Not fit to be tied: Study unravels mystery of weak knots

The scientific evidence is now clear: We tie our shoelaces wrong.

America’s epidemic of untied shoes can be blamed in part on faulty shoe- tying technique, according to rigorous experiments in a newly published study. But there is also a more insidious culprit: The mere act of walking loosens even a beautifully knotted lace and quickly leads to the grim fate the study’s authors call “catastrophic knot failure.”

“What was remarkable to us was how fast it happened,” says study co- author Oliver O’Reilly of the University of California at Berkeley. In treadmill experiments, after several minutes with no apparent footwear malfunction, “it took two strides for the shoelace to untie.”

Through experiments, the team worked out the undoing of a shoelace. The downward spiral begins with the foot’s repeated impact against the ground, which loosens the central knot. Meanwhile, the leg’s swinging causes the lace’s free ends to whip around and gradually slide out of the knot. Finally, one lace end slips free, resulting in “runaway knot failure,” the researchers write in Proceedings of the Royal Society A: Mathematical and Physical Sciences. With laces weighted to mimic the forces of a swinging leg, shoelaces tied the conventional way, using the “bunny ears” technique taught to kids, failed every time. The team labeled this the “weak knot.” The so- called “strong knot” came apart in half of the 15- minute lab trials at the maximum weight. To make a strong knot, cross the left lace over the right and pull it through the resulting loop. Form both the right and the left lace ends into loops and wrap the bottom of the right loop around the bottom of the left.