How does a bicycle helmet protect your brain in a crash?

A bicycle helmet protects your brain from injury in a crash through a combination of components designed to handle both direct and rotational impacts.

1. Outer Shell: A thin, tough polycarbonate shell is the first line of defense. Its main job is to protect the inner foam from damage and reduce the risk of sharp objects penetrating the helmet. [E2]

2. Foam Layer: This is the most critical part for absorbing the energy of a direct impact. The foam, typically expanded polystyrene (EPS), works by crushing upon impact. This process spreads the force of the impact over a much longer time, dramatically lowering the peak force that reaches your head and brain. Because the foam is irreversibly crushed in the process, the helmet must be replaced after a single impact. [E2, E5]

3. Inner Layer for Rotational Forces: Many modern helmets include an inner layer designed to reduce injuries caused by the skull rotating during an angled, or oblique, impact. When the skull stops suddenly, the brain can continue to move due to inertia, causing shearing of brain tissue. [E3] These inner layers work in a few ways:

   • MIPS (Multi-directional Impact Protection System): This system uses a slippery layer, often made of Teflon, that allows the outer part of the helmet to slide about 10-15 mm independently of your head, absorbing some of the rotational energy. [E3, E4]
   • Collapsible Lattices (WaveCel): Some helmets use a collapsible plastic lattice structure that can crumble and absorb the energy from an oblique strike, imparting less rotational force to the rider’s head. [E4]