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Some more details on whip crack physics (Read 813 times)
erricrice
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Some more details on whip crack physics
Jan 11th, 2025 at 11:01pm
 
Over the holidays I was playing around with different crackers on my slings and ended up going down a bit of a rabbit hole with regard to the physics of a whip crack.

Turns out, it is NOT the tip reaching the speed of sound that creates the crack. It is the loop traveling along the cord reaching the speed of sound that creates the crack. At the point of the crack, the tip is actually traveling 2x the speed of sound.

Some folks in 2002 were confused by the 2x tip speed phenomena so they did some research and it was published in scientific american. A very abbreviated version is here: https://www.scientificamerican.com/article/true-cause-of-whips-crack/ but I've also attached the original paper for any other physics nerds that might want to dig through it. They key info is:

Quote:
Previous whip work (one of just three papers on the subject in the past century) had resulted in the puzzling observation that the sonic boom occurs when the tip of the whip is traveling at about twice the speed of sound. But if the tip were truly the cause of the crack, why wasn't the sound heard earlier, when the tip first reached the speed of sound? Goriely and McMillen's calculations have revealed the answer. "The crack of a whip comes from a loop traveling along the whip, gaining speed until it reaches the speed of sound and creates a sonic boom," Goriely says. He notes that even though some parts of the whip travel at greater speeds, "it is the loop itself that generates the sonic boom."


This revelation immediately explained some slinging behavior I had been seeing and led me to another couple of conclusions, so I wanted to share and discuss here. Based on this data and my own experience, these are the things I believe to be true about whip cracks in the slinging world. Certainly up for discussion, I'd like to know what you guys think!

  1. To start, your cracker(or release cord) needs to be long enough to create a clean loop, and long enough to accelerate that loop to the speed of sound. No matter how fast you sling, if it can't create a loop, it can't crack. I imagine defining exactly how long is "long enough" has something to do with the diameter and stiffness of the cord, but that goes beyond my knowledge. Anecdotally, I've heard that longer crackers make for easier cracking, but I haven't tested it.
  2. If you have a separate cracker attached to the end of your release cord, you want the tail of the cracker to exit the attachment point(typically a knot of some kind) paralell to the release cord to help with a clean transfer of energy the same direction so the loop can continue from the release cord to the cracker without interruption. This means simple knots like overhand or constrictor are no good for attaching a cracker. You want something like a common whipping so the cracker exits the same direction the release cord was already traveling. Image below to help illustrate.
  3. As has been discussed here many times, a knot in the cracker may prevent the crack. But apparently for a different reason than we thought. From prior discussions I've seen here, it seems like the generally understood reason for this is because the knot is heavier and thus absorbs the energy necessary to get the tip of the cracker to reach the speed of sound. This may be part of it, but based on this new information, the primary reason is more likely that the knot interrupts the clean motion of the loop traveling down the cord and thus prevents the loop itself from reaching the speed of sound.
    1. I did a simple test to prove this. I tied a knot right at the end of my cracker, and it still cracked(photo below). It seems a knot at the end of the cord doesn't prevent the crack as long as the cracker BEFORE the knot is still long enough for the loop to fully form and have enough length to accelerate to the speed of sound. I wonder if this might be a way to prevent degradation of the ends of release cords? Since it would prevent the tip from traveling 2x the speed of sound by adding weight, but would still allow a crack to occur. This will need more testing.
    2. This also explains why balearics crack so well. They have no knots or changes in stiffness to interrupt the clean motion of the loop traveling down the length of the cord.
  4. Similarly, your pouch design can prevent a crack if it has a very stiff connection to the release cord. This will prevent the smooth unrolling of the pouch and release cord and thus create no traveling loop to crack at the end. More details in my post further down here.
  5. This basically confirms the suspicion I had posted here previously that a properly aligned rifling spin release is a large factor in creating a crack in the first place. If a loop traveling down the cord is required for a crack, then realistically the only way to accomplish that is with the pouch perpendicular to the direction of travel and the projectile slipping "forward" out of the pouch, causing the pouch to be rolled outwards and form the loop that travels down the length of the release cord.
    1. By way of example, think about doing the opposite: supinating a full 90 degrees so your palm is facing up during release(assuming a sidearm release). The pouch would be "in the way" of the projectile and you would get a lot of spin causing the projectile to curve left. In this case, the pouch and release cord would not "unfurl" at all and would form no loop. It would swing outwards away from you like a solid rod with the pouch as the fulcrum. It might still make a loud "whoosh", but it would not have the correct motion to create a crack at all no matter how hard you threw.
  6. This also makes me wonder if perfectly round projectiles would whipcrack more easily than jagged/angled projectiles because it would help form a cleaner loop on release. I feel like this would probably be very hard to test so I'm probably not going to bother - not to mention I've gotten plenty of whipcrack on oddly shaped rocks, so if there is any effect it is probably minimal.
  7. And a couple of things unchanged by this new information, just for posterity:
    1. Heavier slings will tend to crack more than lighter ones due to the greater energy they have to impart to the cracker. Some slings can crack even without a projectile because they have enough energy from the weight of sling itself.
    2. Heavier projectiles tend to create a crack more easily than light ones. They're still imparting more energy to the cord, speeding up the loop more than a light projectile would. This is assuming you have enough strength to accelerate the heavier projectile enough to still create a crack in the first place.
    3. A tapered release cord still helps with this effect since it will allow the loop to accelerate more quickly.
    4. I believe a heavier and/or larger diameter cracker will lead to a louder crack, but would be harder to obtain the crack in the first place(needs more energy). I'm not 100% sure about this, but it seems logical.
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« Last Edit: Jan 23rd, 2025 at 10:51pm by erricrice »  
 
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Re: Some more details on whip crack physics
Reply #1 - Jan 12th, 2025 at 2:11pm
 
Another interesting thing I discovered in some testing that I think others have stated previously: dyneema REALLY does not want to crack. Even very thin/light dyneema(2mm) won't give me a full crack with the same sling/ammo that 90lb paracord will crack quite well with. The 90lb paracord has a similar weight to the 2mm dyneema per length, but is much weaker, thinner, denser, and more supple).

Maybe this has something to do with dyneema's much greater strength/stiffness when compared to its weight. Maybe it doesn't have enough weight to overcome its stiffness and create a smooth traveling loop in the first place. So with this in mind, maybe the ideal cracking material would be both heavy AND supple? Or possibly the crack has more to do with the diameter than weight, assuming similar stiffness. Again, this will require some more testing.
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Re: Some more details on whip crack physics
Reply #2 - Jan 12th, 2025 at 3:35pm
 
That's a great write up, I'll have to look more in detail later on.

Regarding dyneema vs paracord. I think suppleness of the cracker is quite important. At least on slings I've made, supple materials crack much louder and more easily.

I've made quite a few polysteel balearics and at least intially they do not crack very easily, I have put this down to polysteel being quite stiff and solid.

I've made several whips in the past too, the smoothness of the taper is always considered important in whip making videos/content I've seen.
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Re: Some more details on whip crack physics
Reply #3 - Jan 12th, 2025 at 9:33pm
 
Ahh good to know! I'll have to play around with some other flexible materials for crackers and see how they do.
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Re: Some more details on whip crack physics
Reply #4 - Jan 16th, 2025 at 12:34pm
 
Great work!
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Re: Some more details on whip crack physics
Reply #5 - Jan 19th, 2025 at 8:28am
 
Good post.  Very enlightening.
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erricrice
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Re: Some more details on whip crack physics
Reply #6 - Jan 23rd, 2025 at 10:47pm
 
Ooo I have a significant update! Story time.

I was disappointed that my new hiking sling wasn't cracking when I threw rocks. It worked ok for tennis balls(not great, but it would give a little crack), but as soon as I switched to rocks, nothing. I threw hundreds of rocks trying different throwing techniques(balearic, byzantine, greek), different wrist angles, different strengths and couldn't get it to crack a single time.

So after an insane amount of testing different factors, I ended up isolating it down to the pouch design preventing the crack. It's due to the same effect Acroballistics describes here in his LP-4 video about the smooth unrolling of the pouch and pouch/cord connection(also see attached gif of his demonstration): https://youtu.be/rKbaOivX85w?t=78

Acro covers it in his video due to accuracy and release feel, but when combined with my original info in this thread about what actually causes a whipcrack, it becomes fairly obvious what the problem is. If the pouch and pouch/cord connection can't smoothly unroll, it won't create a traveling loop on the release cord, and thus there will be no loop at the cracker to crack. To reiterate from above, no matter how fast you swing it, it cannot crack unless there is a traveling loop.

Back to my story: the reason my hiking sling won't crack at all with rocks(and only barely does with tennis balls) is because it is a standard seatbelt sling design with a big honkin' stiff nail knot(highlighted in my photo below) that holds the pouch to the release cord. This creates a long(nearly two inches), incredibly stiff point in the release cord which interrupts the smooth unrolling of the cord and thus creates no loop to crack further down. The only reason tennis balls would crack slightly is because their diameter is so large that the knot was touching the side of the tennis ball and thus the tennis ball would impart some energy to the knot and some to the release cord directly, enough to get a weak crack.

Aaaanyways, I confirmed this by testing the exact same cracker on a couple of different slings(literally, the same piece of cord untied and retied to a new sling, pictured below). These slings are all exactly the same length(28"), same weight (14-16g) and all use the exact same brand paracord(with the exception of the dyneema sling of course). The only difference is in the various pouch designs, and they each behaved predictably given this new information.

  1. The seatbelt sling we've covered.
  2. The sheet bend sling ("good" in the photo) has a bit of a stiff spot because of the sheet bend, but cracks much better than the seatbelt sling since that knot is much smaller than the nail knot.
  3. The pouch with grommets was next, and I think is better than the sheet bend because the loose bowline around the grommet allows the release cord to actually rotate freely inside the grommet. I believe this negates the small stiffness from the grommet and allows the motion to transfer more freely to the release cord and make a better traveling loop for the crack.
  4. And best is the nearly-knotless which of course has no stiffness at all because there are no knots and creates the smoothest traveling loop.


I'll add this property to my list above, but this is definitely something to consider if you're looking to get a good crack!
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AcroPouchUnroll.gif (2605 KB | 15 )
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Re: Some more details on whip crack physics
Reply #7 - Jan 25th, 2025 at 5:45pm
 
I will say one thing though, the "Bad" image sling you showed has cracked quite loudly for me many times. And with the dyneema vs paracord, I have used uhmwpe before however not the dyneema brand, and I have found that that has cracked the best for me, almost to the point where I can't sling long because my ears start ringing. So it could be differences between brands that causes the difference in cracking, not the material.
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erricrice
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Re: Some more details on whip crack physics
Reply #8 - Jan 25th, 2025 at 9:05pm
 
Ahh interesting! I'm sure there are some differences in throwing style that would account for some of that. But a question for you - what size rocks are you slinging with that seatbelt sling, and how large is the pouch relative to the rock?
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Re: Some more details on whip crack physics
Reply #9 - Jan 26th, 2025 at 12:37am
 
50g to 120g, the lighter ones deff crack more. And it's a 6 inch pouch
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erricrice
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Re: Some more details on whip crack physics
Reply #10 - Jan 29th, 2025 at 1:49pm
 
Interesting. Mine is a 7 inch pouch (including the nail knot), but I wonder if it also has to do with the flexibility of the pouch material. My 1" nylon webbing(the black stuff in my photo) is incredibly flexible so when there is a rock in there, it's folded up almost completely flat. Whereas the orange/silver 2" webbing is very stiff, so it doesn't folder over nearly as much. Maybe the webbing being so flexible/light doesn't have enough energy to overcome the heavy nail knot and continue the cracking loop? It's also very slippery, so maybe it doesn't get as much energy from the friction of the rock rolling out compared to a rougher webbing? Or maybe it's a combination of all these factors?

Hmm. More to figure out here. All of this could be moot since it might be related to my technique somehow.

@GT is your seatbelt sling webbing fairly stiff, or very thin/light/flexible?

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Re: Some more details on whip crack physics
Reply #11 - Feb 4th, 2025 at 3:35pm
 
hey, sorry I did not see this lol. It's pretty stiff, the one I got from mersa is more flexible and it still cracks, albiet less so, I'm starting to wonder if it is a difference in technique more than materials.
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erricrice
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Re: Some more details on whip crack physics
Reply #12 - Feb 4th, 2025 at 4:12pm
 
Oh yeah there is almost certainly a component of technique here. Likely my particular release is contributing to my results, but I haven't had a chance to meet up with others a do a control with the same sling quite yet.

Interesting though that your more flexible seatbelt design cracks a bit less than the stiff one. I do wonder if the super high flexibility of my nylon webbing(it folds over like paper under its own weight) is leading to my results. Maybe it's not as much absolute stiffness vs flexibility that leads to a weaker crack, but any drastic change from one to the other through the pouch and release cord that interrupts a loop from forming and traveling down the length cleanly.
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Re: Some more details on whip crack physics
Reply #13 - Feb 5th, 2025 at 3:25am
 
that would 100% make sense, I need to get out and sling, It's just been garbage weather lately
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