How do slap bracelets work?


Stupidest question ever, but the only one I’ve thought of all day which still makes me wonder…


Have you ever used one of those roll up tape measures? You can see a very similar behaviour in them if you press in the middle. It’s due to the metal being bent upwards in the middle, causing the bracelet to stiffen, but as soon as you pop the middle the entire thing curls up on itself.

The slap bracelet is known as a blistable spring, which is a spring that has two stable states. In it’s long, straight shape, it is stable, but with a strain on it. It makes it want to bend into a circle.

Deforming it a little bit by applying pressure to one part of it causes that part to roll up, and reduces the strength to maintain its shape, causing the rest to roll up in a chain reaction.