This Is Why Your Shoelaces Are Always Untied

By Charles Choi | April 11, 2017 6:01 pm
shoelaces

(Credit: leolintang/Shutterstock)

Hey, your shoe is untied, and now scientists know why: the combination of foot stomping and leg swinging cause the laces to slip apart.

Yes, a child could have told you this, but there’s a reason scientists gave knots a closer look. Knots are everywhere, from stitches used in surgery to steel cables used in construction. Sailors are familiar with the clove hitch, bowline and cleat hitch. Even DNA is a snarled knot. With knots holding so much together, scientists thought it couldn’t hurt to clearly explain how they come undone.

Mechanical engineers at the University of California at Berkeley became intrigued by knot strength after coming across a TED talk that explained there were two different ways to tie the common shoelace bow tie knot, one of which is stronger than the other. The weaker version is the “granny knot”: take a rope, cross both ends left over right, bring the left end under and out, and repeat. The stronger version is the square knot: instead of repeating the first step, finish the knot by crossing the right end over the left.

“I wear dress shoes, and my shoelaces seem to come untied all the time,” says study lead author Christopher Daily-Diamond. “But when we looked into it, while people knew one of these knots was stronger than the other, the mechanics of why that was remained a mystery.”

To help solve the puzzle, the researchers attached sensors to shoelace knots as study co-author Christine Gregg, a runner, walked and ran. They also repeatedly swung a pendulum arm with a shoelace knot tied on it to better analyze forces knots experienced.

Coming Undone

Slow-motion videos of Gregg running on a treadmill showed the granny knot held together for many strides, but when it only slightly loosened, the knot typically failed catastrophically within as few as two strides. Intriguingly, the weak knot did not untie itself when Gregg’s leg was just swung back and forth, nor when the foot was only stomped repeatedly on the ground. This suggested that knot failure is based on some interplay between the swing and stomp.

The death of a knot. (Credit: Christopher A. Daily-Diamond, Christine E. Gregg and Oliver M. O’Reilly

The undoing of a knot. (Credit: Christopher A. Daily-Diamond, Christine E. Gregg and Oliver M. O’Reilly)

The research team found the repeated impact of shoes on the floor during running loosens the knots. The sensors revealed that during running, feet strike the ground with seven times the force of gravity, causing knots to deform.

The whipping motions of the free ends of the laces caused by swinging legs then led the laces to slip, eventually leading to runaway unraveling of the knot, the researchers said. In line with this theory, adding weights to the free ends of the laces, which increased the pulling those ends experienced as they swung, led knots to fail more often. Daily-Diamond, Gregg and senior author Oliver O’Reilly detailed their findings online April 12 in the journal Proceedings of the Royal Society A. 

Why Study Knots?

Daily-Diamond noted that researchers are today building microscopic structures made of DNA and other molecules. “DNA origami,” for example, could yield everything from advanced microchips to smart medicines. “These can be incredibly complicated knotted structures that are subject to a variety of forces, so if you want to start building these structures, you’ll want to know how they can become untied,” Daily-Diamond said.

Future research can perhaps track how shoelaces move during walking and running to unravel the mystery of why square knots are stronger than granny knots once and for all, Daily-Diamond said. Computer simulations of how these knots work could also disentangle the complex role friction likely plays, he added.

Until then, be sure to stick with the square knot.

CATEGORIZED UNDER: Space & Physics, top posts
MORE ABOUT: physics
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  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    Define a shoe’s long axis as heel-to-toe. After wearing your shoes for a while, look down. If the bow is parallel to the long axis you tied a granny knot (odd parity) and it will unravel. If the bow is normal to the long axis you tied a square knot (even parity) and it is secure.

    …1) If you tie granny granny knots, instead tie the first overhand knot in the other wrapping direction to obtain square knots.
    …2) Given (1), when you tie the bow throw two bights over your thumb not one, then pull through the tunnel and tighten. This bow is exceptionally stable, however…if you pull it open and catch one end of the shoelace through a bow rather than around both, it will be a pisser to loosen and untie.

  • OWilson

    Why do shoelaces always come undone, but earphone buds always knot?

    Why can you never hook a coat hanger over a car lock button, but you can snag a garment on any tiny protrusion?

    So many questions, so little time!

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    • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

      DOI:10.1073/pnas.0611320104 for earphone buds.

  • Matthew McDaid

    My shoes are always tied correctly with a square knot rather than a granny, yet I am plagued these days as never before with shoes coming untied. The reason? I’m convinced that the lace manufacturers are at fault. They have changed the specifications on the material from which they are made. The newer materials don’t have a sufficient coefficient of sliding friction. Tie them the same way you always have and even if you’re tying them correctly, they come undone. This will come to an end when someone is seriously injured by a loose shoelace and they sue the manufacturer for faulty material.

    • stluyjuy

      Are you wearing jeans more than you did when your shoe laces stayed tied. My shoes always stayed tied until I started wearing Levi’s. Now it is a constant thing to keep them tied.

    • OWilson

      If they would use ear bud material, problem solved.

      (Except how to untie them!) :)

  • Kold

    After watching that Ted talk years ago my laces were good to go. I had been doing it wrong for 35 years.

  • MaggieMagoo2

    oh please…Grannies have known for years that to tie a shoelace all you have to do is make the rabbit duck through twice. After you tie your regular shoe lace but –before you pull it tight—-stop! Duck that “bunny/bow” through the rabbit hole a second time. Then tighten. Your shoelace will stay firmly tied and yet magically it will untie with the same simple pull on one of the loose ends. Yes, I see the scientific reason for knowing all this. Grannies aren’t anti-science, they’re actually rather strong on innovation.

  • okiejoe

    Velcro is a wonderful thing.

  • patches604

    I had three daughters and one son. When my daughters were young, my wife would tie their dresses in the back with a “granny bow know”. Not only did eventually come untied it would also sort of angle upward and downward no matter how much my wife would try and straighten it out. So, the Boy Scout in me, saved the day. I taught her how to make a “square bow knot”. Problem solved

  • Lorie Franceschi

    Two things…the little plastic ends that help us thread the laces through the holes of our shoes are the “added” weight that helps untie the lace and the biggest reason, is soon they will tell us we have been walk wrong ever since we began walking on two of or appendages. So it will become our fault our shoe laces come untied.

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