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Rigid or Soft Outer Shell
There are many helmets available with Various approval standards such as ECE, SNELL, DOT, etc. All these helmets may look the same, but there is a significant quality difference among these helmets. Some of them are made in such a way, that the minimum standard requirements are also the maximum requirements.
Others are on the contrary made with the standard demands as the minimum requirements! For example, let’s look at two types of helmets to make a comparison for a shock-absorbing test. In this test the helmet is dropped from a certain height with a metal alloy headform to determine impact energy (G) occurring at impact. Also the important ‘HIC’ value (the Head Injury Coefficient) can be determined. The test will be done with two completely different helmets:
Type A: Rigid outer shell with soft density liner (as Arai uses)
Type B: Non-rigid outer shell with hard density liner (as many polycarbonate helmets use)
Either helmet A or B may pass the same standard. But the reason why B may pass is that a non-rigid shell breaks itself and absorbs impact energy in doing so. The remaining energy, which the shell cannot absorb, is then absorbed by the hard density liner, which can also work as part of shell function. Note that the absorption of the non-rigid shell may not be enough to absorb sufficient impact energy if multiple impacts hit the same area, and the harder density liner is not sufficient to absorb the total energy that is passed through. The Type A helmet spreads the force of the impact evenly across the whole of the outer shell area; the soft density liner then easily absorbs the retaining energy that is transferred through the outer shell, even when multiple impacts are applied.
Helmets designed as Type B can save much weight in the shell. However, when we talk about safety helmet shells, there should be another requirement in addition to “light weight” and that is “strong enough”. Although even Type B helmets satisfies penetration tests for shell strength required in some standards, some helmets are constructed in such a way that the outer shell is made partially strong only in those places where the tests impact will occur, in order to pass those tests. As said, there will be many quality levels beyond what is required by the official standards. It is therefore important to know how to identify the better quality helmets. In general two kinds of materials are used for the outer shell of helmets: one is thermo-injected plastic (for instance ABS or Polycarbonate) the other is reinforced resin material better known among the public as ‘fibreglass’. At Arai we only use ‘Super Glassfibre’ (more commonly called Super Fibre) shell material for all our motorcycle helmets.(23)