Why is it easy to balance on a moving bike but not when it stands still?
In: Physics
Because the movement of the bike is predictable: it will go forward.
When static, you have to predict the direction as well .
This is why people fall off skateboards when learning. They stand and don’t move, then they can’t anticipate the direction skateboard is going and face plant, even though they were travelling at 0 miles an hour.
Centrifugal force! Spinning wheels are like magic helpers.
The bike’s wheels act like spinning tops. When they spin fast, they want to stay upright,
These “magic helper” wheels make it easier for you to stay balanced while the bike is moving.
Try standing perfectly still on one foot – it’s tricky, but if you’re running/walking, you don’t even notice you’re only on one foot, even if it’s only for a second,
Bikes work the same way – when they are moving they are spinning, creating a “magicAl helper”
You balance a bike by steering to keep the wheels under your centre of gravity: start to tip left and you steer left to bring the wheels under you. If the bike is not moving then steering does not cause the wheels to move across, and you continue to fall.
Because when you’re moving forward, turning the front wheel a tiny amount in the direction you’re tipping will stop the bike from tipping. Your brain does this automatically after learning to ride. That’s basically what learning to ride is.
It’s not the gyroscopic (“centrifugal”) thing others have mentioned.
The front wheel is on an angle called a castor.
Leaning the bike to one side causes the tire to steer in that direction due to the offset steering angle. Try it by holding the bike still and upright, just let the bike lean over a bit and watch the tire turn on its own.
If falling to the left, the bike also steers left. And when you’re going forward, intertia will push the weight of the bike and rider against that lean.
You’ve felt that intertia before. Most commonly when riding in a car. The car takes a turn to one direction and you feel pushed towards the opposite direction.
Since that push of intertia is against the direction the bike leans, the intertia is constantly working to correct the lean and push the bike back upwards, typically.
But if you’re not moving forwards fast enough, there’s not enough intertia to push you back upwards. So you just fall over.
There’s a little more to it than that. A person riding a bike can influence the steering, so they can contol the way and direction the bike turns and leans, often might be called counter steering. A bike doesn’t actually require that castor angle to stay upright if the rider just does the corrections manually. But that intertia from moving forwards (or backwards) is generally required.
You can also picture trying to balance a broomstick on your hand. If the stick starts falling to the side, you move your hand and the bottom of the stick over to underneath the stick’s center and “catch” the stick before it falls over.
If the bike starts falling to the side, you steer to move the tires underneath the bike’s center and similarly “catch” it before falling over.
When turning, you allow the bike to lean over enough that it doesn’t fully fall over. But you also don’t fully “catch” it. You let the bike keep falling so you can continue turing and moving the wheels underneath it as you go through the turn.
It’s not until you’ve completed a turn that you would fully “catch” the bike and get it back upright again.
The answer. And what will probably get deleted from this reddit is, that they don’t really know. You can easily see the results of all the dynamics of the operation of a bicycle. But they just don’t know why it does what it does.
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Because the bike pretty much balances itself in a way.
Think of it like spinning a top. When it’s spinning super fast it can stand up for a long period of time, the spinning motion creates angular momentum, it’s spinning and doesn’t want to necessarily change *how* it’s spinning.
Now imagine a bike, the two tires are almost ‘tops’ that are just positioned sideways. As you move forward the wheels generate angular momentum, they want to keep spinning in the direction they are going. On top of that there is the gyroscopic effect, they don’t want to really tilt or move, if you try to spin a top slightly angled, it will right itself. You can tilt the top and it stays balanced, the bike also wants to stay balanced.
When you’re not moving, these forces don’t really come into play, at that point you’re basically just trying to balance like you’re sitting on a tightrope. You can, but it’s a little difficult and you have to move your body a bunch.
It’s a surprisingly difficult question. You’ll get lots of disagreeing answers because bikes have multiple mechanisms for staying upright, all of them contribute to it staying upright, so there’s no single right answer.
All of the mechanisms work by steering the bike into the direction it falls.
There’s a myth that bikes stay up due to gyroscopic effects from the wheels and conservation of angular momentum. But there are still gyroscopic effects (gyroscopic procession) that keep a bike from falling over by turning into the direction the bike is falling.
Two other effects are due to the angle of the handle bars and steering column ie the handle bars are behind the front wheel.
This means the centre of mass of the steering column (handle bars + front wheel) is in front of where the handle bars turn from, so the bike turns into falls. You’ll have experienced this if you pick up a bike and don’t hold the handle bars. They’ll flip around and it’s really difficult to balance them straight.
Another effect is due to the position of the contact patch of the front wheel and the angle the handle bars are attached at and how they don’t line up.
But it’s awkward to explain over text so here’s a video. It covers all these effects. There are more, gyroscopic stability (the myth one) will still contribute a small amount.
I think there a papers written about why a bike is stable in motion even when there’s no one on it.. I think there is dispute about the why!
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