Nov 24, 2007

Motorcycle Helmets: What is a helmet made of?

A helmet is generally made of layers. The outermost layer is usually a hard material. The best helmets today use composite materials which include fibreglass mixed will all manner of high-tech new-age materials ranging from aramids to carbon fibre. In every case, the new-age material is highly regarded for its strength and resistance to decay under abrasion and to penetration. The lower rung helmets can be fibreglass, polycarbonate or even ABS plastic. You generally get what you pay for in terms of helmet construction. The hard layer usually looks pretty industrial, so it usually covered with a decorative layer of paint and decals to make it look appealing. Any vents and diffusers on top are usually plastic. Point to remember is that many manufacturers still use industrial double-sided tape or glue to attach these bits. In really hot weather, the glue can literally melt off and the vents can fall off when you try to operate them. The hard layer's basic job is to spread an impact over a wider area, absorb all of the abrasion that comes with falling on the road and the rest of the time, to look good. Oh, and it also protects the EPS liner from minor knocks.

Inside the hard layer is a softer layer, made almost invariably of polysterene, technically called EPS, expanded poly styrene. Polystyrene is a normally a hard white plastic, usually seen in disposable coffee cups and ice boxes. However, when polystyrene is heated and a blowing agent is used (used to be CFC based, but is now eco-friendly), the result is a still hard-ish plastic 'foam' called EPS, which looks and feels a lot like thermocol (probably because it is... wikipedia redirect thermocol to polystyrene). In an impact, the EPS liner works more or less on the same principle as an airbag. It absorbs the energy of the impact, using that energy to crack, break, crush, rather than pass it on to your brain. Obviously, the acceleration of the brain inside the cranium is controlled better by this, and you should, if everything goes to plan, walk away with nothing worse than a headache and destroyed EPS to show for it. With the shells becoming harder and harder, it is today hard for the naked, unpracticed eye to tell if the EPS liner is destroyed or now. So, if you've had a fairly impressive knock on the lid, play it safe and retire it.

Still further inside is what everyone calls the comfort liner. This is usually foam (or several layers of foam) topped by terry-cloth or other new-age materials like MCoolMax (absorbs perspiration, but surface stays dry). Most big brand helmets today do the liner in two stages. The first (outer) stage is fixed and made of the hardest of the foams employed. The second (inner) stage is the one that touches your skin and is designed to be removable and washable. In most cases, the two cheek pads and a large skull cap come off (velcro and snap fasteners are the way to do it) to allow you to wash it. Some manufacturers, like Arai, allow you to separate the foam and the cloth itself.

The removable stuff is actually far more useful than most people imagine. I've personally owned two big-brand helmets one of which had removable liners. And I can vouch for the fact that after three years of use, the removable liner lid smelt and felt far, far, far better in fit as well as hygene terms. Also, removable pieces can be replaced, so a year down the line, you have the chance to alter/refresh the fit of your helmet if you need to.

That leaves the three other elements that make up a helmet, the retention mechanism, the visor and the venting. Which, we shall see in detail shortly.

Related links in this series:

Other links on helmets

1 comment:

theslayer said...




Awesome work there. Loads of info and more through external links. Kudos.