Take your hands off the bicycle handlebars and your bike won’t notice. Hop off and give it a shove, and chances are it’ll keep skimming along all on its own (as long as you don't push it over en route to your faceplant). Ever since bicycles were invented in the 1860s, people have been wondering: what makes bikes so spookily stable? Popular explanations are that the spinning wheels behave like gyroscopes or that the front wheel making contact with the ground just behind the steering axis stabilizes the bike. But take both of those properties away, researchers reporting in Science ($) have found, and the bike still rolls merrily onward.
The team built their own bicycle with extra wheels that rotated in the opposite direction to cancel out the gyroscopic effect and with a steering axis that’s behind the front wheel, and found that even that was not enough to knock the bike off balance. When graduate students tried to tip it over, it steered into a turn and recovered. So what is keeping the bike up? A great sum-up at ArsTechnica explains that the answer is: we’re still not really sure. While the math is clear, how it connects to physical reality isn’t.
What their math can't apparently tell them is why so many different bike designs tend to stay upright...The best they can surmise is that the stability is related to the ability of the bike to steer into a fall if it starts to lean, and that there are multiple ways of constructing a bike that does this.
But here's an idea for next time: taking a bike that falls over all the time and tweaking it until it stays up. Video credit: Jacques Tati, Jour de Fete (1949)