I was wondering if anyone knows why exactly a yoyo “turns over” in mid air during a loop. And why exactly is it impossible to loop with a butterfly shaped yoyo?
Everyone agrees that there are two types of yoyos - butterfly and and disc. And that discs are great for looping and butterflies are for string tricks. But why is that so? I mean I understand that wider yoyos catch the string more easily - so does that mean that with perfect technique, we can do all string tricks with a disc yoyo like say the Yomega Raider?
Similarly, why are looping tricks impossible with a butterfly shape? Is it theoretically possible to do them with a butterfly, or is there some physics reason around the shape that makes it impossible?
Looping with a butterfly shape is certainly not impossible, just a little harder. This is because the moment of inertia around the axis perpendicular to the axle is what counts for flipping during looping. Butterfly shape tends to push it out more from that axis around which it needs to rotate (flip).
For solid wood and lightweight plastic butterfly shapes (like the Duncan Butterfly) the weight is more evenly distributed (because of the density of the material) so it’s pretty manageable for looping to work no matter what the shape, but of course you get less spin time than if it was rim weighted (which is fine).
The yoyo turns over because it is much more stable for the yoyo to unwind when it is spinning in the correct direction. Imagine that the yoyo is rolling along a physical surface in the shape of the loop, so that it rolls out along the underside of the surface, curves up at the end of the loop, and then rolls back toward you along the top of the surface. This is the direction you want the yoyo to be spinning while it is unwinding or winding back up, which also happens to be the way the yoyo spins when you throw a forward pass. You can test this yourself by taking a looping yoyo and trying to throw a forward pass or breakaway with the yoyo spinning in the wrong direction (i.e. turn the yoyo over in your hand before you throw it so it unwinds opposite how it normally does).
The problem is when you regenerate the spin to do the loop, it reverses the direction of the spin. So for the next loop, the yoyo would be spinning in the unstable direction. The yoyo flipping over as you regenerate the spin corrects that and keeps it spinning in the right direction. Like FiveIronBrian was saying, butterfly yoyos are more resistant to tilting over by design, so it is harder to get them to flip over while looping. For the same reason, 2A tricks that don’t involve looping, such as Shoot the Moon, can actually be easier with a butterfly yoyo. Similarly, adding a loop at the end of a trick like Braintwister or Split the Atom is easier with a butterfly yoyo because the yoyo starts off spinning in the “wrong” direction for a loop, so when you regenerate the spin for the loop, you don’t want it to flip over.
It is also possible to loop without the yoyo flipping over, it is just really hard to control and there isn’t really any benefit of doing it that way unless for some reason you want to try to loop with a butterfly yoyo. Depending on the particular yoyo, looping might be easier without flipping over for some butterfly yoyos (but in general those yoyos will not be good at looping and were not designed for it).
The Yomega Raider (and other looping yoyos) would be terrible for modern string tricks for several reasons. The shape making string catches harder is one, but the response system, gap width, weight/weight distribution, and stability of the yoyo would all make the stuff today’s top string trick players are doing prohibitively difficult. Theoretically it would be possible to land the string hits with that small a gap, but that along with all the other factors would severely limit what anyone could practically do with it compared to modern string trick yoyos.
Interesting. So because the moment of inertia of a butterfly is larger (thanks to the flared shape), it’s more difficult for it to turn. Got it.
But why does it turn in the first place? In this series of photographs from yoyo wikia, you can see the yoyo begin to turn in step 9: Loop the Loop | YoYo Wiki | Fandom
Is it because of the direction of the string change so the yoyo will finally end up below the string instead of above it?
To be clear, not the usual moment of inertia around the axle people usually talk about, but rather around the axis perpendicular. They are related though.
I think it turns because of the flicking motion, the fact that it’s not really spinning around the axle when it’s flipping, and because it’s tilted already to assist (at least it should be). Not sure I have a good explanation but those are contributing factors.
Yossarian hit the nail in the head, but for the sake of simplicity, I’ll re-explain it to you.
If you hold a responsive yoyo in front of you, let’s say you’re holding it with the string facing properly, palm facing down, and just let it fall, the yoyo will spin backward… when the yoyo return and you catch it, the string will roll the opposite way (backward) because the yoyo only spins one way. If you let it fall one more time, the yoyo will spin forward and when you catch it, the string will roll forward too.
Now since loops “revolve” around your hand in one way/direction (outward from bottom, return from the top), the yoyo needs to flip to continue the revolution. It’s not impossible to loop with no flipping, BUT it will have to spin forwards and backwards alternatively, and that is more difficult than simply flips.
A butterfly yoyo will be harder to flip because the wall of the yoyo is far from the rims, which means it’s too “stable” to flip, it’s like pushing a door closer to the joints, it will feel heavier. Try doing trapeze with a classic shaped yoyo it can be tilted a lot easier than a butterfly shaped yoyo.
A butterfly shaped Yoyo is wider and has a different center of gravity - farther away from the axis (center), thus it resists flipping. A looping yoyo is opposite. The weight is close to the center of gravity making it easier to flip. If you draw a circle from where the majority of the weight is (like if it was flipping from a loop), the butterfly will have farther to travel. More distance means more difficulty flipping. Move the weight towards the center and you have a smaller circle and easier time flipping.
Thanks for that. Though I’m not sure the spin direction of the yoyo changes because it flips around the string axis - I could be wrong though. You can see the high speed camera shots of the different stages of the loop here: Loop the Loop | YoYo Wiki | Fandom
I think in all shots, the yoyo is always rotating clockwise both before and after the “flip” which takes place at step 9. Am I correct?
Edit: Ah no, I think you’re right. The yoyo spin direction changes in step 8 when it gets pushed out again, but before it’s flipped. Is that’s what’s happening?