Basic question about angular momentum.

[Tigermisu]

Hello, recently I came across this video:

https://dl.dropboxusercontent.com/u/12852456/bike.webm

My question is, why does this happen? The bike simply slides off as if nothing were keeping it up right.

Obviously, the bike is going very fast. Isn't the huge angular momentum of both wheels supposed to be strong enough to keep it upright?

[Bill Phil]

That and the natural tendency for the wheels to steer into turns.

[Camacha]

I would not recommend pulling on someone's head like that right after such a fall. You might just aggravate damage in a major way. Then again, he fell off himself too. He is probably pretty shaken up.

[Kerbart]

Angular momentum doesn't keep a bike upright. What angular momentum does do is turning the front (steering) wheel into the direction the bike is falling. As long as the bike is in a nearly upright position it will keep the bike stable, but once the bike is going it's obviously not enough.

[Tigermisu]

I would not recommend pulling on someone's head like that right after such a fall. You might just aggravate damage in a major way. Then again, he fell off himself too. He is probably pretty shaken up.

Yeah, I noticed that, it was quite risky.

Angular momentum doesn't keep a bike upright. What angular momentum does do is turning the front (steering) wheel into the direction the bike is falling. As long as the bike is in a nearly upright position it will keep the bike stable, but once the bike is going it's obviously not enough.

But, what gets it going down that the angular momentum cant keep it stable?

[LordFerret]

Looks like maybe he hydroplaned. You can see it right at the beginning, the slight drift to the right, and then he tries to correct (turn left, but the front wheel just dances out), and from there it just lays down. I've had this happen; the bike can just slide right out from under you once you're off to the side of its center of gravity even just a little bit (it doesn't take much). About the only thing you can do once it starts to go, if you know it's coming, is ride it down trying to stay over the bike... at least that way the bike takes most of the road-rash and not you.

You can easily test this out without the high speed - just ride on snow, or ice.

[K^2]

The amount of angular momentum in bike's wheels is actually pretty small. It's not what keeps the bike upright, contrary to popular myth.

[baggers]

Looks like maybe he hydroplaned. You can see it right at the beginning, the slight drift to the right, and then he tries to correct (turn left, but the front wheel just dances out), and from there it just lays down. I've had this happen; the bike can just slide right out from under you once you're off to the side of its center of gravity even just a little bit (it doesn't take much). About the only thing you can do once it starts to go, if you know it's coming, is ride it down trying to stay over the bike... at least that way the bike takes most of the road-rash and not you.

You can easily test this out without the high speed - just ride on snow, or ice.

Note that if the front whell lost his ground adhesion (aquaplaning, or overbraking), nothing keep it rotating: if your hand is on the brakes, it will immediatly fully stop rotating.

A usually bad move of young biker is "pull all levers" when in trouble: right lever overbrake the front whell, so lost ground adhesion (espicially when raining), and stop front whell rotation. Left lever make "free rear Wheel" that can too easily overbrake rear wheel in that situation, and stop his rotation too.

That's usually result in a total loss of control of the bike, and naturally, no whell rotation because overbraking = no momentum.

Edited October 2, 2015 by baggers