To prove it, they built a bicycle without any gyro or trail effects that can still balance itself. While gyro and trail effects may contribute to self-stability, they are not the only causes, report Andy Ruina, professor of mechanics at Cornell, and colleagues in the Netherlands and at the University of Wisconsin. This "trail" gives the force of the ground on the front wheel a lever arm to cause steering in a way that can help restore balance. Meanwhile, if the front wheel touches the ground behind the steering axis, it will be pulled into alignment with the direction of travel, just as a wheel on a shopping cart turns to follow whichever way you push the cart. When a bike leans, the gyroscopic effect tends to steer the handlebars in the direction of the lean, bringing the wheels back under the bicycle and helping to keep it upright. If you try to tilt the axis of a gyroscope in one direction, it will turn in a different direction. The accepted view: Bicycles are stable because of the gyroscopic effect of the spinning front wheel or because the front wheel "trails" behind the steering axis, or both.
Now, a new analysis says the commonly accepted explanations are at least partly wrong. Scientists and engineers have been trying to explain bicycle self-stability since the 19th century. Mathematical analysis shows that long-accepted explanations are incomplete. Almost any bicycle will keep itself upright as long as it's rolling.
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