Scientists find "invisible hand" could be why shoelaces become undone

Pablo Tucker
April 12, 2017

As it loosens, the swinging leg applies an inertial force on the free ends of the laces, which rapidly leads to a failure of the knot in as few as two strides.

Robert Matthews, a physicist at Aston University in Birmingham who was not involved in the study, told the Guardian: 'It's provided hard scientific backing for what many people have long suspected: that the traditional way of tying shoelaces is pretty rubbish'.

The experiment with weights on shoelaces, done to replicate similar forces to walking, found the weak knot untied 100 per cent of the time and the strong knot 53 per cent of the time.

"We looked like insane academics because we were just walking the halls of Berkeley, watching our shoelaces come untied", says team member Christine Gregg, an avid runner.

O'Reilly said he was inspired to investigate after spending decades pondering why laces spontaneously unknot themselves - an intellectual niggle that intensified when he came to teach his daughter how to tie her laces. "As a result, the center of the knot deforms", Gregg and her colleagues wrote in their report, published in the Proceedings of the Royal Society A. But I have one pair of shoes I slip on and off without ever touching the knot in the laces that's been there for many months now.

When we talk about knot morphology, it is not just something which is to be confined to shoelaces.

Click to enlarge: An overhead view of how to tie the weak knot (left) and the strong knot (right).

To tackle the enigma, a trio of mechanical engineers at the University of California at Berkeley filmed a knot - on the shoe of a researcher running on a treadmill - coming undone in extreme slow motion. In a square knot, you start by crossing the lace in your right hand in front of the one in your left hand and then threading it under the left one. Testing different varieties of the typical bow knot in both the weak and strong configurations. They found that the foot strikes the ground at seven times the force of gravity and as the fabric of the shoe squashes down on impact, extra lace is freed at the top of the shoe, causing the knot to loosen slightly with each stride.

To help solve the puzzle, the researchers attached sensors to shoelace knots as study co-author Christine Gregg, a runner, walked and ran.

The researchers also tested their theory that increasing inertial forces on the free ends would trigger runaway failure of the knot.

There's a possibility such work could shed light on the mechanics of other kinds of knotty structures, such as suture knots used in surgery, or the folding of DNA and proteins -and especially how they fail. While almost everybody has stopped many times to tie their shoes, no one has asked the relevant question why these pesky shoelaces become loose in the first place.

"You really need both the impulsive force at the base of the knot and you need the pulling forces of the free ends and the loops", Mr Daily-Diamond explained.

Further experiments demonstrated that simply stomping up and down wasn't enough for a knot to fail; neither was swinging it back and forth.

A SHOE LOOSE Two complementary effects tag team to untie a runner's shoelace in this video, shot with a high-speed camera.

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