Bike Helmets are designed to lessen impacts to the head of a cyclist in falls while minimizing side effects such as interference with peripheral vision. They are specified to withstand simple falls onto a flat surface without other vehicles being involved.
Bicycle helmets should always be discarded after any accidents.
There are two main types of helmet: hard shell and soft/micro shell (no-shell helmets are now rare). In both types, impact energy is absorbed as a stiff foam liner is crushed, up to the point where the liner is crushed to its minimum thickness, or the helmet shatters, after which no further energy is absorbed.Full Article