Treebeard's Stumper Answer 11 October 2002

Bikes and Boards

Early autumn is a changing season that brings perfect days to be outside. Biking and surfing are on my mind, and they share a stumper. It's very hard to balance on a bike or stand on a surfboard at rest, but it's not so hard when you're moving. That's the Catch-22 that makes both so hard to learn. You want to start slow, but it's easier fast, but you're just learning so you must go slow. Why is balancing a bike or board easier when you're moving? What other activities are similar? If your first thought is spinning wheels, then keep thinking about surfing!

This DMS student proves that it is possible to stand on a surfboard at rest. When the waves are this flat, there's not much else to do! Mountain bike trials riders do astounding tricks on near-stationary bikes when they attempt to get over an obstacle course with a penalty point every time they put a foot down. But for most of us, it's much easier to balance bikes and boards when moving. Why?

You can balance a broom if you can keep your hand under the center of gravity. Riding bikes and boards is a similar dynamic equilibrium. Tip a bike to one side and the front wheel steers the same way to restore it. Spinning wheels act as gyroscopes, and fork and frame geometry might be even more important. Surfing is like running down the up elevator with opposing motions. Maybe the real answer is that we're wired to balance one motion with another. Walking is really just controlled falling. We avoid the floor by always moving forward. We've been practicing all our lives.

Notes:

Dynamic equilibrium comes into play with walking and running, bikes, surfing, skateboards, skis, snowboards, skimboards, rockhopping up a dry creek, tightrope walking, skating, sliding on wet grass or an icy porch, and maybe life itself. These are all activities we do better in motion. I think it helps being able to walk on two legs. My dog Mojo can balance on my kayak, but not a bicycle. Trained circus bears and monkeys really can ride bikes if they have to, and they can also walk on two legs. Sometimes I feel like a trained bear trying to balance my way through someone else's life!

A three-wheel tricycle has static stability at rest, but it's very easy to tip over during a sharp turn. Two-wheel bicycles have great dynamic stability in motion, but only a trials rider can balance a bike at rest because it has no base of support. Even without static stability, a moving bike is remarkably stable. Kids soon learn how easy it is to ride (and even turn) a bike with no hands.

It's hard to tip over a spinning toy gyroscope. I had trouble understanding angular momentum vectors in college physics and I still do, but you can feel the force of gyroscopic precession if you stand on a lazy susan while holding a spinning bike wheel with the axle between your hands. If you tilt the wheel to your left, you will twist to the left. If you twist the wheel, it will turn. The result is that a moving bike automatically steers in whatever direction it leans. Inertia then tends to bring the weight of the rider back over the line between front wheel and rear and restore stability. We steer and balance by leaning (with our butts) as much as with the handlebars.

This sounds reasonable, but many cyclists and scientists now say that gyroscopic effects have little or nothing to do with bike stability. Instead, it depends on the geometry of the bike frame and fork. You can easily demonstrate that if you hold a stationary bike upright and then tilt it slightly to the side. The front wheel will turn towards the tilt. This is the same effect as gyroscopic precession, but it works at rest too. One source on the Web explains it like this:

This is due to the amount of "trail" in the frame... To find the trail, measure the distance along the ground from the extended centerline of the head tube to the point of contact of the wheel. This distance acts as a moment arm, which turns the front wheel when the tilt of the bicycle causes the upward force from the ground to be out of plane from the axis of the head tube.

English physicist David Jones has done interesting experiments by constructing bikes that should be unridable, e.g. bikes with an extra set of wheels that spin the other way to balance out gyroscopic effects. He found that these bikes are still ridable. There's an account here.

Riding a bike seems to be much easier than understanding it. Biking seems as natural as walking once you learn, and so does surfing. I stand by my general answer that we can easily learn to bike and surf because we've been walking all our lives, so we're good at balancing one motion with another. Of course physics and engineering can and do make it better!

Here are some links for more research:

• I was surprised to find that balancing a bike is such a lively physics topic!
  • Some sites say that gyroscopic precession is the explanation, e.g. The Straight Dope and here.
  • Other sites say it's the bike geometry, e.g. Newton: Ask A Scientist and here.
  • Most sites say it's a bit of both, e.g. Louis A. Bloomfield's How Things Work: Bicycles and section 5.2 of his textbook How Things Work: The Physics of Everyday Life (2000). (This fine book is too expensive, but you won't regret it.) Also check Physics Central and PhysLink

• There's more discussion about balancing bikes at Marilyn is Wrong, Tony Foale Designs, Teaching Kids To Ride, Bicycle for Investigations, and the Exploratorium: Science of Cycling pages. Check out the Steering Geometry Calculator for some of the math. The Exploratorium has a Quicktime movie of Libor Kiras (2.8 MB) showing what an expert trials cyclist can do. There are more links about this kind of extreme biking at Trials Online

• Countersteering is the interesting idea that you actually have to lean left to turn right. That's why it's so hard to steer a bike away from a close curb. I'll save this for a future stuumper, but there are some links here and here.

• There's some info on the physics of surfing here, here, here, and here. I don't know who came up with that nice phrase that "surfing is like running down the up elevator", but it sounds just right. The wave lifts you up and gravity brings you down. Surfing is playing with the balance between the two.

• That idea that we have to move forward to avoid falling is a lesson for life. There are interesting sermons at Evertt W. Huffard's "Balance Requires Forward Progress" and W. Frederick Wooden's "Dynamic Balance". I based my DMS 1995 graduation speech on a demonstration with balancing brooms.

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