How do bikes balance when we ride them?

If you find yourself wondering how the large man riding past you in the tiny yellow shorts can stay up on something as skinny as a bicycle, be heartened by the fact that scientists wonder too. Though they have long known how fast the earth is traveling around the sun, and what deep ocean currents do, and how to get to the moon, the way that different forces combine to produce bike-stability is still a subject of investigation.

Many have agreed that bikes are held up at least partly by two powerful gyroscopes – their wheels. A gyroscope is, appropriately, a spinning wheel or a sphere that rotates so fast, it creates forces that make it hard to change its orientation. Scientists, fittingly, use gyroscopes to measure changes in orientation.

As a bike wheel spins, it is rotating around an imaginary horizontal line that goes through its center, creating a force in the direction of its spin. If you try to change the horizontal axis by falling left or right, the spin translates that push into a turning motion. Lean right – turn right, lean left – turn left. Scientists call this behavior precession.

You can see precession in action with a rider-less bike. Take your two-wheels out to the park and give it a good forward shove. Then, run up alongside and give it a sideways shove. Unless you’re doing a full on rugby tackle, the bike should wobble, but then realign and keep on going. Scientists believe that it’s the forward spinning motion of the wheels that helps to keep the bike balanced.

On the other hand, it has also been shown that the gyroscopic effect of a bicycle’s wheel is all but cancelled out by the weight of the rider’s body as well as the force pushing down on the front handlebars. One ambitious engineer, Dr. Hugh Hunt of the University of Cambridge, built a bike with a second front wheel. Theoretically, if gyroscopes were all that had to do with balance, spinning the second wheel in the opposite direction of the first would cancel out the gyroscopic forces, and make the bike un-rideable. Yet no matter how fast the second wheel was spun, the bike performed exactly the same. This, he concluded, demonstrated that while gyroscopes are part of a bicycle’s movement, their effect on stability is small enough that it is overshadowed by the force of the rider’s weight and movement.

This leaves a bicyclist to depend on small corrections in her steering, body position, and speed to keep her from falling. You can see such corrections at work when watching a beginner zoom around. New cyclists tend to veer back and forth, correcting any leaning with big shifts in direction. Sneakily, experienced riders are doing this, too – just so precisely that you can’t really tell.

This acute adjustment of balance also explains those hipsters in really tight pants doing ‘track stands,’ or staying still on a bike without putting a foot down, at city intersections. Inexperienced track-standers wobble back and forth, slowly inching into oncoming traffic; the well practiced, on the other hand, shift their weight so precisely, the bike stays put. This represents the kind of physical power over the bicycle that Hunt says overshadows the spinning of the front wheel.

The caveat to the balance-shifting explanation is that it only represents the forces responsible for keeping you upright when you’re riding fast in a straight line, or refusing to put foot to asphalt at a stoplight. Gyroscopic action becomes extremely important for maintaining a straight path when you ride “no-hands” and you aren’t using your arm-force to steer.

True to this point, researchers at the Delft University of Technology in the Netherlands built a spider-shaped test bike with a computer on the back that precisely measures forces while rolling on a giant treadmill. Their results suggest that a rider’s skill at balancing and the shape of his bike prevent him from falling over as he rides. Gyroscopic forces help keep him from falling over if you give him an overzealous pat on the shoulder. So go on – give Mr. Yellow-shorts a little push. He’ll be fine.