Right, it’s physics — but we don’t know the exact physics behind it. There’s no theory without a few gaps in it at this point. Certainly not a closed case by any means.
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: http://en.wikipedia.org/wiki/Bike_physics
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: http://en.wikipedia.org/wiki/Two-mass-skate_bicycle
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?
4 Likes
Ugh. This article is so misleading. Scientists have known for decades that gyroscopic effects are not the only thing keeping a bicycle upright. Also, the fundamental physics of the bicycle - Newton’s laws - have been known for centuries. There is nothing “mysterious” about it. The non-gyroscopic bicycle demonstrations that have become popular on the internet in the last few years are more like catchy visual tools for teaching non-scientists what scientists already know, and not earth-shattering discoveries of new physics or cases of mysteries being solved. Bicycle designers know exactly what physics is happening. They must in order to optimize their bikes. But I guess the headline “non-scientists on the internet clear up some of their misconceptions about the bicycle” is not as catchy as “physicists solve the mystery of the bicycle”. Just because something is new to you does not mean it is new to everyone.
“Notice how it falls over.” Um, you apparently have never rolled a bike down the street without a rider. They don’t fall over. That’s the whole point of the study, to figure out why.
No it won’t. Go out and try it. You will surprised if you have never tried this.
Excellent Reply.
I doubt that this is true, though I agree with the rest of your comment. I believe that most bicycle design is based on a few rules of thumb and trial and error: “If we increase trail, it should feel more stable. Let’s build one and see. Uh oh, it develops a shimmy at 25 mph. Let’s try a stiffer fork.” It is better than the random permutations of evolution, but not yet multivariate optimization based on the equations of motion.
Part of the reason is that the common upright bicycle design is so robust, as Jones famously showed: a lot of things can be not quite right, but the bike is still easily rideable. The other main part of the reason is that riders can adapt so quickly to bikes with different handling characteristics. Anyone that rides a couple very different bikes has probably experienced this. Ride one for a while and it feels perfectly normal. Switch to the other, and it feels very weird, but only for a few minutes. Then the second feels as normal as the first one.
I think the issue is that some people are looking for a single answer. This strikes me as a perfect example of that favorite 80’s word, synergy.
Yes. It is far easier and clever sounding to say “oh, did you know bikes stay up because of trail?” or “we don’t actually understand why bikes stay upright as they move,” than it is to say “well, let’s measure the geometry and mass distribution of this particular bike,” choose a particular forward speed and tire inflation pressure, use the benchmarked and published equations of motion to calculate the individual effect of all the known contributors to self-stability, and then declare “in this particular case, the front-end geometry makes the largest contribution to the steering torque necessary to turn the front wheel just the right amount towards the direction it is leaning so that the bike returns to upright forward motion, followed by front-end mass distribution, front tire twisting torque, and front wheel gyroscopic precession, in descending order of importance.”
I was hoping this thread would be about a sequel to The Mysterious Cities of Gold.
I doubt that the physics are all that mysterious, just never taken seriously enough to finally resolve the question. “Only” classical mechanics (a lost art for the fadingly-fashionable QM crowd) has the power and elegance to address hair-raisingly difficult problems. This is one, and I would be surprised if the mystery of dynamic friction isn’t fundamentally involved. Clearly the roadbed-tire and frame-airstream forces need more consideration. It’s a complicated case … a lot of ins and outs.
Physics of a bicycle. Isn’t it remarkable? Physics of a bicycle. Isn’t it remarkable? quo googlus
After riding home in a state of considerable inebriation, I can confirm that a sense of balance is completely unnecessary to keeping a bike upright. I did wobble a lot, though.
(Note, drunk biking is completely legal in my state, as far as I can tell.)
Well it’s a bit late but I really ought to mention that there is at least 40 years of articles covering this in the ‘Vehicle systems dynamics’ journal - http://www.tandfonline.com/toc/nvsd20/current#.UgUgVhYTdbw
I’ve been referring to it occasionally for, ooh, 30 years or so. Lots of single-track scholarliness to enjoy. And tanks, occasionally.
He was an awesome prof. Remember his wager of $100 to the person who could ride the rear-steered bike all the way down the hallway? Did anyone ever collect on that?
This topic was automatically closed after 5 days. New replies are no longer allowed.
