Shell. Not just for turtles. The shell is the first line of defense, designed to absorb and spread impact energy. Two basic constructions are used: fiberglass and polycarbonate. More specifically, fiberglass shells can be made from any fiber-reinforced material, including fiberglass, carbon fiber, or aramid. Most use a cross section of materials, strategically placed to exploit their strengths. The different materials are laid down in a certain order, resulting in a shell that’s thin yet strong. Areas more or less likely to sustain impacts in a crash are made thicker or thinner by adding or subtracting layers in those spots.
Composite shells are labor intensive, which is why less expensive helmets tend to be made from cast polycarbonate plastic. These shells are typically thicker than composite shells, so you either have a larger helmet for any given size or compromise on the density of the liner.
The shell can usually sustain small impacts, such as falling off the seat when you’re parked, without impairing its ability to protect you. A small chip or scratch won’t affect it. Anything more than a very minor drop can do invisible damage, so it’s best to send your helmet in to the manufacturer for inspection before you trust it again. Any hit hard enough to crack the shell signals the end of the helmet’s useful life.
EPS Liner. Skydivers say it isn’t the fall that kills you; it’s the sudden stop at the end. The same goes for your head. When a helmet takes a hit, the head inside it keeps going until something stops it. The liner, made of EPS (expanded polystyrene), often in several different densities sandwiched together, slows that movement by deforming and absorbing the impact energy. Think of it as the difference between hitting air fence or an Armco barrier; one gives while the other is unforgiving. Although some certifications require a helmet to take two hits in the same place, once the EPS is compressed it doesn’t spring back into its original shape, and it’ll never again offer the same level of protection. A helmet crashed hard enough to deform the liner should be retired. They make great lampshades.
Fine-tuning the fit of your helmet by compressing or even removing a small amount of the EPS liner is considered a no-no by the manufacturers even though it’s often done. The issue is that you don’t know how your changes will affect the helmet’s dynamic behavior. If you’re lucky, your helmet will fit without modification. But it’s better to take advantage of any custom-fitting options, such as different cheek pads or comfort liners, before modifying the EPS. If you feel you have to grind a big chunk of liner out, you’re in the wrong size or shape of helmet.
Comfort liner. Here’s where fit happens. The comfort liner’s main job is to hold the helmet in place on your head and keep it from rotating or moving around in a crash. It also has an effect on the noise level inside the helmet and acts as a barrier between you and the liner, absorbing the sweat and skin oils that attack and degrade the EPS. Good helmets come with removable and washable comfort liners. A lot of riders judge helmets on how they fit around the face, but it’s important for the rest of the head to fit snugly too. Face shield. Clearly important, it not only lets you see the road, but it keeps stuff on the road from bouncing up and hitting you in the face. Good ones are made of polycarbonate and are tested for penetration resistance.
Retention system. Or as non-engineers call it, the chinstrap. It’s riveted to the helmet shell and must pass several tests that measure how resistant it is to being pulled out. There’s no safety difference between double D-ring and ratchets or buckles—all types pass the retention tests or they don’t get sold here—but there are no springs or hinges to break in a D-ring, which is why many riders and almost all racers prefer them. The placement of the rivets should be such that you can’t tilt the helmet forward off your head with the strap buckled.
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