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> Even today, few bicyclists understand how a bicycle is able to turn and remain upright. I’d go so far as to say no cyclist understands. AFAIK there’s no simp

> Even today, few bicyclists understand how a bicycle is able to turn and remain upright. I’d go so far as to say no cyclist understands. AFAIK there’s no simple physical explanation.

Smoosh 3y ago
Fortnine recently made a video (motorcycles, but still applicable) which I think aids in understanding the issues and debunks some incorrect ideas. https://www.youtube.com/watch?v=vSZiKrtJ7Y0 https://www.youtube.com/watch?v=vSZiKrtJ7Y0
- aaronharnly 3y ago
  Thank you, great video.
beryilma 3y ago
Motorcycles and bicycles turn with the same principle: countersteering. When I started riding motorcycles, I bought a well-known motorcycling book. It had a whole chapter on countersteering because there are even today many seasoned motorcyclists who don't believe or understand that you turn using countersteering.
- kqr 3y ago
  You're answering the wrong question. The question is not "how does the rider initiate a lean" which is usually through countersteering. The actual question is "Why, when you give a bicycle a good shove, does it stay upright on its own, continually changing its direction to balance itself?"
  - barrkel 3y ago
    The counter steering answer to that question is rake, the fact that the steering axis hits the ground in front of the contact patch if the front tire, so that when a bike is leaned over it self-steers into the turn (the contact patch torques the wheel around the steering axis) and countersteers itself upright. It's well known in motorbikes that the amount of rake is what makes a bike more stable vs more responsive to steering. Some folks lower the front of the bike - tighten the triple tree lower on the forks - to increase responsiveness on otherwise more boring bikes. Of course there's more to it all, gyroscopic effects and so on, but this is the countersteering perspective.
    - barrkel 3y ago
      Reviewing this comment I didn't get all the terminology right - when I said rake, I was conflating trail, rake and steering head angle, where the latter two combine to define the amount of the former. There's more stuff here: https://calfeedesign.com/geometry-of-bike-handling/ https://calfeedesign.com/geometry-of-bike-handling/
abdullahkhalids 3y ago
Here is a simple simulation of a bicycle's stability [1]. The simulation more or less predicts what happens in real life, so we do understand why bicycles are stable. As you can see from the simulation, this is a multibody dynamics problem. The bicycle and human have lots of moving parts, and as they move there are forces in 3D [2] amongst them. So there can't really be a simplified, short and mostly complete English explanation (like for why planets go around the sun). That doesn't mean we can't predict. [1] https://www.comsol.com/blogs/simulating-the-motion-of-a-self-stable-bicycle/ https://www.comsol.com/blogs/simulating-the-motion-of-a-self... [2] i.e. you can't find a 2D plane in which all forces act.
- oskarkk 3y ago
  And here's another, extremely good explanation of how a bicycle works: https://ciechanow.ski/bicycle/ https://ciechanow.ski/bicycle/
  - mr_mitm 3y ago
    The amount of detail on that site is breathtaking. Beautifully done, really. However, on the question at hand, it mostly says: >Bicycle stability can’t be explained using just one or two mechanisms. It’s a combination of many different intertwined factors, like the mass distribution of individual components, size of the tires, geometry of the frame, and others. [...] What keeps bicycles balanced with or without a rider is still an active area of research, and even the seemingly basic idea that, for a bicycle to be self-stable, it needs to turn the handlebars into the fall, has not yet been proven.
- School-Cotton 3y ago
  From your link: > Their study shows that there isn’t one simple reason for this phenomenon. A combination of factors, including gyroscopic and caster effects, bicycle geometry, speed, and mass distribution come into play to keep an uncontrolled bicycle upright. I think we’re saying the same thing. There is no simple physical explanation that humans can intuitively understand, because the interaction of forces is so complex. I’m not claiming that it’s a mystery causing fundamental problems for physics.
rob74 3y ago
It's also amazing how stable a bicycle is, that it allows you to steer with one hand or even "look ma, no hands". I only came to appreciate that after I started riding e-scooters - those are very hard to control if you don't keep both hands on the handlebars, and taking both hands off will probably throw you off instantly.
- pjerem 3y ago
  Yep, e-scooters are maybe 7 years old (I start counting from the release of the Xiaomi M365) and so it’s real a shame that e-scooters with turn signals are finally here since only one year or two. Back when I used to do my commute with e-scooters, indicating my direction was a real pita and, most of the time, totally impossible. In fact I used to use the sidewalk and pedestrian crossings if I needed to go left through the traffic.
dessimus 3y ago
Probably on the same percentile as the number of people that understand how the Otto cycle works to power their car, or how differentials in the axles function to apply said power to the wheels through turns, etc. Understanding the physics is not a requirement to operate the vehicle.
WalterBright 3y ago
It is simple. A bike will be stable without a rider because if it tilts to the left, the front wheel will also tilt to the left (as the hinge is behind the center of gravity). This results in a left turn, and the centrifugal force of the turn will bring the bike back upright and straighten the wheel. The rider turns the bike by moving the handle bars slightly in the opposite direction he wants to turn. Then the bike falls in the direction he wants to turn, and the handle bars are then rotated in that direction. To straighten out, the handle bars turn a little tighter, and the centrifugal force of the turn pushes the bike back upright.
_carbyau_ 3y ago
Simple? No. But the universe is not a simple place. Definitive explanation? Yes - to a significant degree of definitive. As with all science/engineering-understanding there are always further details but this we've got pretty well down at the macro level humans operate at. A site beloved by HN to explain this: https://ciechanow.ski/bicycle/ https://ciechanow.ski/bicycle/ A beautiful site, check out the archives for other explanations and animations of things too.

> Even today, few bicyclists understand how a bicycle is able to turn and remain upright. I’d go so far as to say no cyclist understands. AFAIK there’s no simple physical explanation.

Smoosh 3y ago

Fortnine recently made a video (motorcycles, but still applicable) which I think aids in understanding the issues and debunks some incorrect ideas. https://www.youtube.com/watch?v=vSZiKrtJ7Y0 https://www.youtube.com/watch?v=vSZiKrtJ7Y0

aaronharnly 3y ago
Thank you, great video.

beryilma 3y ago

Motorcycles and bicycles turn with the same principle: countersteering. When I started riding motorcycles, I bought a well-known motorcycling book. It had a whole chapter on countersteering because there are even today many seasoned motorcyclists who don't believe or understand that you turn using countersteering.

kqr 3y ago
You're answering the wrong question. The question is not "how does the rider initiate a lean" which is usually through countersteering. The actual question is "Why, when you give a bicycle a good shove, does it stay upright on its own, continually changing its direction to balance itself?"
- barrkel 3y ago
  The counter steering answer to that question is rake, the fact that the steering axis hits the ground in front of the contact patch if the front tire, so that when a bike is leaned over it self-steers into the turn (the contact patch torques the wheel around the steering axis) and countersteers itself upright. It's well known in motorbikes that the amount of rake is what makes a bike more stable vs more responsive to steering. Some folks lower the front of the bike - tighten the triple tree lower on the forks - to increase responsiveness on otherwise more boring bikes. Of course there's more to it all, gyroscopic effects and so on, but this is the countersteering perspective.
  - barrkel 3y ago
    Reviewing this comment I didn't get all the terminology right - when I said rake, I was conflating trail, rake and steering head angle, where the latter two combine to define the amount of the former. There's more stuff here: https://calfeedesign.com/geometry-of-bike-handling/ https://calfeedesign.com/geometry-of-bike-handling/

abdullahkhalids 3y ago

Here is a simple simulation of a bicycle's stability [1]. The simulation more or less predicts what happens in real life, so we do understand why bicycles are stable. As you can see from the simulation, this is a multibody dynamics problem. The bicycle and human have lots of moving parts, and as they move there are forces in 3D [2] amongst them. So there can't really be a simplified, short and mostly complete English explanation (like for why planets go around the sun). That doesn't mean we can't predict. [1] https://www.comsol.com/blogs/simulating-the-motion-of-a-self-stable-bicycle/ https://www.comsol.com/blogs/simulating-the-motion-of-a-self... [2] i.e. you can't find a 2D plane in which all forces act.

oskarkk 3y ago
And here's another, extremely good explanation of how a bicycle works: https://ciechanow.ski/bicycle/ https://ciechanow.ski/bicycle/
- mr_mitm 3y ago
  The amount of detail on that site is breathtaking. Beautifully done, really. However, on the question at hand, it mostly says: >Bicycle stability can’t be explained using just one or two mechanisms. It’s a combination of many different intertwined factors, like the mass distribution of individual components, size of the tires, geometry of the frame, and others. [...] What keeps bicycles balanced with or without a rider is still an active area of research, and even the seemingly basic idea that, for a bicycle to be self-stable, it needs to turn the handlebars into the fall, has not yet been proven.
School-Cotton 3y ago
From your link: > Their study shows that there isn’t one simple reason for this phenomenon. A combination of factors, including gyroscopic and caster effects, bicycle geometry, speed, and mass distribution come into play to keep an uncontrolled bicycle upright. I think we’re saying the same thing. There is no simple physical explanation that humans can intuitively understand, because the interaction of forces is so complex. I’m not claiming that it’s a mystery causing fundamental problems for physics.

rob74 3y ago

It's also amazing how stable a bicycle is, that it allows you to steer with one hand or even "look ma, no hands". I only came to appreciate that after I started riding e-scooters - those are very hard to control if you don't keep both hands on the handlebars, and taking both hands off will probably throw you off instantly.

pjerem 3y ago
Yep, e-scooters are maybe 7 years old (I start counting from the release of the Xiaomi M365) and so it’s real a shame that e-scooters with turn signals are finally here since only one year or two. Back when I used to do my commute with e-scooters, indicating my direction was a real pita and, most of the time, totally impossible. In fact I used to use the sidewalk and pedestrian crossings if I needed to go left through the traffic.

dessimus 3y ago

Probably on the same percentile as the number of people that understand how the Otto cycle works to power their car, or how differentials in the axles function to apply said power to the wheels through turns, etc. Understanding the physics is not a requirement to operate the vehicle.

WalterBright 3y ago

It is simple. A bike will be stable without a rider because if it tilts to the left, the front wheel will also tilt to the left (as the hinge is behind the center of gravity). This results in a left turn, and the centrifugal force of the turn will bring the bike back upright and straighten the wheel. The rider turns the bike by moving the handle bars slightly in the opposite direction he wants to turn. Then the bike falls in the direction he wants to turn, and the handle bars are then rotated in that direction. To straighten out, the handle bars turn a little tighter, and the centrifugal force of the turn pushes the bike back upright.

_carbyau_ 3y ago

Simple? No. But the universe is not a simple place. Definitive explanation? Yes - to a significant degree of definitive. As with all science/engineering-understanding there are always further details but this we've got pretty well down at the macro level humans operate at. A site beloved by HN to explain this: https://ciechanow.ski/bicycle/ https://ciechanow.ski/bicycle/ A beautiful site, check out the archives for other explanations and animations of things too.