The rotating wheels act as gyroscopes, such that the axles of the wheels resist tilting. The faster the wheels are rotating (and the more mass is situated out where the tire and rim are) the greater the stabilizing effect.

Plus, when the bike is in motion, you can make constant small steering corrections to compensate for leans.

Someone will answer “God”.

@Brian1946 – when mine is stationary, it does fall over. It doesn’t stay upright.

Balance.

The answer is actually not well known. Bicycles have been made that counter all gyroscopic forces and they still ride fine. I honestly am humbly stumped by them. Rake, trail center of mass over the wheels are thought to be involved. This is why dynamics was never my thing.

Watch this video. It is interesting. https://www.youtube.com/watch?v=MFzDaBzBlL0 It may not fully answer your question but you reminded me of it.

@ARE_you_kidding_me What kind of bicycle? Can you post a link or a video? Curious.

@flip86 that was a cool video. I’d like to put forward some evidence by Rupert Sheldrake that may be a different answer than neuro polasticity.
Sheldrake experimented with several generations of rats with a water maze. Two possible exits, one lighted, one dark. It took the first group of rats something like 40 weeks to learn to go to the dark exit. The next group took significantly less time, like 26 weeks. Some of the rats were descended from the first group, some were not. They all took the same length of time. The next group took even less time, and so on. Sheldrake is the one who debunked the 100th monkey hypothesis, but not in the way that completely rules out the learning behavior, ie; when a critical mass is reached, all monkeys can now wash the sweet potatoes, Rather he showed evidence that there is some sort of plane of knowledge that they were actually uploading the information into that the others were then able to access. Kind of like how fish school and birds flock.
This guy’s bike experiment should be really expanded upon, because it is possible that it is just as he said, but it is also possible that what he learned is now part of a universal database that the kid was able to tap into.

@Rarebear here is one that I found
I did just come across an article that stated a group of scientists and engineers cracked it and they believe it was like twenty something contributing factors that make it possible. If you have ever tried to ride on rollers then you realize quickly that the physics is complicated.

Here is another

As mentioned above, it’s a lot more complicated than you might expect. Another oddity of two- wheelers is counter-steering.

To turn right you first turn the front wheel left, and vice versa.

It’s brief, and the little flick leans you into the turn.

So several hours ago I was reading this thread and then shortly after hopped on my bike to do an errand—I found myself so conscious of how I was adjusting my weight and approaching turns…

@flip86—that video makes me wanna try that bike just to try and prove that I can learn it faster than 8 months. Every time he says “you can’t do it” makes me wanna try that much more… I imagine the hard part is that you have to anticipate the corrections to keep balanced, and it would be hard to anticipate corrections that are so unfamiliar…

Resuming a family bike ride, I tried to get back on my bike in a fancier way than usual, fell, and the handle bars twisted askew—holding the wheel straight meant having my handle bars point ~30* left. I was biking around the parking lot and telling my family I was good to go—it was a lot of fun with the off-direction handle bars, and I was able to bike around without much difficulty—they disagreed. They said if I got into a problematic situation I’d revert back to muscle memory, yank the handle bars forward, and cause myself to fall or veer into something… they made me sit there while one person went home for a car. Not the same as opposite-direciton turning, but as close an experience as I have.

A bicycle can be thought of as a tripod, with the rear wheel acting as one leg and the front wheel acting as the other two. The front wheel switches between the duties of the other two legs especially at low speeds or even at a stop through a technique known as trackstanding.

https://www.youtube.com/watch?v=nRR4paQnUsQ

At speed an experienced rider is using the bars to support the upper body, steering angle is achieved through body lean, not direct manual input.

Riding a bicycle in a truly efficient and straight line requires skill and practice. Correct pedal stroke and a quiet upper body is key and can only be learned over time.

@ARE_you_kidding_me Huh. Interesting. I don’t understand the physics well enough to comment with any authority. It seems to me, though, that having an opposite wheel doesn’t completely counteract the effect. So having two oppositely rotating wheels somehow doesn’t seem the same to me as having no rotating wheel.

I’ll forward your link to a geeky physics group I am in and ask them.

It’s a stumper for sure. Thankfully I work with electronics, software and not this.

Found this Minute Physics video that explains it perfectly.
https://www.youtube.com/watch?v=oZAc5t2lkvo

That was a good vid, it at least went into counter steering and admitted we don’t exactly understand bike stability 100%

I wonder if the usually symmetrical bilateral air currents generated by the rider are balancing factors.

Has anyone ever tried riding a bike through a vacuum?

@Brian1946 That would be pretty hard to do

@ARE_you_kidding_me I know, right? I love it when my misconceptions are corrected by the facts.

http://xkcd.com/1673/