Physics of Bicycle Self-Stabilization

April 18, 2011 11:03 AM

The physics of bicycle self-stabilization [Science Friday Video] With bonus SCIENCE!
posted by odinsdream (16 comments total)

With bonus SCIENCE!

Quantum fixies and recumbent DNA?
posted by pracowity at 11:28 AM on April 18, 2011 [3 favorites]

Takes a rider to make 'em crash!
posted by cccorlew at 11:36 AM on April 18, 2011 [1 favorite]

A comment on the paper (which I can't seem to find the URL for at the moment--read this a few days ago) claimed that the contraption the built had "negative trail" (steer axis behind the front wheel) and to truly test their thesis, they should have built a device with zero trail.) That's science, folks.
posted by Obscure Reference at 11:44 AM on April 18, 2011

This is only somewhat related, but I have to confess that I never understood on an intuitive level why conservation of angular momentum implies any of the properties that we associate with gyroscopic forces. My physics sections in college coincided nicely with a global minimum in my academic trajectory.

So, if I'm reading the supplementary materials paper correctly, the only simple reason a bicycle stays up is because torque on the handlebars occurs in a direction opposite the direction the bicycle is turning? To what do we the owe the greater ease of remaining upright when the bicycle is going fast, then?
posted by invitapriore at 11:54 AM on April 18, 2011

This is interesting, but wasn't sure of the analogy they made between balancing while running and biking. - That is, when falling to the right, you put out your right foot while running, and turn to the right while on a bike. I know you do do this, but I'm not sure they are equivalent in terms of underlying principle.
posted by carter at 11:58 AM on April 18, 2011

Negative trail still provides a refutation of the common understanding - it's believed that a bicycle with negative trail should be markedly unstable, attempting to flip the wheel around 180 degrees to become positive trail. This is the castering effect you'd see in a shopping cart, for example.

Also, while the common factors are not necessary, they are still important. The final paragraph of the full paper has this to say:

Although we showed that neither front-wheel spin angular momentum nor trail are necessary for self-stability, we do not deny that both are often important contributors. But other parameters are also important, especially the front-assembly mass distribution, and all the parameters interact in complex ways. As a rule we have found that almost any selfstable bicycle can be made unstable by mis-adjusting only the trail, or only the front-wheel gyro, or only the front-assembly center-of-mass position. Conversely many unstable bicycles can be made stable by appropriately adjusting any one of these three design variables, sometimes in an unusual way.

posted by lantius at 12:01 PM on April 18, 2011 [1 favorite]

I thought that the gyro stabilization of most bicycles was so weak that there was no way it was the predominant (or I thought even major) factor in stability. It looks like it can play a larger part than I thought. I really like the idea that the stability of a bike is actually quite complex.

And reading the paper, they mention stable rear-steering bicycles. Which is fantastic, as I always wanted to build one.
posted by Hactar at 12:14 PM on April 18, 2011 [1 favorite]

Takes a rider to make 'em crash!

No, just a turn and a fixed object to hit.
posted by Mental Wimp at 12:33 PM on April 18, 2011

recumbent DNA?

oh, well played.
posted by FAMOUS MONSTER at 12:41 PM on April 18, 2011 [1 favorite]

uh-duh! i discovered this when i was 8, ghost-riding bikes down the hill into the park next to the canal.
posted by RockyChrysler at 12:59 PM on April 18, 2011

Trail may not be essential to bicycle stabilization, but it certainly plays a role in overall stability. I have bikes with straight forks and very little rake, and they are much twitchier than bikes with a more relaxed architecture.
posted by TheWhiteSkull at 1:03 PM on April 18, 2011

Fucking bicycles, how do they work?
posted by chavenet at 1:04 PM on April 18, 2011 [1 favorite]

In 1970 David E.H. Jones built a bike with an extra counter-rotating wheel to prove that stability wasn't a gyroscopic effect.
posted by alby at 1:57 PM on April 18, 2011 [2 favorites]

I took a course (Dynamics?) from Andy Ruina, one of the study authors, in undergrad. He had a reputation for being blazingly smart (I think he was studying friction at the time) and he was a fantastic instructor. I found his method for managing his chalkboards was the best I'd ever seen. He would enter class with a large bucket of water that had two large (foot+) sponges tied together. There were three boards. He would wash one board with the oversized sponges -- just a few quick strokes -- and move on to the free board. By the time he got to the washed board it would be dry and have that perfect ebony shine.

I also remember a few weeks into class he had us all stop what we were doing, clear our desks and get out a clean sheet of paper. Everyone was freaking out, thinking we were having a pop quiz. Instead, he asked that we write down what we didn't like about the class. The following week he made some radical changes to the course and his teaching style. I'm forgetting what they were -- this was nearly two decades ago -- but I remember being so impressed that anyone could change so drastically.

He was intimidating though. Whenever anyone would lean over and ask a question of the person sitting next to them in class, he would ask them what their question was. Initially they would demur, but he would insist. Usually it was a valid point, and he would turn it into a teachable moment.

Anyway, it was fun to hear his voice in that video. Glad to see he's still at and working on my favorite form of transportation.
posted by funkiwan at 2:03 AM on April 19, 2011 [1 favorite]

I came in here for the bike science and got a Daedalus link as icing. Awesome.
posted by DU at 11:51 AM on April 19, 2011

I don't get the science but I see the funky little bikes built for the experiment being used in the new sport of Bicycle Curling.
posted by Rarebit Fiend at 5:46 PM on April 19, 2011