Between the bicycle and the tricycle sits an unbridgeable gulf. They seem like two different flavors of the same thing, but they steer in entirely different ways. So much so that when researchers at Cornell University tried to hybridize the two in a new study, they got an utterly uncontrollable four-wheeled beast of a bike—and a dramatic demonstration that gravity is indispensable for steering.To understand how a bike turns, imagine a box sitting on the floor of a bus that is moving straight ahead. When the bus suddenly turns to the left, the box keeps moving straight. But to passengers on the bus, it appears that the box slides to the right. We call this effect centrifugal force. Unlike boxes, bus passengers don’t slide around or fall on even the windiest roads. Instead, they lean to the left as the bus turns left, shifting their center of mass to use gravity to counter the centrifugal force pushing them to the right. We use the same principle to steer a bicycle—the rider turns the wheel to the left and simultaneously leans to the left, balancing the forces on the bike to stay upright. Tricycles, on the other hand, don’t tip to turn. They stay upright while the front wheel turns left and the right rear wheel exerts more force on the ground.“I thought it would be cool if we could just turn a knob and move from a bike to a tricycle,” says engineer Andy Ruina of Cornell University. Ruina points out that on a bike, gravity is often perceived as the enemy, the force that makes you fall. But it’s also what creates the forces that allow bikes to turn without falling. It’s necessary for tricycles, too; they need gravity to exert force against the ground with their rear wheels. 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He wasn’t surprised when they built it anyway. “When you haven’t seen it with your own eyes, when you haven’t actually done it … there’s a risk you have a big blooper” in your thinking, he says.Ruina knew Papadopoulos was right, but he has conducted several experiments over the years in which the “athlete is smarter than the natural scientist,” where a human did something that a hypothesis said was impossible. That forced him to find the flaws in his own thinking—and sometimes led to breakthroughs. Similarly, he wanted to see if the bike/trike dichotomy was really true in practice: A vehicle perfectly balanced between tricycle and bicycle would negate the effect of gravity by both preventing it from exerting force with its rear wheels like a trike, and by allowing the rider to lean the bike at any angle without shifting her center of mass.Ruina’s “bricycle,” as he calls it, is a bike equipped with two training wheels attached by means of a spring. When the spring is stiff, the bricycle turns like a trike. When the spring is loose, the bricycle turns like a bike. But at a certain point when the spring is just stiff enough, the training wheels and rear wheel offset the force of gravity on each other. (See video here.) At that stiffness, the bike becomes unsteerable and falls over if the rider tries to turn, Ruina reported today at the American Physical Society meeting in Denver.The bricycle was never meant to be practical. But its designers hope it will shed some light on the design of a new trend in automobiles, called narrow-track vehicles, whose footprints fall somewhere between two-track vehicles like cars and single-track vehicles like motorcycles. Two-track vehicles turn as tricycles do, by exerting force against the outer wheels, while motorcycles turn by leaning in the same way as bicycles. Narrow-track vehicles, such as three-wheeled motorcycles, would ideally have the stability of tricycles at rest but be able to lean like a bike when turning. Leaning makes them fun to drive and adds to the appeal of these smaller, more maneuverable, and fuel-efficient machines.It’s theoretically possible to build a wheel suspension that could constantly adjust its stiffness to create the perfect narrow-track vehicle. But as Ruina’s contraption shows, there would be a dangerous intermediate stiffness where gravity was offset and the vehicle could tip at any angle without falling over—unless the driver tried to turn. Both Ruina and Papadopoulos believe the bricycle demonstration will help narrow-track vehicle designers avoid wasting time working on a “neutral balance” for their machines. That particular balance might be called the bricycle point, something that needs to be avoided for narrow-track vehicles to take to the streets.