Comment by bambax

Thanks! Corrected!

Here's what I originally typed out before googling:

The "something about the geometry" is called caster, and is the same effect that makes the front casters on a shopping cart go straight: the point where the steering axis intersects the ground is ahead of the contact patch of the tire. On a bike, this is mostly determined by the angle of your head tube when looking at the bike from the side (if the fork is "bent" from the side view, this would also contribute to the caster effect).

But I've now googled, and found a paper that says that a bike can be stable without gyroscopic or caster effects [1]. It seems like the specific mass distribution of the steerable mass (front wheel, fork, handlebars, etc) vs the rest of the frame matters, and of course all of these variables interact in complex ways. They do agree that caster plays an important role though.

Vehicle dynamics is notoriously tricky stuff. I can say with experience that it doesn't get easier when you go to four wheels.

[1]: https://www.researchgate.net/publication/51051995_A_Bicycle_...

> but there is something about the geometry of a bike that is sort of self correcting.

Yup. Plenty of videos on youtube where they send a bike down a hill with no rider and as long as there is forward motion it will self-correct and stay upright.

Bikes in motion are self-balancing, and with no rider on, will continue indefinitely until the forward momentum has been exhausted.

https://www.youtube.com/watch?v=NcZCzr9ExKk

This is because gyroscopic forces are not significant except at high speed, and most of the forces relevant to cycling and which direction the bicycle goes are generated between the front tire and road surface. Turning the handlebars to change direction is also only relevant at low speed for the same reason.

Can you clarify what is it exactly that you consider counter-steering?

My understanding is that it means "briefly turning the handle-bars to point the front wheel in the opposite direction of the intended turn, causing the vehicle to start tipping over in the direction of intended turn", which is exactly how you steer both motorcycle and bicycle.

I too am a motorcyclist (and now, mostly, a cyclist) and think I may have misspoke (steering vs counter-steering).

When I learned to ride a motorcycle I was taught to push the handle bars with the hand on the side I wanted to turn (so, if trying to turn right, push with the right hand); this causes the bike to "fall" on the side of the turn, and follow the turn.

This is what I meant by "counter-steering" but 1/ it only works at relatively high speeds (above, say, 20 mph, which isn't high on a motorcycle, but pretty high on a bike) and 2/ it doesn't "prevent" the bike from falling, it makes it fall, which is what we want.

Following the same principle, staying upright on a bicycle involves steering, not counter-steering: when a bike starts falling to one side, turning the wheel to that side makes it want to fall to the other side; and if done fast enough and often enough (as all riders to), maintain the bike upright.

Counter-steering is how bicycles steer, whether you're doing it consciously or not.