Why does a bicycle stay upright when moving?

This turns out to be a genuinely subtle question, and the explanation most people were taught is largely wrong.

The common answer — gyroscopic effect — holds that spinning wheels resist tipping. There is a real gyroscopic effect, but research has shown it is not necessary for stability. In a well-known 2011 study published in Science, researchers built a bicycle with counter-rotating wheels that cancelled all gyroscopic effect and eliminated the usual steering geometry. It still balanced itself when rolling.

The second common answer — trail, or caster effect — refers to the front wheel's contact point trailing behind the steering axis, like a shopping trolley castor. This does contribute, and the same experiment showed it is also not essential.

What actually keeps a bicycle up is steering. Balance is maintained by continuously steering the wheels under the centre of mass. When the bike leans left, the front wheel turns left, which curves the path leftward and brings the contact points back beneath the falling mass. It is the same as balancing a broom on your palm by moving your hand under it.

A moving bicycle does this automatically because of how mass is distributed and how the front assembly is geometrically arranged. Several factors — trail, gyroscopic precession, and crucially the fact that the front assembly's centre of mass sits forward and low — combine so that a lean produces a steering input in the correcting direction. No single one of these is required; what matters is that the overall design produces self-correcting steering.

This is why a stationary bicycle falls over — with no forward motion, steering cannot move the contact patch anywhere useful.

And why speed helps: faster motion means corrections take effect over a shorter distance.

A rider adds active balancing on top, which is why people can ride bicycles that are not self-stable.