But what's inside your helmet and how does it work?
Human beings are pretty fragile. You've probably broken a few bones and had some serious scrapes over the years, but there's one body part we can't mess with-the brain. The whole essence of who and what you are is tied up in this one squishy blob. Give this three pounds of gray jelly anything more than a firm tap and it swells up, breaks down, and turns to mush. Then you're gone.
Luckily, we've all got a reasonably tough carrying case in the form of a skull. The brain floats in a bubble of fluid inside this solid bony dome, meaning you can take a certain amount of abuse without it flaking out. But it's not too hard to exceed the protective capabilities of this stock setup: if you fall and hit your head on concrete from as little as five feet you can sustain a head injury bad enough to kill you. Bust an arm, leg, even smash some ribs, and you're still here. Bust yo' brain and you're finished.
So it's clear that the most important protective item you can wear is a helmet. By adding a solid extra layer of impact protection to your head you stand a much greater chance of surviving a knock on the noggin. And if you're going to engage in a risky activity - such as riding a motorcycle - then a helmet has to make sense.
The first thing to realize about a helmet is what it can't protect you from. And what it can't do is let you head-butt a car or wall at 75 mph and survive. Helmets aren't there to protect you from high-speed impacts with other vehicles or the local scenery-they just can't be made big enough to absorb all the energy in such an impact. All the helmet can do is defeat more moderate hits such as landing on your head after a highside or dropping off the back of a wheelie.
How a helmet works
The essence of a helmet is pretty simple. There are two main parts. There's an outer shell made from a tough, strong material such as fiberglass, carbon fiber, or polycarbonate plastic. Inside this outer shell is a thick lining of expanded polystyrene (EPS) like the stuff your fancy DVD player was probably originally packed in. There will be a super-strong strap firmly attached to the outer shell that's designed to fasten around your chin so it can't be knocked off in crash.
During an impact the two main components do two specific jobs. The outer shell absorbs some of the initial knock. It deforms around the impact point as it decelerates and also prevents anything from penetrating into the helmet (like a footpeg or a fence post). Meanwhile, the inner shell is compressed by the force of your head inside the helmet trying to continue on the path gravity has sent it on. It's this inner shell deformation that protects your brain. It allows your head to decelerate from the impact speed to a stop in a greater distance. This prolonged slowing reduces the forces acting on your brain and skull, reducing the amount of damage and, hopefully, preventing serious brain injury.
Though a seemingly basic function, it's been developed to a very high level by modern helmet designs. Outer shells are strengthened by clever structural design-Arai uses an extra stiffening ring around the base of its helmets, and a net-like fiber weave inside its composite shells. The inner lining is normally made of various densities of EPS, allowing the designer to fine-tune the amount of protection in different points, according to how likely they are to suffer impact, how much area there is to spread the impact, and how delicate that part of the head inside is. The forehead area of the helmet is usually made of stiffer foam than the crown area since the crown has a larger area to absorb the impact.
Make it fit
The most important part of any helmet isn't visible. If your helmet isn't the correct size for your head then it can't offer the proper protection in a spill. The worst case scenario is when a helmet is far too large, so it can actually come off in a crash. Even if it doesn't come off, a helmet that's too big won't absorb impact energy as efficiently, increasing your chances of a brain injury. Conversely, if your helmet's too small, it can cause headaches and fatigue, ruining your fun and your concentration.
For the best helmet fit, make sure you go to a proper dealer with a trained staff who can advise you. Try on different brands and models (they all vary subtly in terms of shell size and shape) until you find one that's snug, can't be rolled forward off your head yet doesn't pinch at the forehead or temples. Remember that the inner comfort padding will bed in a little after the first dozen wears or so (depending on brand), so a snug fit is ideal.