Expanded Polystyrene (EPS) Foam Liner
Expanded Polystyrene is a rigid and tough closed-cell foam, used as the helmet’s primary safety mechanism in the event of a crash. On impact, the dense foam deforms and transfers kinetic energy around your head instead of through it. This severely damages the foam and its ability to redirect energy, and consequently, almost all helmets are rated for a single impact use.
Additional Liners:
MIPS
MIPS Brain Protection System separates the shell and liner with a low friction layer. When a helmet is subjected to an angled impact, the low friction layer allows the helmet to slide relative to the head, providing additional protection against the rotational motion. Multiple manufacturers have adopted MIPS as an added level of safety.
Koroyd
Koroyd construction utilizes thousands of extruded co-polymer tubes to create a structure that complements the EPS liner. Upon impact, the tubes collapse in a controlled manner to decelerate energy to your head. Koroyd is used exclusively in Smith helmets.