Bicycles are Surprisingly Stable

Think about riding your bicycle… you have one, right? When you have it up to speed, it feels stable. Minor bumps don’t put you at risk for falling over. Isn’t that odd? After all, you have something between 40 and 100 kg of mass with a center of gravity probably a meter or more above the ground, and only two small bits of tire in contact with the ground at any one time. Why isn’t riding a bike more like trying to keep a broomstick balanced upright with just the tip resting in the palm of your hand and the brush part way up in the air?

It turns out that David E. H. Jones did more than idly wonder about it. His 1970 article, “The Stability of the Bicycle”, was reprinted this month in Physics Today (pp.51-56). Jones considers a number of hypotheses about why bicycles should be stable, and sets about constructing a variety of bicycles, aiming to produce the “unridable bike”, a bike that eliminates those features that yield the stability of the ordinary bicycle design.

Jones constructed a bicycle with an extra counter-rotating front tire that did not contact the ground. It was somewhat more difficult to ride, but still worked. He wrote a FORTRAN program to help with finding out about steering geometries. Along the way, the gyroscopic hypothesis of stability took several hits.

Jones’s masterpiece, his “unridable bicycle Mark IV” (URB IV) had an extension to set the front wheel about four inches ahead of its normal position, making the bicycle very unstable and just barely ridable.

It seems a lot of tortuous effort to produce in the end a machine of absolutely no utility whatsoever, but that sets me firmly in the mainstream of modern technology. At least I will have no intention of foisting the product onto a long-suffering public in the name of progress.

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Wesley R. Elsberry

Falconer. Interdisciplinary researcher: biology and computer science. Photographer. Husband. Christian. Activist.

4 thoughts on “Bicycles are Surprisingly Stable

  • 2006/09/15 at 2:14 am

    Maybe motorcycle wheels get enough gyroscopic stability, but i doubt bicycle wheels, because, as you mention, there’s high torque involved when the center of mass gets a little off-plumb. You can use this, in fact, to throw yourself into scary-hard turns. Whichever direction you want to go, briefly turn the handlebars the opposite way and let the bike run out from under you on that side for a fraction of a second. As you start to fall, turn in the direction you want to go. Sorry if that doesn’t make sense, it would be much easier to show with a graphic.

    Anyway, I always thought bicycles were stable at speed for the same reason that it’s unstable to stand on one foot, but very stable to alternate feet every half-second or so.

  • 2006/09/15 at 8:20 pm

    “At least I will have no intention of foisting the product onto a long-suffering public in the name of progress.”

    If the road to hell is paved with good intentions (not sure who said that), I wonder where the road paved with no intentions leads to.

  • 2006/10/03 at 7:33 pm

    Yeah, Steve S., ‘cept it works with bicycles, too. We call it – are you ready – countersteering.

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