Why Do Shoelaces Always Come Untied? It's Simple "Knot Mechanics"

Mechanical engineers at UC Berkeley are doing you a huge favor: They just completed a study on why your darn shoelaces keep coming untied. While they couldn’t identify every factor that causes the unraveling — or how to prevent it, beyond the “tie ‘em tighter!” axiom — they did uncover some fascinating information about the process.

The study, titled “The roles of impact and inertia in the failure of a shoelace knot,” which will be published on Wednesday in the journal Proceedings of the Royal Society A, concludes that the invisible forces of physics act upon your laces in exactly the same way your hand does when you untie them. While different types of knots have varying levels of effectiveness, these researchers believe that they all fail the same way:

First, the repeated impact of the shoe on the floor during walking serves to loosen the knot. Then, the whipping motions of the free ends of the laces caused by the leg swing produce slipping of the laces.

The study had two components. The researchers observed a runner’s shoelaces while she jogged on a treadmill; they also simulated the collision of a shoe hitting the ground by attaching a tied lace to an impacting pendulum. They filmed these experiments with a slow-motion camera and shared the footage with the public. Check out the physics in action below.

Here’s how the untying process works: The force of your shoe hitting the ground is seven times stronger than gravity, and it causes your knot to stretch and relax with each impact. At the same time, the force of your swinging legs (called “inertial force” because it resists the change in velocity) acts upon the ends and loops of your laces, pulling them like a hand.

You have probably noticed that your laces fail quickly; one moment, they’re tightly tied, and the next, you’re at risk of tripping. This study is here to validate your frustrations: It notes that laces can untie within mere seconds. “The interesting thing about this mechanism is that your laces can be fine for a really long time, and it’s not until you get one little bit of motion to cause loosening that starts this avalanche effect leading to knot failure,” says study co-author Christine Gregg.

“Some laces might be better than others for tying knots, but the fundamental mechanics causing them to fail is the same, we believe,” Gregg says.

Of course, sometimes laces don’t come untied. Depending on the knot, how tightly you’ve tied it, and other factors, you may finish walking or running before completing the number of strides it’d take to loosen the ties. The researchers weren’t able to identify the catalyst of that moment when the unraveling snowballs and you notice the laces coming undone.

Let’s be real. Understanding why our shoelaces come untied will help solve a minor day-to-day frustration, but it won’t change the world, a fact the researchers acknowledge:

The study is more than an example of science answering a seemingly obvious question. A better understanding of knot mechanics is needed for sharper insight into how knotted structures fail under a variety of forces.

However, the research from this study can help shed light on the physics that act upon other knots — even DNA.

“When you talk about knotted structures, if you can start to understand the shoelace, then you can apply it to other things, like DNA or microstructures, that fail under dynamic forces,” says co-author Christopher Daily-Diamond. “This is the first step toward understanding why certain knots are better than others, which no one has really done.”

Abstract:

The accidental untying of a shoelace while walking often occurs without warning. In this paper, we discuss the series of events that lead to a shoelace knot becoming untied. First, the repeated impact of the shoe on the floor during walking serves to loosen the knot. Then, the whipping motions of the free ends of the laces caused by the leg swing produce slipping of the laces. This leads to eventual runaway untangling of the knot. As demonstrated using slow-motion video footage and a series of experiments, the failure of the knot happens in a matter of seconds, often without warning, and is catastrophic. The controlled experiments showed that increasing inertial effects of the swinging laces leads to increased rate of knot untying, that the directions of the impact and swing influence the rate of failure, and that the knot structure has a profound influence on a knot’s tendency to untie under cyclic impact loading.