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Structure of helmets

Normally helmets are composed of outer shells, inner shell(EPS) and liner.
Generally speaking, black EPS is more durable and energy absportion than white EPS.
PC shell is lightweight and bette than ABS shell, but more expensive.


Why we have to use a helmet?

Helmets are useful as safety gear to prevent injuries in an uncontrolled environment. If you can't prevent a crash or impact, but you know it will occur, a helmet can prevent or minimize injury to the head and brain.

We can not emphasize enough that the first step in preventing injury is to minimize the probability of crashing or being impacted. Not hitting something hard is infinitely better than hitting with a helmet on! 

Safety always involves compromises. A helmet that could protect completely against every impact might be huge. A strong strap that keeps the helmet in place in a crash may strangle a child catching that helmet in the monkeybars on a playground. Helmets are designed to keep expected impacts within the range of human brain tolerance. But what if the brain is in a senior citizen and has become more brittle and less flexible, or what if it has already been injured in the past and is easier to re-injure? Or what if it is just more fragile than the average due to hereditary factors? Current helmet standards do not even attempt to address this problem. Concussions are acceptable as long as catastrophic injuries are reduced. Eliminating the crash or eliminating hard objects in the crash environment may be a more effective means of addressing a head injury problem than wearing a helmet. The benefits of that approach extend to those other parts of the body that a helmet does not even attempt to protect.

How do helmets work?

Human brains can be injured by impact, of course, or by exceptionally violent rotation of the head, when the brain remains stationary, giving blood vessels and nerves a yank. Internal blood vessels and nerves yank parts of the brain around too in different ways, straining the vessels and nerves in the process.

Helmets designed to handle major crash energy generally contain a layer of crushable foam. When you crash and hit a hard surface, the foam part of a helmet crushes, controlling the crash energy and extending your head's stopping time by about six thousandths of a second (6 ms) to reduce the peak impact to the brain. Rotational forces and internal strains are likely to be reduced by the crushing.

 

Somewhere about half way up that spike is where permanent brain damage begins.

Thicker foam is better, giving your head more room and milliseconds to stop. If the foam is 15mm thick it obviously has to stop you in half the distance of a 30mm thick foam. Basic laws of physics result in more force to the brain if the stopping distance is shorter, whatever the "miracle" foam may be. Less dense foam can be better as well, since it can crush in a lesser impact, but it has to be thicker in order to avoid crushing down and "bottoming out" in a harder impact. The ideal "rate sensitive" foam would tune itself for the impact, stiffening up for a hard one and yielding more in a more moderate hit.