Short Wave - All Tied Up: The Study of Knots

Episode Date: April 29, 2022

Climbing enthusiast and producer Thomas Lu has long wondered what makes knots such a powerful tool. Today, Thomas digs into the research with the help of Matt Berry, Quality Assurance Manager at the outdoor gear company Black Diamond Equipment, and researcher Vishal Patil.Reach the show by emailing [e-mail] pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hello there, world. Aaron Scott here with Shortwave producer Thomas Liu. Howdy, howdy, Aaron. Howdy, Thomas. Hi. Today, I have a story to tell you. Love story, Tom. So there once was a bunny sitting under a tree. He's crying because he can't keep his shoelaces tied.

So Mama Rabbit calls him over to her tree. Across the way, he crisscrosses over. and goes under the bridge to meet her. He gets excited to see her so his ears perk up and he hops around Mama's tree before popping down into their rabbit hole home. Aw, that's so sweet. But, Thomas, that is also the story we tell children

to teach them how to tie their shoes. Are you implying that I don't know how to tie my shoes? Do you, Aaron? I mean, are you really tying your shoes the right way? I think so. The knot doesn't come undone as you go about your business. Oh, yeah. They always come untied.

Then you're likely tying your shoes wrong. I hate to say it. Ouch. But a group of researchers at the University of California, Berkeley, might have figured out why. Basically, they found that a strong knot will hold longer than a weaker knot. Okay, that sounds great, but what exactly makes it not strong or weak? So, Aaron, dear shortwave hosts, that is the question that, and other researchers are trying to understand.

In this specific study, the Berkeley researchers found that when a shoe strikes the ground and the knot of the shoe laces contend with the forces of physics, aka the swinging of the laces as we walk or run, the common square knot is superior over the common granny knot, both of which are knots most of us do when we tie our shoes. Okay, so this I'm actually familiar with, because these are the super basic knots most of us use when tying string or rope together. they look pretty much the same

except the direction in which you cross the strings before tightening them differs. Right, yes. So a quick way to tell which knot you've tied is to look at the bunny ears, Aaron. If they sit parallel to your shoes, you likely tied a granny knot, the less stable knot. And if those bunny ears sit perpendicular to your shoes,

there's a high chance you tied the more stable square knot. You're crushing my soul here, Thomas. My bunny ears are parallel. to my shoes. I have been doing this wrong my entire life. See, I told you you weren't tying your shoes correctly. So today on the show, Aaron, we get a little naughty. And get all tied up in the study of nuts. I'm Thomas Liu. And I'm Aaron Scott and you're listening to Shortwave, the Daily Science Podcast from NPR. Okay, T. Lou, you started this episode by telling me, I tie my shoes wrong.

So I think I've earned the right to ask, what got you even thinking about these existential questions in the first place? That's a fair question. I will give you that. I'm somewhat of a rock climbing hobbyist. And in climbing, something that I notice is that there is a lot of trust placed on the ropes to hold us on mountains or walls. Right.

But there's a lot of trust that the knots won't come undone while folks are out on their adventures as well. Yeah, I've always marveled at what seems like a dire simplicity of rock climbing knots. Like if you cross one way in the knot, it'll save your life. If you cross the other way, let's just hope you don't fall. Isn't that amazing? Yeah. Yeah. Do you find any answers? So yes and no, Aaron.

When I first looked into knots, I realized that knot research can be sectioned off into a few categories. Among them is something called knot theory, which is a mathematical and theoretical look at knots. However, there's also a couple areas of research focused more on applications like DNA structures or surgical sutures in medicine. All of which sounds like it's getting pretty far away from the knots that you are interested in. Yeah, a little bit. Yeah. Yeah, I'm more interested in the everyday knots, like the kind we use in tying our shoes, keeping our running shorts up, sailing, cooking, you name it. But few researchers have really looked at these kinds of knots from a physical engineering perspective.

Well, let's get into it then. What exactly is a knot? Yeah, so let's start with the basics. Knot is actually more of an umbrella term, as Matt Berry tells us. It also covers hitches, which require either an object or another rope that is tied around it or a bend, which is essentially when you're tying two ropes together. Matt, a mechanical engineer and quality assurance manager at the Outdoor Gear Company Black Diving Equestiming. And although it's generally okay to group the three together, he says a knot, a hitch, and a bend are technically three different things. Okay, so this is probably why there are so many different knots.

I mean, you flip through a knotbook and there's just this endless variety of ever more elaborate knots. I love it. Yes, exactly. That's one reason for it. And another reason, Matt tells me, is that there are countless uses for these knots. I mean, you could think of a knot like, you know, a tool in your toolbox. And in climbing, you want something that's extremely strong, but you often need to untie. Or if you're repelling down the mountainside, you might want to be considering some other things like how likely that knot you're tying between the ropes is going to get snagged and put you into a more dangerous situation.

Now, Matt is talking specifically about ropes and knots used in climbing because that's his expertise. But the point stands, if you see... think of knots as tools. Oftentimes, you could use a knot to secure things, to hoist things up, or merely to make a rope longer by tying two together. And there are different knots depending on if it needs to hold a lot of weight or a little bit. And yeah, if you want it to be pretty permanent or you want to be able to undo it really quickly. Aren't they amazing? But get this. Over the course of my reporting, I also learned that the knot is often a rope's weakest point. Wait, stop, You just spent the last few minutes telling me how amazing knots are.

And now you're going to say it's the weakest point. Yeah, I know, I know. But let me back up a bit here. Ropes used in climbing and adventure sports are all safety tested and must meet certain standards set by the manufacturing companies, as well as external organizations. In fact, one of Matt's jobs is to simulate extreme test failures in a lab and quote-unquote misuse scenarios in the field.

And it turns out the knot can be a problem. Not efficiency, which is essentially another way of just expressing the strength of a knot, is basically a proportional strength to a rope in its, if there is no knot in a rope and you pull on it until it fails, and then if you tie a given knot in that rope and pull it again and compare those two data points that would tell you your reduction in strength and your knot efficiency. So, Aaron, if a rope were held in a taut straight line without a knot, the strength of that rope would be a baseline of 100%. But when you add a knot, that adds stress to the rope.

Once that knot is tightened, you have some compression going on on the insides of the bends. You're putting tension on the outsides of those bends. And the less efficient the knot, the more it weakens the rope. And depending on the knot, you could reduce the strength of a rope by more than 50%. Wow, 50% is huge. It is. Like, we're depending on these knots. That seems like a big problem.

I know. Isn't that mind-boggling? But remember, Aaron, these ropes are designed to hold more weight in the field than often is needed. And there's a variety of factors that affect rope strength, like whether it's wet or damaged in any way. Right. That'll make sense to me. But something I'm not yet understanding is what exactly makes a knot strong or weak? Like, how do I choose the right knot for a job?

Good question, and here's the man with the answer. Hi, my name is Vishal Patil. I'm currently a postdoc and Stanford Science Fellow at Stanford University. I think there's still a lot we don't know about why particular knots is strong or weak theoretically. Empirically, we have a lot of data from using these knots from thousands of years. People who do adventure sports, like climbing or those who work with knots in their daily jobs like fishermen, they know these knots well. But there's currently only a handful of researchers studying everyday knot usage.

The shoe-tying study, in fact, is one of the first of its kind. Sounds like science is playing catch-up to the knowledge of people like Matt who work extensively with knots every day. Indeed. So tell me about Vishal's research. Right. When he was a graduate student at MIT, his team had a goal, to figure out a way to come up with a mathematical rule that could be used to predict aspects of a knot's stability. simply by looking at a diagram of the loose knot.

So for this study, they worked with a special color-changing fiber that. When you bend them and stretch them, you can actually see the strain in the fiber. In other words, when you put a knot into this fiber, you can see the tension and stress that are being applied to the knot. That sounds so cool. I'm picturing a hyper-colored knot. It's pretty amazing. And with this visual aid, Vishal and his team created a computer program that demonstrated the effects of stress on a variety of knots,

ranging from a simple granny knot to more complicated ones like the Zeppelin. The Zeppelin. I looked it up and it looks like two pretzels in love. Oh, isn't that adorable? What did Vichelle find? Playing with these simulations and with mathematical properties of these knots,

we came up with some simple rules. The first rule is the number of strands crossing with any knot. They found that generally, the more windings and crossings are in a knot, the more friction that knot has, and the stronger that knot will likely be. Got it. Then, from there, they add on two other rules

to compare knots with the same number of strands crossing, circulation and twist fluctuation. The circulation fluctuation is looking at how the ropes are dragged across each other in the same plane, whereas the twist fluctuation is torque, so it's when one rope imparts a twist on the other. And Aaron, of the three parameters, Vishal stressed the importance of twist fluctuation.

He's said to think of the twist like gears in a machine. If two gears are spinning in the same direction, there is less friction. While if the gears spin in opposite directions, they tend to lock. So in a rope, as you're pulling a knot tight, if the strands at the cross points are twisting in opposite directions, their models seem to suggest that this knot will be less likely to slip. All right, and to pull this around to my lack of shoe tying skills, this seems to complement

and support the idea that a square knot is better for tying your shoes compared to a granny knot. This is true indeed. Well, thank you, Thomas, for schooling us with this little primer on knots and some of the preliminary research that's going on. Of course, happy to get naughty anytime, Aaron. This episode was a lot. produced by Rebecca Ramirez, edited by Stephanie O'Neill, and fact-checked by Margaret Serino.

The audio engineer for this episode was Quasi Lee. Special thanks to the folks at the International Climbing and Mountaineering Federation, UIAA. Giselle Grayson is our senior supervising editor. Andrea Kisick runs the Science Desk. Edith Chapin and Terence Samuel are the executive editors and vice presidents of news. And Nancy Barnes is our senior vice president of news. I'm Thomas Liu. And I'm Aaron Scott.

And you've been listing this. a shortwave, the Daily Science Podcast from NPR.