A new study from University California Berkeley showed why shoelaces might keep coming untied. People often wonder why shoelaces keep untying once you tie them, and a group of mechanical engineers has conducted a series of tests to figure it out.

The researchers found that the force of a foot hitting the ground stretches and relaxes the knot, while another force caused by the leg swinging acts on the ends of the laces. They suggested the understanding of how shoelaces work can be applied to other structures such as DNA.

Sport Shoe
“This is the first step towards understanding why certain knots are better than others, which no one has done,” said the study authors. Image credit: Business Insider.

“When you talk about knotted structures, if you can start to understand the lace, then you can implement it to other things, like DNA or microstructures, that fail under dynamic forces,” said Christopher Daily-Diamond, a graduate student at Berkeley and study co-author, according to Berkeley News. 

Shoelaces untie because of inertia forces

The study was published April 11 in the journal Proceedings of the Royal Society A. The researchers point out that there are two ways to tie the shoelace and although one of the ways is stronger than the other, no one understands why. The strong version of the bow tie knot is based in a square knot, which consists of two lace crossings of opposite handedness, placed on top of each other.

The weaker version has a false knot, as both laces have the same handedness, causing the knot to twist when tightened. The investigation found that both versions fail, and it also provided a groundwork for researchers to analyze why two similar structures have different structural integrities.

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Research expect to conduct more research to clarify all the variables involved in the knot failure process. Image: Shutterstock.

“We are trying to understand knots from a mechanics perspective, such as why you can take two strands and connect them in a certain way that can be very strong, but another way of connecting them is very weak,” said Oliver O’Reilly, professor of mechanical engineering at Berkeley, whose lab conducted the study.

O’Reilly says that they were able to show that the weak knot will always fail and the stronger knot will also fail, although after a longer timescale. The purpose of the research was to determine the mechanics of how shoelace bow tie knots come untied under dynamic force. 

To achieve this, the first step involved recording the process of a shoelace untying in slow motion. Christine Gregg, a graduate student and study co-author, tied a pair of running shoes and ran on a treadmill while the team recorded her shoes.

Researchers found that shoelaces untie because when you are running -or walking- one of your foot strikes the ground at seven times the force of gravity, which causes the knot to stretch and then relax in response to that force. When the knot starts to lose, the swinging leg applies inertial force on the free ends of the laces, which consequently leads to a failure of the bow tie knot in as little as two strides after inertia acts on the laces.

Gregg explains that to untie her shoelaces, she only pulls on the free end of a bow tie and it unties. The forces that cause that are not from pulling the free end, but rather from the inertial forces of the leg swinging back and forth. The knot is loosened because the shoe repeatedly hits the ground, and the knot unties.

Some laces are better than others, but they all untie eventually

The footage from Gregg running also helped researchers find that there is a large magnitude of acceleration at the base of the bow tie knot. To further analyze this, the team used an impacting pendulum to swing a shoelace knot and to test different knot mechanics using several types of laces.

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

The team also tested their theory that increasing inertial forces on the ends of the shoelaces would cause runaway failure of the knot. To do so, they added weight to the free ends of the laces of a knot they swung back and forth and found the knots failed at higher rates when the inertial forces on the free ends were increased.

According to Daily-Diamond, you need both the impulsive force at the base of the knot as well as the pulling forces at the free ends of the laces to get knot failure.

Researchers note that not all knots come untied when a person is running or walking. They explain that tight knots require more cycles of impact and leg swinging to come undone. 

Source: Berkeley News