I keep seeing reference to the idea that bicycles don’t stay up because the wheels are gyroscopes. People even have built bikes with extra wheels that “cancel out the effect.” I thought that spinning objects don’t like to change the direction of their axes, and that more wheels would only add to that effect. I’m finding that the whole thing is much more complicated than I thought, but I don’t understand why oppositely spinning wheels would cancel out the effect. Is this an answer than can be communicated through text? Does anyone have another thing to look at? Putting that term into youtube didn’t find anything.
Is this the specific work being referenced: How do bicycles balance themselves?
My understanding is that the high-school physics claim that bicycles are stable solely because of conservation of angular momentum of the spinning wheels is overstated (i.e. the effect contributes, but it’s not the only reason for stability).
A rotating object will “prefer” to stay rotating around the same axis in the same orientation. The effect on the bike is what happens when you change the direction of the axis anyway. The force resulting in that situation is dependent on the direction of rotation, and opposite for the opposite direction. Check out this video on gyroscopic precession:
Guess nobody’s interested in the question the OP asked. Here’s what I found.
Man! Don’t mess with your bike! You can actually train your brain to not know how to ride a normal one.
Concerning the Veritasium video, that has very little to do with why bicycles stay up, and does not answer my question about canceling out gyroscopes.
Even worse, I have watched many precession videos now, and none of them do a very good job of it actually explaining it. I’m a science teacher, although not a physics teacher, and I still don’t know how it works, because they make glib references to stuff like the torque goes out the axle, and the angular momentum goes points out the axle. These are essentially nonsense as far as I can tell. Clearly there is not a torque out the axle, because a torque is a rotational force. They then want to argue that the torque is in a direction that the wheel does not turn. I get almost angry at these explanations. Even my physics major/teacher brother has spent some time with me and he can’t actually explain what that means. Apparently it’s just a convention that people use because it’s convenient. But if I use what amounts to catchphrases, that don’t really mean anything, I’m only going to get blank stares. As you can tell, I’m a little annoyed by the level of these videos. No one seems to be able to explain exactly why the wheel turns around. I understand that the wheel tilts at 90° to the force being applied to it, but they don’t actually explain then why it turns around the string. They hand wave about the right hand rule and so forth, but don’t want to get into the details. Teaching it to students is hopeless with their explanations.
A spinning gyroscope has (angular) momentum. If you add an oppositely spinning wheel, its angular momentum would be minus that, for a total of no angular momentum. But doing so wouldn’t make a bicycle impossible to ride; I’m pretty sure someone on YouTube has filmed his or herself doing precisely that.
No, the only convention is whether to use the right-hand rule or the left-hand rule. Pick one and use it consistently and you’ll get the same answers either way. But whether there is a torque on-axis vs. off-axis is not a convention–it’s how angular momentum works.
I don’t have an intuitive explanation of how gyroscopic precession works, unfortunately.
There seems to be some confusion about precession vs torque. The axis of rotation of a spinning body moves around even in the absence of any torque:
That’s only true if all of the momentum is on the axis with an intermediate moment of inertia. If it’s spinning on the axis with the highest or least moment, it’s stable. See the tennis raquet effect. Well, mostly: if it’s spinning on the axis with the least moment, and there are energy-dampening forces, it’ll transition to the axis with the highest moment (see the Explorer I satellite for a famous failure).
The torque they’re talking about isn’t the torque that causes the wheels to turn. Imagine, for simplicity, a bike with the rider clamping down on the brakes, so the wheels can’t turn at all. And imagine that the rider is trying to balance on this bike, without putting his foot down. Well, he probably won’t be able to balance perfectly, and will end up leaning slightly one way or the other. And once you start leaning one way, you end up falling and leaning ever further in that direction, until you’re flat on the ground and the bike is sideways. The gravitational force that causes the bike to fall over on its side is the source of the torque that we’re talking about here.
Well, with the brakes locked, the bike will fall over straight sideways: If you were facing north before you fell, then you’d still be facing north after you fell. But now, put a big gyroscope on the bike, where the wheel would be (but with something other than the wheel touching the ground, so you can spin it without driving the bike forward). Spin it quickly, and let the bike start to fall over again. Now, because of the angular momentum of the gyro, the bike won’t fall over straight sideways: It’ll instead twirl on the ground, so the direction it’s facing will move around the compass points. If there’s not enough friction to prevent the twirling, it might not even fall over at all, just keep on twirling. This twirling is precession.
If you added another identical gyro, but spinning he opposite direction, it would also try to make the bike twirl, but in the opposite direction. The two twirling effects would cancel out, and it would just fall straight sideways again.
And this effect is present in a real bicycle, and would in fact help to prevent the bike from falling over. Except that it turns out that it’s a fairly small effect, and that most of the stability of the bike comes from other effects, so that if you do make a bike with no gyroscopic effect at all, it’s still nearly as easy to keep it upright while you’re moving forward.
Ski bike video (start at 3:30 to see the action). Seems to be ridable without any gyroscopic effect. And in this it is not cancelled out, it does not exist. Besides balance it seems a lot has to do with the front wheel being turnable that introduces a torque to offer minor corrections that are amplified due to the fowrard motion.
Though not quite as pure as the ski bike, the kick scooter also demonstrates the effect of minimal gyroscopic effect (due to very low-mass wheels). And to ride one, your bicycle skills transfer readily.
Sure. Though (more of an issue with a Frisbee than a bicycle) a misalignment of the angular velocity and angular momentum can still result in a “wobble”:
In fact, there is a cute story by Feynman where some guy was tossing plates in the cafeteria, and, Feynman being Feynman, he calculated the ratio of the wobble rate to the rate of rotation of the plate.
That Veritasium video is utter shit. Not because it’s wrong as such but because it is useless, as you say.
You can find a video on youtube of Feynman saying the rigid rules taught in school about how to solve algebraic equations are for people to be able to do so with no understanding of what they are doing. That video is a classic of the genre and would have Feynman spinning in his grave (har har).
If we counterspin another Feynman, in the opposite direction, it cancels out and the video is good.
Actually I think if you did this you would discover that its a myth that the video is bad due to the spinning Feynman. The surprising thing is the persistence of the myth despite it being shown experimentally that the video was bad - even if one added a counter spinning Feynman - as early as 1970*.
The video is actually bad due to a combination of factors.
*The ethics of the experiment - first conducted using live Feynmen - could be regarded as questionable. However repetition of the experiment has since become easier due to the availability since 1988 of deceased Feynmen.
If you could locate a single Feynman, you could have him construct ten 1/4 scale Feynmen. The mass required is tiny, of course, so there is no question that such a thing is possible. Now, each of those ten Feynmen can construct ten more Feynmen at 1/16 scale–and so on, until you have a billion of them. At each stage you are using less mass than the previous, so at the final stage they are only a tiny fraction of a percent of a full-size Feynman.
At this point you may pair up the spinning Feynmen however you wish. But there are entropy problems to sort out, I suspect.
Veritaserum, in general, is not very good.
It’s not just the fact that the front wheel accepts steering inputs - for most two-wheeled vehicles, stability is also designed into the steering geometry. Depending on the position of the contact patch relative to the steering axis, you can create a vehicle that’s really stable in a straight line but fights against you when you try to navigate through a turn (like a cruiser motorcycle), or one that is eager to lean over into a turn when you merely think about turning (like a sport/race motorcycle).
I say “most” two-wheeled vehicles because some smart people made a two-wheeled vehicle that lacks the usual steering geometry and also lacks gyro-induced stability, but is nonetheless stable. Instead of relying on steering geometry to make the contact patch induce a corrective torque to the steering assembly when the bike leans, the bike in this video relies on clever placement of mass to induce corrective steering torque when the bike leans: