We’ve all been there. You’re walking along, when suddenly your shoelaces come untied for some reason. Cue the awkward moment where you’ve got to find a place to stop, bend down, and nonchalantly re-lace them.
Well, it turns out there’s actually some science behind the annoyance. In a peculiar but equally fascinating study from the University of California, Berkeley, researchers studied the mechanics of a shoelace knot.
"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," said Christopher Daily-Diamond, study co-author and a graduate student at Berkeley, in a statement. "This is the first step toward understanding why certain knots are better than others, which no one has really done."
The researchers performed various experiments with a slow-motion camera, including lacing up a pair of running shoes with a bow knot and running on a treadmill. This enabled them to see how the knot was failing. Here’s what they found.
As you run, your foot hits the floor at seven times the force of gravity. This causes the knot to loosen as it stretches and relaxes. Coupled with the inertia from your swinging leg, the free end of the laces is able to pull the knot apart in as few as two strides once it is loose. Adding weights to the free ends of the knot, the researchers further confirmed this theory as the knot fell apart even more quickly.
"Some laces might be better than others for tying knots, but the fundamental mechanics causing them to fail is the same, we believe," said study co-author Christine Gregg.
So, what can us mere mortals do to prevent our shoelaces untying? Tightly tying the knot will slow the process down, allowing for more swings of the foot. But ultimately, all traditional bow knots run the risk of failing eventually. It appears that it’s just a matter of time until they do.
"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," said Gregg.