Why do shoe laces keep coming undone?
When I was a small boy one of the rites of passage was to be able to demonstrate the ability to tie up one’s own shoe laces. It was a tricky business and required great perseverance and phenomenal powers of concentration. Eventually I cracked it and have never looked back since. These days, with Velcro fastenings and the penchant for wearing trainers without laces it is less of a vital accomplishment and, I’m sure, we will all be the poorer for that.
When you think about it, though, and I have the luxury of being able to, tying shoe laces is a rather odd and inefficient way of making sure that your shoes stay on your feet. Invariably, the laces work loose and at some point in the day you find that you have to bend down and tie them up again. I find round laces the worst and when I buy a pair of shoes, try to avoid them. For the enquiring mind the obvious question is why do shoe laces, however well tied, work loose of their own accord.
Fortunately, some research carried out by Oliver O’Reilly, a professor of mechanical engineering at the University of California Berkley and published in the Proceedings of the Royal Society may provide the clue to understanding this conundrum. As often is the case, the starting point was to take a couple of PhD students who were rich and uncool enough to own a pair of lace ups. They were asked to sit on a chair and swing their legs and stamp their feet. What they found was that these tow actions independently do not cause the laces to loosen. However, it is when you combine these two actions that trouble begins.
A runner was put on a treadmill and their actions were filmed using a slow-motion camera. What the scientists found was that when running, the foot strikes the ground with a force that is seven times that of gravity. As the fabric of the shoe squashes down on impact with terra firma, extra lace is freed at the top of the shoe, causing the knot to loosen imperceptibly. The trailing leg causes the free ends of the laces to move backwards and forwards, resulting in them being tugged outwards. The knot loosens causing a reduction in the friction which is holding the knot in place and eventually the free ends lengthen and the knot unravels.
It doesn’t happen all the time, the scientists say, but once the tension holding the knot decreases as a result of the movement of your feet, you will soon be bending down to tie your laces up again. It seems that some types of knot are more prone to coming undone than others. I use a granny knot but the tests conducted by O’Reilly show that these knots are five times more likely to come undone than a square knot. With a square knot you cross the end that is in your right hand behind the one in your left rather than passing the ends of the bow and knot over each other.
I doubt whether I will be able to obliterate a process that has been hardwired into my subconscious for over half a century. I find double knots help immeasurably but at least I now know why my shoe laces come undone. Perhaps I should invest in some slip ons.