It’s not the knot

Why do shoelaces with a circular cross section come undone more easily than those with a flatter cross section? ■  The thing that holds a shoelace in place is friction. Lots of friction means the lace is difficult to undo. Little friction and the lace can slide undone as you walk along. If you rub two wooden rods of circular cross section together, there is very little friction because the point of contact between the rods is small. Compare this with rubbing two flat, wooden slats against each other. There is much more friction because there is a much greater contact area. The same principle applies to shoelaces. Added to this, flat laces have an extra degree of freedom in that they can warp across  their width, in effect wrapping around other bits of shoelace and increasing friction even more. David Muir Science department, Portobello High School, Edinburgh, UK ■  I would suggest the problem of shoelaces coming undone has less to do with the cross section of the laces and more to do with being tied incorrectly. The standard shoelace knot can be described as a doubly slipped reef knot. It comprises a reef knot in which both loose ends are fed back on themselves to create a method of rapidly undoing the knot. The crucial point in tying a reef knot is that the two overhand knots of which it is made are of opposite handedness – left over right, then right over left. If the handedness of each overhand knot is the same, then one ends up with a similarly doubly slipped granny knot. Such knots have a tendency to come undone more easily than a correctly tied shoelace knot. The way to tell if you’re tying a granny knot or a correct shoelace knot is to see how the laces and the loops lie. With a shoelace knot, they will naturally lie across both sides of the shoe. With a granny knot they will lie perpendicular to the tied lace, “If you have a granny habit, the way to correct this is to reverse the initial twist of your shoelaces” pointing up your leg and to your toes. If you have a granny habit, the easiest way of correcting this is to simply reverse the handedness of the initial twist of your laces. The tying of one’s shoelaces is discussed in wonderful detail on Ian’s Shoelace Site, along with helpful pictures (at bit.ly/ TheGrannyKnot). My ongoing straw poll would suggest that about half of people tie their shoes with a granny knot and that problems of them coming undone – be the cross section round or flat – invariably disappear once they start tying them correctly. Henry Gomersall Oxford, UK