Bicycles

[SimonInSimon]

question guys,

 

why does a bicycle remain standing when it is moving while if falls to either side when it's stationary? thanks!

 

i just want to add a question here. where is the center of gravity of the bicycle?

[swansont]

Angular momentum is conserved when there isn't any net torque.

[Mr Skeptic]

Like what swansont said. Think of the two wheels as spinning tops. They add stability in the same way. Also like the flywheels in spacecraft.

[bombus]

But like how does it actually work? Why does it work?

[Klaynos]

Hasn't someone debunked that idea swansont? I'm sure I read an article with someone who built a bic with a third wheel spinning the other way...

[Country Boy]

The crucial point is the front wheel. As I am sure you know, if you tilt a spinning wheel, downward to your right, say, the wheel will tend to turn to the right. That means it turns to "catch' the fall.

[swansont]

Hasn't someone debunked that idea swansont? I'm sure I read an article with someone who built a bic with a third wheel spinning the other way...

 

Can you find it? It might be an interesting read and we can possibly pick it apart.

[Klaynos]

After a bit of searching... And ending up on wikipedia... a quick look at the references gave me:

 

http://www.losethetrainingwheels.org/default.aspx?Lev=2&ID=34

 

http://socrates.berkeley.edu/%7Efajans/Teaching/MoreBikeFiles/JonesBikeBW.pdf

[swansont]

Two things leap out at me on a quick scan:

 

The empirical significance of Zero Gyroscopic Bike I is that the argument advocating sole dependence upon the “gyroscopic action” for stabilization of a bicycle is smashed and thrown out the window.

 

and

 

The only significant handling difference discovered in riding trials, compared to conventional bikes, was that the Zero-Gyroscopic Bike I was not capable of being ridden “no-hands.”

 

I don't think anyone is/was advocating that conservation of angular momentum was the only reason one can ride a bike, but that's not actually the question at hand. A stationary bike can be held upright by a good rider. I was thinking about without a rider there to provide feedback. A two-wheeled system is inherently unstable, and while it's moving you have Newton's first law helping out. But the observation that you can't ride "no-hands" is a strong indication that conservation of angular momentum is very important to the stability of a no- or reduced-feedback system.

[insane_alien]

yeah, that 'can't ride it no hands' does imply that it has taken quite a stability hit. and from just sitting on a stationary bike, you can keep it standing and at quite weird angles too.

[Rocket Man]

the shaft that holds the front wheel is at an angle and the front axle is even further forward from that axis.

that causes the front wheel to point in the direction the bike leans for shallow cambers.

if you turn the forks around, the stability drops right out and it's near impossible to ride no hands.

the gyro effect smooths that out and assists the same motion

 

the center of gravity on a fairly normal bike is just infront of the seat post and just below the centre of the frame. you can put drinks there

[alan2here]

If a bike was built that couldn't steer and it was projected forwards at a high speed then it would fall over fairly quickly, therefore it must be to do with the turning of the front part of the bike that does it. I think maybe the front turns right slightly as the bike leans right and vice versa with left, when moving fast the handlebars will turn faster.

 

I found I was unable to keep a bike upright for more than a few seconds when it was stopped without putting my feet on the ground. Riding no hands is also rather difficult.

 

Therefore it is not too much of a surprise to me that this

can't be ridden no handed.

[SimonInSimon]

could you expound on this line from wikipedia?

 

At high speeds, the input required to return the bike to upright only needs to be small; a much greater input is required to maintain balance at low speed. As such, it is easier to maintain balance at high speeds

[swansont]

could you expound on this line from wikipedia?

 

At high speeds, the input required to return the bike to upright only needs to be small; a much greater input is required to maintain balance at low speed. As such, it is easier to maintain balance at high speeds

 

Sounds a lot like conservation of angular momentum. Higher speed means a smaller fractional effect of torque due to gravity.

[Rocket Man]

that bike has a higher centre of gravity. no wonder the stability has dropped out.

if you've ever ridden a bike with a heavy pack above the seat post, it's almost impossible to ride without hands. panniers take the same load and keep the mass low to the ground so the bike is easier to ride.

 

i'd still say the front acts as a tilt controlled castor and the gyro effect aids resonse time.