Why do shoelaces come untied?
The short answer
Shoelaces can come untied because each step slightly loosens the knot when the shoe hits the ground, while the loops and free ends swing and pull the lace through the knot. The process starts slowly but can finish suddenly.
In 30 seconds
When we walk, an impact reaches the shoe and its knot every time the foot lands. At the same time, the loops and free ends swing back and forth with the movement of the leg.
After many cycles, friction between the laces may no longer be enough to stop their motion. If a free end slips out a little, the next swing can pull out even more lace.
What holds a knot together?
A knot crosses and folds a lace so that parts of the lace press against one another. That contact pressure and friction resist sliding and keep the knot in place.
But a knot is not a solid fastener. Repeated forces can change its shape. When the center of the knot opens or deforms, the friction that resists sliding can become less effective.
What happens while we walk?
One important idea is inertia. Inertia is the tendency of a moving object to keep moving. When the foot changes direction, the loops and free ends do not stop immediately; they tend to continue forward.
Another factor is the impact as the shoe meets the ground. The impact can briefly deform the knot, while the swinging loops and ends move in a direction that opens its center. The free ends can then slide through the knot more easily.
Why can it come undone suddenly?
At first, a free end may move only a tiny amount. But as the free end becomes longer and the loop becomes smaller, the difference in the forces caused by their swinging can grow.
The result is a change from slow slipping to rapid failure. High-speed video in a research study showed knots that changed very little for a while and then came apart within only a few steps.
Does the way we tie it matter?
Yes. The structure of the knot affects how easily it comes undone. One study compared a form based on a square knot, which tends to lie flat, with a form based on a granny knot, which tends to twist when tightened. The latter “weak knot” came undone more readily under repeated, walking-like impacts.
That does not mean a stronger form can never fail. Lace length and material, how tightly the knot is pulled, the shoe, and the way a person walks can all affect the result.
A common misunderstanding
A shoelace does not come undone simply because there is “vibration” while walking. In the experiments in this study, the combination of impact from the ground and the swinging motion of the laces was important to explaining the failure.
The lace usually has not broken either. In many cases, a free end slides through the center of the knot and the knot structure collapses.
One more thing...
Shoelace failure has two recognizable stages: gradual loosening followed by a sudden collapse. This small everyday annoyance is also a mechanics problem involving moving strands, friction, impact, and inertia.