> It's not the polystyrene which cracks, but the plastic shell holding it in place. The separation will happen after the compression force is removed, because t

> It's not the polystyrene which cracks, but the plastic shell holding it in place. The separation will happen after the compression force is removed, because the compression will keep the split parts close together. Meaning that the helmet falls apart after it's done the job. Nope. As a cyclist, I've seen numerous people in social media groups, friends, etc post pictures of their helmets that "saved their lives." Every single time, it's cracked to pieces, with no visible denting to the polystyrene foam. Deformation of that foam is how a helmet absorbs impact force, and cracking apart is a failure of the helmet. Bike helmets in the US are required to pass one test - a weight being dropped directly on top of the helmet that simulates a detached adult male head falling onto the ground from about the height of an average adult male. The test makes absolutely no sense, because the whole thing is a sham.