My theory: self-correcting action on the front tire when moving forward.
When the wheel pitches to the either side, the road under the bicycle pushes the wheel back to center.
When the bicycle leans to the side, the wheel pitches as well. Now the road under the bicycle pushes the wheel back towards center, but the angle of the tire to the road is also skewed, so the some of the force also gets translated into pushing the bike upright.
It's a little reminiscent of the self-centering action you get when you have a double-cone (or cylinder tapered on both ends) rolling down two rails.
I think if you fixed the front tire so there was no steering, you wouldn't get any stability from speed.
Two conditions are necessary for a bicycle to remain upright (at least with most riders.)
One is gyroscopic forces. Ever picked up a spinning hard drive? Notice that it feels strangely hard to turn in some directions? Same idea.
The other is the feedback loop consisting of the bicycle and its rider.
If the bike is stationary, it's hard to keep it upright because you have no assistance from gyroscopic forces. At low speeds, you have some assistance but not enough. At higher speeds, the bike wants to maintain its current orientation, and it's easy to feed in the slight corrective forces needed to keep it that way. Hop off the bike and it will keep going until something causes it to veer off course.
You can throw a ton of math at it, as in the paper mentioned elsewhere in the thread, but at the end of the day, gyroscopic forces and negative feedback are all that's necessary. The Schwab paper appears to show that the gyroscopic forces aren't necessary, but no bicycle in the real world is ever going to work that way except in rare corner cases, e.g., if you're one of those riders who can stay upright at a standstill.
That is indeed one of the authoritative explanations, and in a few seconds of ad-hoc search we may find at least two refutations from authoritative sources.
For example, gyroscopic-forces-as-stabiliser don't need the Schwab paper's "ton of math" to be undermined. A simple counter-rotating wheel was used empirically at Cambridge to show as much, alongside notes that gyroscopic forces are relevant to the dynamics of a loaded bicycle, but misconstrued; far from assisting to hold it upright whilst ridden, they induce instability at the beginning of a change in direction, and more so at speed, a phenomenon (counter-steering) familiar to cyclists and relied upon by motorcyclists.
Then there's a simpler observation that can be made: people have to learn to ride a bicycle. The fact it stays upright when rolling unloaded, but not when loaded, is indicative of how small the gyroscopic effect is, not how significant it is. Ergo, that argument would suggest, it is tiny shifts body position that contribute all of the stability.
And then others, proposing further explanations, etc etc ad nauseum.
I have come around to the view that in fact they don't stay upright, and they are almost always falling over, but in a many-branched configuration of the universe our observer effect sends us preferentially down the vanishingly unlikely path where they didn't, and there are an uncountably many alternatives of Me that have nothing but knee scars to show for it.
Sorry, I don't buy anything beyond what I stated. Those papers are exercises in violating Occam's Razor. Your quantum model makes more sense. :)
When you learn to ride a bike, you're simply training the feedback mechanism. Much like a PID controller, your brain has to keep track of the amount of error and null it out with proportional and integral terms (at least). Once those constants are dialed in, it's "just like riding a bike" -- they're yours for life.
Then there's the matter of learning which way to lean so that the gyroscopic instability inherent in turning doesn't send you into the nearest ditch...
Occam’s razor is for hypotheticals. You’re rejecting an actual physical demonstration, by a reader of engineering dynamics and vibration at Cambridge.
The feedback mechanism you’ve described is likely correct, and also a complete furphy, since the central nervous system is not part of the bicycle.
All of which is par for the course and rather confirms the point, viz. that people will happily hold forth on any explanation they care to latch on to, secure in the knowledge that the total absence of consensus makes it impossible to say, definitively, “that is wrong”
Great discussion, I agree with inopinatus who, if I am not mistaken, supports my position that the science is not quite out on this topic yet. Many explanations exist, none seem to be all-encompassing.
That being said, it doesn't answer the question posed in this forum: "What scientific phenomenon do you wish someone would explain better?"
Instead, it answers the question: "What scientific phenomenon do you wish someone could explain?"
I guess my question on this specific topic would be, "What aspects of bicycle operation are not adequately explained by gyroscopic action under the control of a human rider?" That would give us a good basis for inquiry into other possible principles.
Sure, a bike could work some other way, but my point is, it doesn't need to. Anyone who has ever picked up a hard drive should understand how a bicycle remains upright. What else is there to know? It's not like an airplane wing, where the "obvious" conventional wisdom is inadequate, misleading, or incomplete.
My hunch is that the science is not quite out on it yet and that the intuitive explanations are not accurate.
For example, see "Some recent developments in bicycle dynamics" (2007). Especially the folklore section:
"The world of bicycle dynamics is filled with folklore. For instance, some publications persist in the necessity of positive trail or gyroscopic effect of the wheels for the existence of a forward speed range with uncontrolled stable operation. Here we will show, by means of a counter example,that this is not necessarily the case.
For every authoritative-sounding, in-depth explanation, there is an equally plausible, yet conflicting and contradictory alternative.