You see these stories in the news all the time. A bicyclist gets hit by a car and suffers a major head injury. It could have been much worse, we are told, but thankfully he was wearing a helmet. A shattered bike helmet is shown, with the implication that this could have happened to his skull:
While pictures like this are scary and dramatic, they are actually further evidence that bike helmets are ineffective. This helmet did not function in the way it was intended. In fact, it failed spectacularly.
Let’s review the physics of a bike helmet. They are constructed out of polystyrene foam (styrofoam) which is supposed to compress in a collision. This compression spreads the force out and reduces the acceleration of the brain as it smashes into the inside of the skull. At least that is how it is supposed to work — under ideal laboratory conditions where the impact force is perpendicular to the helmet. Actual collisions are chaotic events involving complex interactions. When subject to an oblique impact, the styrofoam will typically crack and break off without compressing. This is what appears to have occurred in the helmet shown above. If there is no compression, then it is unlikely the helmet absorbed the impact.
What is unfortunate about these kinds of news stories is that they give false hope about the effectiveness of bike helmets. And rarely do these stories ever mention the dangerous road infrastructure that caused the crash to happen in the first place.