Why is it that I can balance myself on a bicycle when it's moving, but I can't when it's stopped. When you think about it, that doesn't make sense. Or does it?
-- Bikeman, Bay Park
Depends on who's doing the thinking, apparently. The answer used to be simple but for some reason, that wasn't good enough. You should have asked this a couple of years ago, before some big thinkers got ahold of it. The elves won't give up their training wheels, and they don't care what the answer is, so I'm on my own here.
I assume there's no mystery about why a stationary bike is so unstable. Same reason two-legged bar stools never caught on. You're trying to balance yourself along a line that runs from the contact point of the front wheel to the contact point of the back wheel. You're perpetually a victim to gravity. If somehow you do manage to find the precise point of balance, all you have to do is switch your bubblegum to your left cheek, and you fall to the left. But when the bike's wheels are spinning, the physics of the situation changes. The front wheel acts like a gyroscope, and one thing a spinning gyroscope wants to do is remain stable. Now if you promise to go along with me on this explanation, I promise I won't go into the bloody details of angular momentum. Nobody wants to hear that, believe me.
So this time the bike is moving when you shift your gum (and the center of gravity of you and your gum and the bike). In response, the bike tilts slightly to the left. But instead of caving to gravity, the stabilizing influence of the gyroscopic front wheel (the steering wheel) causes it to turn automatically to the left, under the moving center of gravity. It's the effects of the angular momentum of the wheel and the pressure of the ground against the wheel that cause the shift. The faster you're moving, the more stable this system is. The design of the bike frame and the wheel fork also influence stability. The fork creates a steering axis, and when the moving bike tilts to the right or left off this axis, the wheel automatically tries to correct itself by turning into the direction of the tilt. You can see this even when the bike is stationary; tip it to one side, and the front wheel will pivot to that side. All in all, riding a bike is sometimes described as a whole series of these tiny automatic corrections being made continuously as you pedal along. Personally, just thinking about all this is plenty exhausting, and I'm going to take a nap.