(Image: Matthew Richardson)
IN 2011, an international team of bi-pedal enthusiasts dropped the bombshell that, despite 150 years of analysis, no one knows how a bicycle stays upright. Across the world, riders dismounted and stared at their bikes in disbelief. What they had been doing for years was a feat inexplicable by science.
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Well, sort of. 鈥淲hat we don鈥檛 know are the simple, necessary or sufficient conditions for a bicycle to be self-stable,鈥� says , an engineer at Cornell University in Ithaca, New York.
We have relied on trial-and-error engineering to construct stable bikes that aren鈥檛 prone to toppling while in motion. Explaining how they work mathematically requires around 25 variables, such as the angle of the front forks relative to the road, weight distribution and wheel size.
Before 2011, researchers had reduced this profusion to two things. One was the size of the 鈥渢rail鈥�, the distance between where the front wheel touches the road and where a straight line through the forks would meet the ground. The other was the gyroscopic restoring force that acts on a spinning wheel to keep it upright.
Ruina and his colleagues, including of the Delft University of Technology in the Netherlands and Jim Papadopoulos of the University of Wisconsin-Stout at Menomonie, not only revisited this mathematics, but also skewed the trail and gyroscopic forces in prototype bikes to make them technically unrideable. To everyone鈥檚 surprise, the bikes were still stable (Science, vol 332, p 339).
The researchers haven鈥檛 been resting on their saddles since. Last year Ruina unveiled a 鈥渂ricycle鈥�, a cross between a bicycle and a tricycle with spring-loaded stabilising wheels that can be adjusted to vary the rider鈥檚 perception of contact with the ground. By studying the influence this has on how the rider steers and remains stable, he hopes to gain new insights that might lead to more easily controllable bicycles.
It鈥檚 still an uphill struggle. 鈥淚 think the real understanding of bikes requires a mix of what we did, plus some kind of brain science,鈥� says Papadopoulos. Human riders act in extremely complex yet intuitive ways to keep a bike balanced and on track. At very low speeds, for example, we recognise that the handlebars become useless for steering, and instead .
Why? 鈥淲e don鈥檛 know,鈥� says Schwab. Yet another bike-based mystery that could be around long after we鈥檝e worked out the origins of the universe.
Read more: 鈥�10 mysteries that physics can鈥檛 answer鈥� yet鈥�
This article appeared in print under the headline 鈥淗ow do bicycles stay upright?鈥�
