Those crazy folks at Cornell produced a hybrid between a bicycle and a tricycle, and ended up with a vehicle that has a very weird steering capability.
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.More info on this can be found at the YouTube page where they have uploaded a video of this device.
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. 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 is really the same as the gravity-free pendulum. Assuming friction and so on are negligible, if we start from an upright position, the lean and the sideways displacement of the ground contact point are always in proportion to each other. So changing direction would cause both an ever-growing distance for the original line of travel, and an ever-growing lean angle. The riders don't tolerate this. Instead, they maintain balance and thus are stuck going about straight.Here's the video:
So gravity, superficially the thing that makes it hard to balance a bicycle, is the thing that allows you to steer it.