Bicycle physics: Everything you know is wrong
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Give a riderless bike a push, and chances are it will cruise along for a bit without tipping over. Scientists have long thought they understood the physics of what keeps the bike upright. But a new study shows that much of what they believed is wrong.
Researchers assumed that two mechanisms kept a riderless bike upright. First is the “caster effect,” which relies on the position of the front wheel relative to the steering axis—the imaginary line that extends from the front forks downward. On most bikes, the front wheel meets the ground just behind this axis, so it tends to center itself like the casters on a shopping cart. The other mechanism is known as gyroscopic precession: Because the front wheel is spinning forward, any tilt to the left or right creates a force that will steer the bike in that direction, pulling it out of the fall.
To show that neither of these mechanisms is in fact crucial, engineer Andy Ruina of Cornell University and colleagues used a computer to design a bicycle that excludes both mechanisms. …
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Joel Fajans, a physicist at the University of California, Berkeley, who has studied bike stability, calls the work a “very thorough, well-done study.” The most surprising result, he says, is that the caster effect isn’t necessary, as previous experiments had already suggested that gyroscopic precession isn’t as important as many people think. “It’s possible that these results might influence bicycle design,” he says.
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Balancing act. You might not want to sit on it, but Ruina and his colleagues’ bike stays upright without so-called gyro or caster effects. Credit: Sam Rentmeester/FMAX Click here for a larger version.