How bicycles keep the rubber on the road

Engineers try to explain the surprising stability of two wheels

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An unusual riderless bike with two wheels that spin backwards demonstrates that the physics behind the stability of bicycles is more complicated than previously thought. Sam Rentmeester/FMAX

Bicycle abuse isn’t something you’d expect from the Dutch. But engineers in the Netherlands who love bikes enough to hurt them are challenging long-held beliefs about how a moving bike keeps its balance even when slapped, shoved or otherwise insulted.

The team has found previously unidentified factors that help a bike stay upright and has developed a slew of unusual designs that wouldn’t have been thought to be stable.

“We believe there is room for improvement in the handling qualities of bikes,” says Arend Schwab, a professor of engineering mechanics at the Delft University of Technology.

A conventional bicycle is remarkably stable when moving. Even without a rider, it can coast for long distances and catch itself from falling. As early as 1910, scientists credited this stability to the front wheel behaving like a gyroscope. As a spinning wheel leans, it should naturally swivel in the direction of the lean, guiding the bicycle into a curve that keeps it upright.

In 1970 David E.H. Jones, then a spectroscopist at British chemical company ICI, tested this explanation by trying to build an unridable bike. An extra wheel mounted to its frame spun backwards and canceled out the gyro effect. This bike was less stable — but still ridable, even with no hands.

Jones looked for another stabilizing effect and found one similar to that which keeps the casters of shopping carts lined up. Hold a still bicycle by the seat, lean it to the side and gravity turns the wheel. This “trail effect” is based on the front wheel’s position relative to the angle of the steer axis that connects the wheel to the handlebars. Move the wheel forward a few inches, Jones discovered, and a traditional bike becomes less stable.

Published in Physics Today, the paper describing these experiments circulated widely and was read by a high school junior in Corvallis, Ore. Jim Papadopoulos, a competitive cyclist, didn’t understand the math at first. But later, in graduate school, its conclusions would bother him.

“It took me 30 years to put my finger on the big flaw, ” says Papadopoulos, now an engineer at the University of Wisconsin–Stout in Menomonie. “Jones’ paper wasn’t based on the physics of something falling but on the physics of something being held.”

Papadopoulos teamed up with a researcher from Cornell and a team in the Netherlands that built a bike with no gyroscopic or trail effects. Their riderless contraption, which sports two extra backwards-rotating wheels and a front wheel that touches the ground in front of the steer axis, can still coast stably. Give it a smack, and it curves, swerves and recovers.

“You don’t need gyroscope or trail to make a bicycle self-stable,” says Andy Ruina, a professor of mechanical engineering at Cornell and coauthor of the paper describing the bike in the April 15 Science.

Bicycles, the team suggests, are more complicated than previously thought. While gyro and trail effects can contribute to stability, other factors such as the distribution of mass and the bike’s moment of inertia can play a role as well. Computer simulations that take all of these factors into account could lead to improved designs for folding bikes with small wheels or bikes that carry cargo, Ruina says.

To demonstrate the possibilities, the researchers sketched out several new exotic bicycle designs. One is predicted to remain stable even with a negative gyro that tries to turn a falling bicycle in the wrong direction. In another, the steer axis is reversed such that the handlebars are farther forward than the center of the front wheel.

“They found a design with rear-wheel steering that can be ridden and is self-stable,” says David Gordon Wilson, a retired MIT professor who designed the modern recumbent bicycle in the early 1970s. “That’s quite amazing.”

In the simulations, these new design principles still work when the weight of a human being is added. But the real test is waiting out on the open road.

“The next step would be to study a bicycle with a rider in the real world: on actual roads under varying conditions, on a fully instrumented bicycle,” says Joel Fajans, a plasma physicist who also studies bicycles at the University of California, Berkeley.  To find out how we really ride a bike.”

How bicycles keep the rubber on the road from Science News on Vimeo.

After being slapped, a moving bicycle steers itself to recover and straighten out.

Credit: © Science/AAAS

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