Anon starts asking questions

Fuckin magnets
How do they work?
- Tide comes in, tide comes out, You can't explain that.
  
  The dumber a civilization is, the more sense God makes.
  
  For those that don't know: Bill O'Reilly Doesn't Understand How Tides Work

Gyroscope effect. You ever do the experiment where you spin a bike tire really fast and then try to tilt it? Shit's nuts.
- That only causes part of the effect, most of it is the bike's steering countering the momentum of your fall.
  You wouldn't be able to balance on a bike with just the wheel spinning, you're too heavy. That is why bikes on those indoor rollers allow the bike to move left and right a bit.
  
  So biking is just falling with style?
  
  Just wait till OP learns that you need to counter-steer bikes to balance
- Hmmm... Both things involve bicycles... Maybe they're just magic?
  
  My next DnD character will be a bicyclist
- The gyroscopic effect of slowly spinning, light bicycle wheels is negligible compared to the weight of the bike and it's rider. If it was what keeps you upright, riding a tiny scooter-thing with skateboard/inliner wheels would be impossible. I mean those without motor, pedals, where you push yourself forward with one foot on the ground), often for kids.
  What actually keeps you upright isn't a physical effect, but just training your brain to instinctually keep you upright. While you're moving, turning the handlebar effectively moves the bike below you left and right. So if you start tilting to the right, you turn right (slightly) so the bike/scooter is moving below you to compensate. That's why learning to ride anything that is balancing on 2 wheels takes a relatively long time, but only once. Then your brain knows what to do, and it just works without thinking about it.
  
  The design of the front forks also assists with stability - having some rake and trail means the front wheel has a tendency to self centre (particularly at speed).
  
  But then, why would they be more stable when moving, even without a rider? If the steering is tight enough, you can push a bike to someone a good distance away. You can do it with a scooter too, although it's a lot harder.
  You can also look at a motorcycle. Their mass is far greater than a human's, a person could never manage that. Those little RC motorcycles are the same, they don't need some crazy balancing system to mimic humans, they just need to stay upright enough to get some speed going, then they balance themselves.
  It's the same with a wheel - the speed makes it stay upright, they can balance on the tiniest edge so long as they're moving
  It's not a gyroscopic effect either, though that's present. It's a balance between rotation speed and the friction with the ground - the object as a whole has momentum, the rotation has momentum, and the contact with the ground balances the two. It'll try to put it's center of mass in line with these forces acting on it
  Add in a human, and they can shift the center of mass on the fly. The vehicle's speed is still pushing you upright - get on a bike with some good speed, and you can lean very far into a turn and ease off to return upright. Way more than you could if it wasn't moving

Brah just discovered conservation of angular momentum
- It's not that. Gyroscopic action exists of course, but it's fairly weak against the weight of your body. Balancing a bicycle is just like balancing an umbrella on your finger, except you can easily move your finger any direction you need. To move the bicycle sideways, you need to already be moving forward.
  
  Track stands! Not a contradiction to your statement at all though: you need to be moving just ever so slightly.
  With a fixie it's easy, because you can pedal forwards and backwards in tiny amounts. With a freewheel, it's trickier but you get the hang of it with practice. Ideally you'll have an incline, so you pedal forward to go forward, and ease up to slide back. After some practice I can use the raised reflective paint from e.g. crosswalks as the "incline." This miniscule motion is enough to balance --- and like you said, it ain't the angular momentum that does it.
  
  ~~it really is that though, have you not done the rotating wheel experiment? it is fucking hard to tilt the axis of a wheel rotating at a speed which is comparable to biking speed.~~ come to think of it maybe not:
  https://m.youtube.com/watch?v=oZAc5t2lkvo
- Which is just regular conservation of momentum plus a force directing it towards some center point it oscillates around, which feels weird because you can hold a lot more of it in your hands than you normally can without that oscillation.

first, and less importantly, your wheels are gyroscopes
second, and much more importantly, at speed you use your steering to compensate for imbalance. You lean a little right? slight steering to the right compensates. When standing still, steering is no longer an option (duh)
- i tought the gyroscope theory had already been debunked?
  
  It has as the sole cause. But when you have a couple big spinny bits, there is going to be some gyroscopic effect, and it does help keep it upright. It just can't on its own, it provides a small assist.

It's the central pedal force
- my bike doesn’t have a central pedal. how does it stay up right?
  
  Aliens.
  
  Magnets
  
  Does it have a central fugue? Classical physics teachers don't want to talk about that but if you look back to the Baroque interpretation it was all the rage.
  
  Magnetic aliens

Can confirm. Last week, I got home from a ride, stopped in front of the garage, couldn't unclip, and promptly fell over. It turned out one of the bolts fell out from the cleat during the ride, so the cleat just rotated, instead of unclipping. D'oh. Fortunately, I mostly landed in grass, though I did scrape my ankle a bit.
- Over engineering problem
  
  How so?
- Yeah, I can relate to this...

Same principle as a gyroscope: a turning wheel will tend to stay perpendicular or parallel to the direction of the gravity vector because if it starts tilting away from such orientation there's a force that pushes it back.
Also works better with bigger wheels (if I remember it correctly the effect is related to spinning momentum).
I was pretty surprised when learning Physics and they show us how to derive the formula for that (which I totally forgot since that was over 3 decades ago).
Edit: Actually the gyroscopic effetc is just a part of it. See this article
- Actually, it's the bike's geometry rather than a gyroscopic effect. Try rolling a bike backwards rather than forward - it'll topple quickly
  
  Yeah, you're mostly right: Why bycicles stay upright.
  There's some gyroscopic effect, but per that article it's not the main reason.
  
  Freestyle BMX riders go in reverse all the time and they don't fall over.
- Gyroscopic effect is not even significant. Lock your steering and you will fall over no matter how fast your wheels are spinning. (Which can happen with a badly pitted headset)

Having the pivot point (steering) for the front wheel behind it's axle helps
- Yes there are demonstrations on YouTube of bikes just wanting to remain upright. You can role it down a hill and it will self correct. Something to do with physics but I forget the terms.

But pedaling on a treadmill make you fall over.
What the hell?
- They're called rollers, and they're pretty easy to ride on.
  
  I wouldn't say easy... But yeah, you don't just fall over.
  
  I have heard that it is VERY hard to get started and then very quickly becomes thoughtlessly easy
- Does it? I bet some weirdo on YouTube has done this.
  
  you're absolutely right. I searched it before making that comment and there's one random dude who did exactly that, he even wondered himself if anyone has ever done that.
- And pedaling in space makes you balance!
  How the hell?
  
  No, you’d start spinning

I'm surprised how much I'm seeing gyro brought up in these comments. It's a factor, but it's practically negligible. It's all in the steering. Start to tip right, and you'll subconsciously steer slightly to the right to correct your balance. Try to ride as slow as you can and you'll find yourself doing these corrections much more frantically and dramatically. The reason for that is because it takes longer for the wheel to roll under your center gravity and "catch" you when you're going slowly so you have to turn in quicker to maintain balance.
Notice that on almost every bike you see, the front axle on the bike is slightly ahead of the neck's axis of rotation. That offset does two things: 1. It stabilizes the steering so that the bike will tend to steer straight and 2. (more important to my point) It makes the balance-correcting effect of steering more immediate and dramatic, making it much easier to ride at slower speeds.
As a counter argument showing why gyro is barely a factor, these exist:
Edit: if you're not seeing the image like I'm not, Google "ski bike".
- It's pretty common to bring up gyroscopes for this when people know a little bit about physics. It's all over motorcycle forums, for instance.
  As you say, it doesn't work. Experiments have been done where they attach a counter rotating wheel to cancel out the gyroscopic effect, and while it's a little wonky to ride, it works fine.
  IIRC, we're not 100% sure how bikes work just yet. Every time somebody comes up with a model that seems to be good, someone finds a counterexample that throws it in the bin. Even your explanation of bike trail isn't all the way there; Razer-type scooters still work without trail on the front wheel.
- I figured it was pretty obviously the rider that's making the bike not fall over, not the bike itself.
  If the bike's ability to remain upright while moving was a natural feature, then why would you ever need to learn how to ride bikes? You could just sit on it and go if that was the case.
  
  It works on its own. If you push your bike along with a good run and then let go, it'll stay upright until it slows down too much.
  Learning to ride a bike is mostly about being confident enough to let the bike work itself out. It gets more stable as it goes faster, but it's natural to be afraid to go faster when it already feels unstable at low speed. Then there's a little bit to learn about countersteering, but most people figure that out without being told it's even a thing.
- Then why does going faster also make it easier to balance on something without steering, like a snowboard?
  My assumption is that when you're standing still relative to the ground you can fall in many directions, but once you start moving momentum limits the directions you can fall to the ones in line with your motion. So the faster you're moving the fewer directions you need to worry about.
  
  Snowboards definitely have steering, you twist the board and shift your weight to manipulate how the edges contact the snow, it's just not quiet as explicit as a bikes front wheel. But whether it's a bike, a board, or literally any moving thing on land, the steering happens because you applied a lateral force to the ground and an equal and opposite force was applied back to you.
  The snowboard uses different methods of applying that force, but other than that it's the same concept as described in my first comment: Greater speed allows more subtle corrections to take effect more quickly.
  Now the snowboard does have a wider contact area with the ground, but that really only helps you on flat ground at very low speed, or standstill. Advanced boarders will carve transitioning from edge to edge most of the time.

A Veritasium video about the topic, if someone is actually interested: https://youtu.be/9cNmUNHSBac

Dynamic stability

Wait til anon hears about launch loops or space elevators
- How long does it take to go up the space elevator?
  
  What floor are you getting off at?

I don’t get it when people (usually chavs) can just sit back with their hands in their pockets - when I try it my handlebars twist out to one side instantly.
- Most bikes are stable and a cyclist can weight shift to turn gently without touching the handlebars
  Mine is a recumbent that steers with long handlebars. The handlebars prevent the bike from balancing itself.
  Yours might have barely stable geometry, or badly placed weight on the handlebars

What's really hard is getting a 5 year old to understand this before you run out of energy from trying to hold their seat and run at the same time.
- That's how I learned it. My dad got tired, let go and stopped.
  I noticed it was suddenly much easier to pedal, so I turned around to see him standing 30ft behind me, then I crashed.
  
  This is the way.
  Getting my 5yo to accept that he can peddle and balance by himself; takes longer than the ability. He used to crash once he realized I had stopped balancing him, sometimes it would take a minute or more.
- My kid screams "un-let go" at me while balancing perfectly well. She understands, she does not accept.

Everyone is wrong. It's the encabulation effect.
- Modern bicycles include an isotropic harmonization manifold to achieve the same thing without an encabulator.
  
  How do they account for side fumbling?

If you’ve forgotten high school physics (like me), this is a legitimately strange phenomenon.

Think of this as you inch forward until the green light with a motorcycle behind you. Just stop. Riding at 2 mph is misery.
- Get a lighter bike, mate. Scooter goes brrrr! But also, yes. Please god, the misery of traffic jams must stop. Let me lane split!
  
  I got a tenere 700 coming from a rebel 500, both relatively light bikes but the tenere is so tall it's hard to waddle. I'd honestly love a scooter just for commuting and errands.
  I live in a state where splitting is illegal but I can get away with filtering at stoplights. :)

Momentum + Gyroscopic effect
- Steering keeps you upright in the same way a broom handle balanced on your hand is kept up by moving you hand around
  Gyroscopic effects are negligable
  
  There is a word for what you're describing, it's called "momentum".
  More specifically, the force vectors for gravity and forward motion combine angles in a ratio of their magnitude and the bike becomes less likely to topple than go forward (in the assumption the horizontal force is the greater of the two values), or in other words as long as it has forward momentum.

Real talk, science doesn’t have the answer to this yet.

Vy two

not magic, gyroscopes. the wheels act as gyroscopes once they have sufficient rotational speed.