Eli5: Why is it so easy to balance a moving bike but not a stationary one?

Question

Just a thought I had today while biking home from work and I don’t know enough physics to answer it

In: 48

Answer 1

Bicycles and motorcycles balance themselves when they are in motion. It’s more noticeable in motorcycles though. You probably have seen videos of motorcycles riding away after the rider fell.

When the bike starts slightly leaning to one side two main effects counter this leaning by turning the bike towards the side it’s leaning:

– The contact surface of the tires with the ground now is curved, because of the shape of the tire.

– Due to geometry of the front assembly (angle and relative position of fork, axle, pivoting point and contact patch), the handlebar turns to the leaning side.

There’s also a gyroscopic effect of the wheels turning, but it gets quite complicated.

When these effects make the bike turn to the leaning side, you can look at the effect in two ways:

– The mass of the bike was going straight, and by the magic of inertia it wants to keep going straight. So let’s say it was leaning to the right, it started making a curve to the right, so “straight” is now to the left of the bike, so now there’s a force towards left, with the equivalent point of application at the bike’s center of mass, reacting to the force to the right, applied by the ground at the tires’ point of contact. This torques the bike to rotate (roll) to the left, neutralizing the right lean.

or

– A “centrifugal” force (which is the reaction to the force required to make a moving mass change direction) appears pushing the bike at its center of mass towards the outside of the curve, which is opposite to the leaning, straightening the bike up.

This effect in reverse can be easily tested by countersteering: You slightly turn the handlebar to the left, the bike “tries” to turn to that direction, but inertia (or centrifugal force) leans the bike to the right, making it change direction to the right.

Edit: typo and botched auto list numbering

and reference:

https://en.m.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

>long-standing hypotheses and claims that any single effect, such as gyroscopic or trail, is solely responsible for the stabilizing force have been discredited.