The researchers made sure that gyroscopic torque of the wheels was negated by placing a matching set of wheels on the bike that spin backward, canceling the forward-spinning wheels' angular momentum. And trail effects were eliminated simply by designing the experimental bike so that its steering axis meets the ground behind its front wheel's contact point.
Both of those factors are indeed major contributors to making a rolling bicycle self-stable, and once appeared to be necessary for a bicycle's self-correction. "There were people who said that each of them were essential," says Mont Hubbard, a professor of mechanical and aeronautical engineering at the University of California, Davis, who did not participate in the research. "And they're not."
"It would be very common to presume that if you don't have them you've got nothing," Klein says. "The bicycle won't stay upright."
The demonstration of self-stability without gyroscopic or trail effects does not mean that today's bicycles are somehow flawed. Bikes are highly evolved machines, improved by strategic, incremental advances—as well as trial and error—over the decades, so a new insight into stability is not likely to usurp well-established standards of bicycle design. "If you look at the bicycle they built, it's a bicycle, yes, but it doesn't bring us to any design suggestions about ways to build a bicycle better than the ones we have now, I don't think," Hubbard says.
And Klein, whose own research is focused on applications of bicycle technology, questions whether the findings will ever make an impact on real-life designs. "What they've done is a tremendous piece of work," he says. "If I were in an investor, would I put money in their company? No."
"We're not claiming that this crazy-looking machine is useful in itself," Ruina says. "It's just to prove a point." Still, mass distribution might play a role in designing better cycles outside the standard design realm, such as folding bikes or recumbent bikes. Small-wheel bikes, where gyroscopic torques are minimized, might be especially amenable to increased stabilization. "It gives you sort of a new way to think about what the design possibilities are," Ruina says. "The story is more complicated—there are other design parameters that have effects on stability."
But the long-standing question of what causes a riderless bicycle to balance itself has not been answered—at least not in a clean way. The researchers note that they have found no straightforward set of attributes that are necessary for self-stability.
For Schwab, after years of research and countless international Skype chats with his co-authors, simply adding a new facet to something most people take for granted is gratifying. "Every time I bring this subject up I have the same problem. Everybody shrugs their shoulders and says, 'We have bicycles, we know how to ride bicycles,'" he says. "I still have to fight that now: 'What's new about bicycles?'" Now, he says, he will have something to tell them.