gyroscope and you push down on the left-hand side, the gyro will turn left. If you push down on the right side, it will turn right. This is known as gyroscopic precession.

It seems as though the force you apply takes effect 90 degrees from where you applied it. So, bikes are stable primarily because of steering. They have built-in mechanisms for steering themselves.

In fact, you don't need all three mechanisms to create a stable bike. Researchers created this weird-looking bicycle to prove a point. It has no gyroscopic effect thanks to counter-rotating wheels above the wheels that touch the floor. Plus, there is no caster effect because the front wheel touches the floor in front of the steering axis.

But this bike is made stable by its mass distribution, the force of gravity on which steers it in the direction of any lean. Understanding how bicycles work is still an active area of research. There is a program you can use to input all the different bicycle parameters and see the range of speeds over which it is self-stable.

And this research is leading to better bikes. This prototype has a smart motor in the handlebars to actively help steer, keeping the bike upright even at low speeds.

I guess it's fitting that we are still learning new things about bicycles since most of us are able to ride one without any knowledge of how we're actually doing it.

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