johnny99

Scientific American does not know:
https://www.scientificamerican.com/ar...lf-stable-bike

Doohickie

In other news: Scientific study seeks to answer the question "How do we know how to breathe?"

Why overthink this? The physics can be modeled. That it turns out to be something other than (or in addition to) conventional wisdom, why is it a big deal?

phoenix

A bicycle doesn’t stay upright, that’s what they have us for.

Commodus

Wait, wait, I got this.

Why can't a bicycle stay up on its own?

...

Cuz it's two tired.

bikerjp

Know what a gyroscope is? Same principle. Balance helps too.

johnny99

The article specifically refutes the gyroscope theory. And the balance theory, too, since they are testing a bicycle without a rider.

johnny99

Because scientists are nerds.

JTGraphics

A wheel doesn't need a frame a single wheel uses the gyroscopic effect and will stay upright on its own so as far as I'm concerned I'll take the gyroscopic theory for it staying upright once rolling.

prathmann

The gyroscope hypothesis can be tested experimentally by mounting a second wheel alongside that of the bicycle but not quite touching the ground. The second wheel had the same mass and was set up to have the same rotational speed but in the opposite direction thereby canceling the gyroscopic effect. The experiment found that it was still easy to ride the bicycle - so the gyroscopic effect is not needed for balance.

Tom Stormcrowe

Even with counter rotating wheels to offset the angular motion, Gyro effect still applies, because the gyro effect works regardless of spin direction and counter rotation doesn't affect that.

The only affect where counter rotation offsets is to stop torque spin, like with a twin rotor counter rotating helicopter, which can fly without a tail rotor, or on the P38 Lightning, which had so much prop torque that they had to counter rotate the props to make the aircraft more controllable, or on the old Russian Bear Bomber, where they used twin counter rotating props to save stress on the engine hard points on the wing, due to the props being turned at an RPM such that the prop tips were traveling faster than the speed of sound.

Phantoj

https://socrates.berkeley.edu/~fajans.../bicycles.html

explains it all, particularly in the technical paper: https://socrates.berkeley.edu/~fajans...eerBikeAJP.PDF

NOT gyro, mostly.

Dan The Man

It is actually pretty well known. The key is that the fork is free to turn about an axis less than vertical, and the wheel contact point is behind the fork rotation axis.

First of all, what would happen if you just turned the handlebars of a bike while not moving? Nothing right, well a slight change in where the contact point of the wheel is, but almost nothing. Now what happens when you turn the handlebars while riding straight, no leaning or anything? Don't test this. You would fall over away from the side that you turned towards. This is because you are moving the centre of support away from the centre of gravity.

Now to get back to why bikes balance, you need a bit of a visualization ability or comprehension of force vectors. Imagine a front wheel that has some serious rake (angle) and that is straight. The supporting force comes from the contact point between the tire and the pavement. The weight on the wheel is perfectly in line with the supporting force, everything is stable. Now imagine that same wheel when you lean the bike over to the side, say 30 degrees. You can actually try this one at home. Now the contact force is offset from the weight on the wheel; they aren't in line. This produces a resultant moment (torque). Because the wheel has a single degree of freedom (rotation about the head tube), the resultant torque can only create a motion in that direction, so the component of that torque which is in line with the axis of the head tube will cause the wheels and handlebars to turn, towards the direction of lean. This is why rake and offset are important, they determine what component of that tipping moment turns the wheel.

Now lastly, to get back to the first idea, what happens when you suddenly turn the handlebars and wheels, it shifts the centre of support. Now however, since your centre of gravity was already offset, this actually returns the system to stability. So you have a self stabilizing system. The important thing is that it needs motion to be self stabilizing. The slower your speed, the less the impact of the motion of the handlebars has, and at some limiting velocity, the bike becomes unstable, unless you add another feedback mechanism (like a rider balancing or doing a trackstand).

Just read the Scientific American, and it is kind of a misleading article. What they have created is a separate self-stabilizing mechanism that stabilizes a bike, but not one that applies to a normal bike (adding a heavy low mass in front of the handlebars). It does not discount the standard stabilization system of most bikes.

Brian Ratliff

The bike stays upright the same way a ruler balances vertically on your finger. You have to move your finger around to keep the balance point under the center of gravity. Same for a bike. You have to steer the front wheel in minute amounts to keep the balance point of the bike under the center of gravity of the bike and rider. If you want to test this theory, overtighten your headset next time you are out on your bike so the fork doesn't turn freely. It feels like an invisible hand is trying to tip you over. If you actually immobilize your headset, you will not be able to balance at all.

This is wrong. Two giros rotating in opposite directions in the same plane cancel each other out. The gyroscopic effect is just a natural outcome of F=ma. If you take the two giros as a pair, all the accelerations cancel out, so no gyroscopic effect.

JTGraphics

These questions never end with an agreement so I leave with this LOL its funny thats all https://youtu.be/9ewqeheLL_I

bored117 

Wasn't there something about rake for centering as well?

Brian Ratliff

The headtube angle and rake relates to trail, which relates to stability, which is a somewhat different subject. Bicycles used to be made with zero trail. You could ride them, heck, you could ride them fast, but they took some attention to stay under control and you couldn't steer them with your hips, which means riding no-hands was out of the question.

Dan The Man

Also worth noting is that the gyroscopic effect has no preference to keep a bicycle upright, it just tends to keep the bicycle and handlebars at the same angle that it was at before.

Phantoj

Not so fast -- I believe it applies a torque to the wheels about a vertical (yaw) axis as the bike falls over (roll axis); this causes the front wheel to steer in a stabilizing way.

rufvelo

What Ratliff said

AEO

front steering, 4 wheel vehicles need toe-in to be self-centring.
If you don't understand what this means, then throw your car in reverse and turn the steering slightly. You'll end up going in circles as the steering does not self-center.

front steering, 2 wheel vehicles self-center automatically, because your front wheel turns into the lean. IMO, the gyroscopic effects are very minute on bicycles.

thump55

Even if we all could agree, I do not believe such knowledge would have any positive impact on my ability to attract the opposite sex, therefore, I am bailing on this sausage party. Carry on.

carpediemracing 

+1 (no trail, rake necessary; tiny wheels with virtually no gyroscopic effect okay)

I just went through this with the Cat 5 clinic at Bethel, doing a last minute bonus tip for all the 5s on how to do a track stand.

I'm not a scientist, I failed out of Physics. I'm not coordinated at all, I can't do a lot of intramural sports well at all. But I can do a trackstand for a while.

If I had to do a trackstand on a bike that didn't roll, I wouldn't be able to. If I had to do one on a bike where the headset was locked, I wouldn't be able to. But give me a bike that rolls forward and backward and whose front wheel turns a bit and I'll be able to do a trackstand.

My grandfather (who I should point out was a noted scientist) tried to explain to my 6 year old brain but I didn't get it. He put it in a slightly different way - steer towards the side you're falling.

You do that minutely when you ride. You may not notice it when you ride or when you watch other adults ride but it's more exaggerated when you watch a 4 year old ride.

achoo

Are you kidding me? Isn't this article posted about two weeks too late?

Rotating in the opposite direction does not cancel out the effects of conservation of angular momentum, which is why a gyroscope works in the first place. If anything, adding ANOTHER wheel just increases the effects.

One might as well ask the question, "Why can't ANY physical object start and stop instantaneously?"

CONSERVATION OF MOMENTUM! IT'S THE LAW!

Sheesh.

renton

I lean mine against the wall.

Essex

The winters at Cornell/Ithaca are long. Ya' gotta do something besides being on your stationary trainer.