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...
[2] i.e. you can't find a 2D plane in which all forces act.
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.
Actually the intuitive human reactions on a bike are often the exact opposite of what you need to do to balance the bike. F.ex. if you look at kids learning to bike their intuitive reaction to the bike falling to one side is to lean to the opposite side, but that has the exact opposite effect of what they want to achieve. When you lean to one side you exert force force on the bike the other side and the bike will fall quicker. What you need to do is to use the handlebars to turn a little bit to the same side the bike is falling to re-balance it. Ehm, at least I think so? The more I think about it the more uncertain I become.
Veritasium does an experiment on this exact topic: https://www.youtube.com/watch?v=9cNmUNHSBac
Let me restate. There are many natural phenomena that are complex. Say the details of interstellar nuclear reactions connected to the behavior of stars. There is no simple explanation, only complex mathematical models that predict the behavior very well.
So don't say "no astrophysicist understands stellar reactions" because that is false. They understand fully, and can write the model and simulate it to answer any question.
Ditto for bicycles. Just because there is no simple explanation for bicyles does not mean we don't fully understand the physics of bicycles. It just can't be put in English. Only in complex mathematical models.
> no astrophysicist understands stellar reactions" because that is false
Cyclists are not exactly the equivalent of "astrophysicists" in this situation though. There are of course physicists who are also cyclists, but basically no cyclist who is not (or isn't very interested in the field) would be able to explain it.
>> There are of course physicists who are also cyclists, but basically no cyclist who is not (or isn't very interested in the field) would be able to explain it.
This is probably a case where not being a scientist was an advantage. Turns out they didn't need a simulation or mathematical proof it could work in order to try it.
And here's another, extremely good explanation of how a bicycle works: https://ciechanow.ski/bicycle/
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.
After looking at that page, I understand something, but I am still curious about other things.
There is no mention of air resistance at all. Does it play a role in (de)stabilizing the vehicle in higher speeds? Would a bicycle work as well in, say, lunar vacuum (I imagine that it would be hard to pedal in spacesuit, but let's assume a spherical cow and a lunar bicyclist in normal clothes). Would a lunar bicycle be more stable at lower speeds because of the lower gravity on the Moon?