Being raised and trained on cars and trucks, we sometimes take stability for granted.
If you're in your car and you come to a stop on a level piece of land, shut off the engine, and get out of the car, your car stays where you put it. Do the same thing with a motorcycle and it will immediately fall over to one side. (which is why motorcycles come with sidestands)
Statically speaking, bikes have no upright static stability. For a bike to be statically stable, you need at least three points of contact on the ground to prevent gravity from pulling the bike to the ground. That's why we slap training-wheels on our childrens' bicycles. When they first jump on, they're too nervous to get the bicycle moving at any speed, so we add a point of contact to keep the bike upright until they learn to ride.
The strangest thing happens, though, as we get the motorcycle moving. Once the bike gets up to a certain speed, it seems to stand upright on its own, without any input from the rider. In fact, we have to TRY to get the bike to fall over now.
While it's true that a two-wheeled vehicle has no static stability, as it gets moving, it acquires dynamic stability.
The Front Wheel - A Dynamic Stability Engine
I like to call a motorcycle's front wheel its "dynamic stability engine". It's the physics of the front wheel that stabilize a motorcycle once it gets moving. In fact, your motorcycle is constantly trying to "fall over", even when moving. It's the small corrections of the front wheel that prevent it from doing so.
The simplest way to explain this is that as the bike tries to fall over to one side, the physics of the bike's front end cause the front wheel to turn in the direction of the fall, essentially steering your wheels back under you.
This is happening all the time when you're riding in (what you think is) a straight line. The bike is actually performing a very mild weave, constantly correcting to keep the motorcycle stable and moving relatively straight.
A Little More on the Physics
Two elements are responsible for most of the front wheel's propensity to steer the motorcycle's wheels back underneath it. I'm not going to go too much into the physics, since talking too much about them seems to confuse more people than it enlightens.
The first has to do with the rake and trail of the front forks. The forks of a motorcycle are never straight up-and-down. Rather, they are angled forward. When a motorcycle's wheel starts to tilt to one side (i.e. the bike starts to fall over), the weight of the bike pushes down on its front end. Because of the shape of the tire, this pushing force causes the tire to turn in slightly so the motorcycle "settles" on a different part of the tire.
The second has to do with the spinning motion of the wheel. If you hold a spinning bicycle wheel by the axle and try to tilt it to one side, you'll feel the wheel wanting to turn in the direction it's being tilted. This is called gyroscopic precession. If you've ever sent some loose change rolling, you'll notice that as it loses momentum, when it falls over, it turns in the direction it's falling, causing it to roll in a tight circle. The concept is the same for motorcycles. As the bike falls over, the force of gravity pulling the front wheel over generates a 90-degree offset force that makes the wheel steer into the turn.
Fast vs. Slow Speeds
So basically, a motorcycle is dynamically stable because the physics of the front wheel cause it to turn in the direction that the motorcycle falls, essentially "keeping your wheels underneath you".
At faster speeds, this provides enough stability that you can essentially cruise down the street without your hands on the handlebars. (except to give the bike gas to keep it moving) At slower speeds, however, you may notice that the bike is more likely to fall over. That's because your "dynamic stability engine", your front wheel, is no longer moving fast enough to provide a positive steering action or gyroscopic force.
What a lot of riders do to make up for this loss of stability is to turn the bars in the direction that the bike is falling. Essentially, when their "dynamic stability engine" is not running at full-steam, they take over its responsibilities manually. If you've ever watch children ride a bicycle at slow speed, this motion is obvious to spot.
Here's the problem...most people who learn to ride a bicycle, motorcycle, or other tandem 2-wheeled vehicle learn to ride at slow speeds before they ride faster. Because of this, they start believing that the motion that they're doing with the front wheel to help balance the bike is a steering motion, rather than a balancing one.
So many of them arrive at a mistaken idea that they are turning the bike by steering into the turn. And if they never understand what's going on, this can lead to trouble later on, when they need to steer the machine quickly at high-speed.
to be continued...