A riderless bicycle can travel for some distance without falling — an ability generally assumed to be due to the gyroscopic forces of the spinning front wheel keeping it upright. But this is not the whole story, according to a paper by Andy Ruina of Cornell University in Ithaca, New York, and his colleagues. Following a theoretical model, the team built a bicycle (pictured) in which the gyroscopic forces were cancelled by a second set of wheels, off the ground, that spun in the reverse direction.
The authors found that the bike was relatively stable at speeds of more than 2 metres per second, showing that gyroscopic forces are not necessary for bike stability. In the case of their custom bike, the low steering assembly — handlebars, fork and front wheel — fell faster than the rest of the bike, tending to steer the bike in the direction of the fall, just as a rider would do.
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A stable, riderless bicycle. Nature 472, 263 (2011). https://doi.org/10.1038/472263a