Well known fact: a drunk person can ride a bike well beyond the state when he is unable to walk. This is not, of course, the reason for cycling's burgeoning worldwide popularity.
The same applies to heroic bike racers who must often be caught and carried away once they finish. There's clearly more to bike stability than meets the eye. So goes the old story of an Indian yogi whose strategy for learning to ride began with balancing on the machine, motionless. Only then, he surmised, would riding be possible. Such is the illusion of bike stability.
The Public Spectacle
When cycling appeared in the 1870's, huge crowds gathered to gawk. In England, 10,000 would show up to see someone cycle. So gravity-defying was the achievement. Nothing in our collective experience prepared humanity for the spectacle of two wheeled stability. Little did anyone know how easy the trick.
Turns out the front wheel plays a special role. A spinning wheel is a gyroscope providing feedback to a rider. Hold a wheel by its axle ends and get it spinning between your hands. Any attempt to tip the wheel to the left or right, the beginning a a fall, generates a powerful force to turn in that direction.
This gyroscopic force works best with just a wheel. Rolling by itself at a brisk pace, a wheel cannot tip over. Anything to cause a lean forces a turn. The turn is an arc exactly sufficient to produce centrifugal force to arrest the fall. The wheel now travels in a large arc. As it slows down, the centrifugal force becomes insufficient and the wheel leans further, which causes a tighter turn (smaller arc) to support the lean. Left to itself, the wheel travels in smaller and smaller circles until its speed is too low to generate the force to stay up. At last, it falls down and stops.
With a complete bike and rider, these forces are too small to hold up the entire machine but feedback through the handlebars is strong. Ride hands-free and your front wheel gyro nearly balances for you. Even hands-on, balancing the bike is easier with positive feedback. Here's a large part why learning to ride is so easy for children. Bikes need to be fun and efficient but they must also be supremely easy to master to be used by the hundreds of millions.
Diameter and mass of the wheel are key ingredients. The larger and heavier, the greater the forces. Tiny wheeled bikes are far less stable because gyroscopic forces are much smaller. No wonder we historically tend to large wheels in spite of the cost in weight, portability, economy, and aerodynamics.
The Mathematical How
Engineers have puzzled for 150 years about the physics underlying bicycle stability. Lengthy papers from nearly every developed country have tackled the complex dynamics of steering. Great mathematics thinkers are challenged by the number of simultaneous variables. Einstein enjoyed his riding and the elusive puzzle of stability.
Most attempts to unravel the mystery of bike balance focus on the gyroscopic inputs just described. An clever outcome of this pursuit is a stable bike riding robot. No small feat.
Lately, however, another line of explanation is helping us understand the complexity.
The Anthropological Perspective
Have you noticed how close cycling posture is to normal standing? Touring, training, and commuting is a position very similar to walking. Head height, spine angle, hip and leg relationship, center of gravity, all akin to walking. As a nomadic mammal, moving about is a highly adapted activity. Humans are bipedal which is very similar to single track (cycling) mobility.
Bipedalism is an unstable undertaking. Contact with the ground is very narrow. Most native peoples walk in straight lines, one foot directly ahead of the other. We overcome serious balance challenges simply walking around. These are the same as with cycling. You don't learn to ride, we simply adapt our well honed bipedal skills to pedaling.
Other long leg animals share the geometry. Dogs and horses (as opposed to turtles and rodents) are essentially single track movers as well. Cornering is a matter of precise leaning to avoid tipping over. None of us can run around a turn without leaning. Same as cycling. We evolved to make accurate calculations of the correct lean angle depending on speed and turn radius. These skills are required for cycling.
Recent research illuminates that, while front wheel gyroscopic forces are useful, they are not required for stability. Physics related to our bipedal posture is involved. To lean one side or another requires a quick move to get our feet beneath us. Here is the most important stability mechanism in cycling.
I'd like to give front wheels sole credit for the magic of cycling dynamic balance. Instead, we appreciate the contribution of wheel gyroscopic force but understand how intimately related the bicycle is to human physiology. No wonder bikes feel like extensions of our bodies. They are designed the same way.
In the end, the drunk cyclist is better resting on a couch than pushing the limits of bicycle stability!
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