The above video is a demonstration of a cycling robot built by Japanese engineer Masahiko Yamaguchi. The tiny mechanical cyclist keeps its balance in the same way a human does by steering in the direction the bike and rider is leaning or falling.
The robot has a sensor in its back pack that detects a lean to the left or right, and this in turn causes the robot’s arms to steer the bike in the required direction.
A human will do this intuitively, as we fall to the left; we steer to the left bringing the bike back under the center of mass, or balancing point. Rather like balancing a broom on the palm of our hand, we do this by constantly moving the palm of the hand in the direction the broom is falling.
Conversely, if we want to turn to the left we lean to the left. We automatically steer to the left to correct the lean and therefore make a left turn. Even when running we lean in the direction we wish to turn, all animals do it.
By leaning we automatically step in that direction and in doing so make a turn the way we are leaning.
For both the runner and the cyclist, the lean also counteracts the centrifugal force, pushing us outwards, that the turn generates.
You never rarely hear of a bicycle or motorcycle tipping over on a corner as a three or four wheel vehicle might do; or for that matter a runner falling outward on a bend. Rather the bike, or the runner’s feet may slide out from under them.
There are two ways to balance on a bicycle. The easiest way by physically steering the bike in the direction we are falling. (The way the robot does it.) Or we can even ride “No hands” and both balance and steer by shifting our body weight.
This requires more skill but as most of the body’s mass is in the upper body; the rider mostly has only has to move their hips to achieve balance. This too is somewhat of an intuitive movement.
Have you ever been riding near the edge of the road and find that the asphalt drops off several inches? Your bike is leaning towards the edge and normally you would steer in that direction to correct it; only you can’t because you will steer yourself off the road.
Instinctively, you throw your opposite knee out away from the edge; move your hips towards the edge and your shoulders away from the edge. You have maintained your balance by moving your body weight rather than by steering. It was an intuitive movement probably accompanied by a moment of sheer panic.
This is why the bicycle is such a remarkable yet simple invention. It is relatively easy to balance and ride; after all a child can do it with a little practice.
It is a similar to the intuitive thing that we do while walking or running; the bicycle could be described as a mechanical extension of the human body.
Footnote: There is more to steering and steering geometry than this simplified explanation. There are several articles in the bike tech section of the archives.