Why is it easy to balance on a moving bike but not when it stands still?
In: Physics
It’s a surprisingly difficult question. You’ll get lots of disagreeing answers because bikes have multiple mechanisms for staying upright, all of them contribute to it staying upright, so there’s no single right answer.
All of the mechanisms work by steering the bike into the direction it falls.
There’s a myth that bikes stay up due to gyroscopic effects from the wheels and conservation of angular momentum. But there are still gyroscopic effects (gyroscopic procession) that keep a bike from falling over by turning into the direction the bike is falling.
Two other effects are due to the angle of the handle bars and steering column ie the handle bars are behind the front wheel.
This means the centre of mass of the steering column (handle bars + front wheel) is in front of where the handle bars turn from, so the bike turns into falls. You’ll have experienced this if you pick up a bike and don’t hold the handle bars. They’ll flip around and it’s really difficult to balance them straight.
Another effect is due to the position of the contact patch of the front wheel and the angle the handle bars are attached at and how they don’t line up.
But it’s awkward to explain over text so here’s a video. It covers all these effects. There are more, gyroscopic stability (the myth one) will still contribute a small amount.
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