The mysterious physics of bicycles

We don't actually understand why bikes stay upright as they move, writes physicist Michael Brooks at The New Statesman. A 2011 paper, published in the journal Science, poked big holes in the old theories about gyroscopic effects, and nobody has come along with anything to fill them yet.

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  1. Hi Maggie, look, I saw this article published a few years ago on Ars and, frankly, I was appalled. I posted there at the time, but I'd like to set the situation straight, here, again. People DO know how bikes 'right' themselves. Its not magic, its physics. Specifically, its friction. Why does a car 'right' itself? FRICTION! Its the same concept. The interface between the tires and the road cause the phenomenon. It literally takes MORE ENERGY to turn a wheel then keep driving along the same vector. Likewise, if you blow a tire in your car, it will vear off to that direction. A first year Engineering student can impart these basic facts. The real scandal here is that these people were allowed to graduate.

    So, Maggie, feel free to post this response to Boing Boing on my behalf. Together we could blow the top off Bikey-Stay-Uppy-And-Go-Straighty Phenomenon!

  2. We had a whole room of modified bikes at UIUC in the early 90's. Many probably dated to before this. Including several that had canceled gyroscopic effects. They rode just fine.

    The collection was created by a professor, Richard Klein. I worked with him and took one of his classes. His specialty was bicycle dynamics and control, so the subject came up a time or two. Most certainly, the gyroscopic effect was debunked as critical for riders. The bikes rode just fine.

    A big part of his study was how much control was required to keep systems stable. What I gleaned by osmosis was that, for a bike, the mass location and steer axis tilt was within a certain range was important to stability. Just what this article states.

    Here's a nice article that has some nice discussion and pics:

  3. That article, even though it was written by a physicist, sounds a lot like that apocraphal story about "scientists prove that a bumble bee can't fly! haha dumb scientists" but which is actually a story about a scientist describing how simplified models only work within the scope they were intended to apply, and fail when applied to other systems (aerodynamics models meant to apply to aircraft don't work when applied to something that flies like a bee).

    Of course we know how bicycles work. People have known for a long time that gyroscopic effects don't keep bicycles upright - in fact precession would actually tend to make the bike LESS stable during steering. The study with the counter-rotating wheels just put the nail in the coffin on that one.

    Tire friction matters, as mentioned, as does the geometry of a bicycle - due to the rake and trail of the fork, a bicycle in a "turning" position has a higher center of gravity than a bicycle in a "straight" position, so gravity literally pulls the bicycle straight again, by lowering it's center of gravity.

    I've made and ridden a lot of "freak bikes" and you realize pretty quickly how small changes, like making a bike with no rake and trail, make it nearly unrideable, since it is no longer dynamically stable.

    I suppose it may be true that no one has written up the whole model that describes the bicycle (and motorcycle's) behavior, but that doesn't mean that it is some kind of mystery, just that it hasn't all been generalized yet.

  4. I know so little physics, I'm not even going to try to argue with the physics and engineering folks... but I do want to mention the experience of Bob Mellin in his book about railbiking:

    He was a unicyclist with excellent balance... so he figured he could balance a bike on a rail without an outrigger. Wrong. He said he learned very quickly, we don't balance (rolling) bicycles by shifting our weight... we balance them by steering in the direction we're falling. Subtly. And we're falling all the time.

    Constraining the bike's wheels to go in a straight line is, I think, an interesting manipulation of the variables...

    Personally, I ride a tricycle. Problem solved.

  5. I agree with FlyingProfs' comments above. The people who study bikes know why they stay upright. At this point, the only reason someone wouldn't be aware of this is because they simply haven't looked into it. All the recent research is mentioned in the Wikipedia article:

    The simple answer, of course, is that the front wheel steers into a lean to keep the contact patches under the weight. (Yes, there is a more-complicated dynamic explanation that includes accelerations, but the gist of it is the same.) The reason that some bikes can exhibit this behavior under some circumstances without a rider is a complicated interplay of mass, geometry, tire properties, and gyroscopic effects. Most of the remaining small gaps in understanding are due to difficulties with modeling small effects and difficulties with measuring the size of those effects in a physical example. In the shimmy of a particular bike, which matters more? Frame flex, wheel flex, or tire flex? That's hard to say exactly.

    Evidence of just how well the big picture is understood can be seen in the so-called two-mass-skate bike, which can stay upright with no gyroscopic effects and no trail, when rolling forward at the right speed, even after being given a sideways whack. The bike was created after specifically searching the design space, as defined by the equations of motion, for just such an unusual bike. The equations predicted it would work, they built one, and it works:

    The questions that are being researched now are more along the lines of exactly "how do riders control a bike?" Of course, they steer in the direction of a lean, but how are they sensing that lean? Visually? With the inner ear? Are they actually sensing lean angle, lean rate, or even changes in lean rate? Some combination of all of the above? Also, do riders steer by sensing the torque they apply or by sensing the steering angle of the handlebars?

    All of these things are being investigated in hopes of finding some way to systematically make a bike that handles exactly the way riders want it to, as opposed designers applying a few rules of thumb and hoping for the best, which is how things are done now. Also, with the huge cohort of aging baby boomers wanting to stay active and finding conventional bicycles less than comfortable, is there some design, which has not yet been stumbled upon, that is as comfortable as a recumbent and as stable as a traditional European city bike?

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