Gravity is the sworn enemy of any cyclist. Succumb to its force while you're on two wheels, and you'll end up bloody and bruised. But like any superhero's nemesis, gravity justifies a bike's very existence, because without it, you'd never be able to steer. Strap on a helmet, this is gonna get weird.

You probably didn't realize the delicate dance you performed the last time you dipped your bike into a graceful arcing turn. To turn left, the first thing you do, counterintuitively, is steer slightly to the right. This countersteering act sends your body and bike leaning left, the same way a right turn in a car pushes you against the left side of your seat. Once you've started that lean, you steer left, into the turn, but not sharply enough to fully straighten upâ€”balancing a leftward lean that steers you in a smooth arc. A little added jab of left steering gets you going straight again.

Don't be surprised if you never thought about those many stepsâ€”you've been pretty much doing them intuitively. As mechanical engineering professor Andy Ruina explained to PhysOrg, "we are much smarter from the ears down. Really the rider has essentially no direct control of leaning. [A]ny leaning the rider intends is accomplished by doing the right sequence of steering maneuvers."

But what if gravity wasn't there trying to turn your lean into a crash? Dr. Ruina and his team built a nifty spring-loaded training wheel setup that allowed riders to lean without toppling, simulating a zero-gravity environment where the bike's wheels just happened to be magnetically attracted to the road (so you wouldn't just go flying off into space). Functioning like something between a bicycle and a tricycle, the team calls the vehicle the bricycle.

As Dr. Ruina explained at a recent meeting of the American Physical Society, the zero-gravity bricycle can't be steered: "the controllability of a bicycle depends on gravity. Without gravity, lean and direction cannot be controlled independently [...] Indeed, experiments with the bricycle show problems when the total effective gravity is about zero. People can then still balance easily but can no longer turn the brike."

Dr. Ruina's research team shows how the bricycle works, and how completely un-steerable it is, in a detailed video:

Check out what happens at around 1:22 in the video: when the rider steers the handlebars, the bike leans in the wrong direction. Countersteering to try and correct the outside lean gets the handlebars straight, but the bike is still leaning. Basically, the rider can't control the lean and the steering, and so while the bike might veer with a vigorous turn of the handlebars, it's not at all a controlled maneuver.

The team built the bricycle to investigate the behavior of a vehicle that's somewhere between a two-wheeler and a three-wheeler. It turns out, while both bicycles and tricycles steer naturally, the freak-o crossover is impossible to control. PhysOrg points out that the lessons learned about balance and control could be applied to everything from narrow-track vehicles like the DeltaWing race car to physical rehabilitation techniques for people with balance deficits.

So the next time you take a two-wheeled tumble, don't blame gravity. The same force that brought you to the ground is the one that would've kept you upright, if you just could've managed the right amount of countersteer. [American Physical Society via PhysOrg]