Motorcycle helmets consist of a shell, a foam layer and padding. The shell disperses the impact across a wide area, while also preventing anything from piercing the helmet. Meanwhile, the foam absorbs as much of the impact as possible, so your head doesn't have to. Lastly, the soft padding ensures a good fit against your head.
The difference between basic and advanced helmets normally lies in the shell . In contrast, a wide variety of helmets will employ very similar foam layers. Most use some variation of EPS – expanded polystyrene foam. This stuff is stiff and lightweight, but also perfectly crushable during impact. Some advanced helmets will use multiple densities of EPS in various layers and locations. This creates a smarter helmet, which can absorb the crash differently depending on the severity and location of impact.
Thermoplastic Shells
Thermoplastic is exactly what it sounds like – hot plastic. That means it can be poured into a mold and cooled into a solid shell. The plastic is usually some form of polycarbonate, which isn't the hardest material out there. So, these helmets require more foam padding to meet safety standards. For this reason, thermoplastic helmets are larger and heavier than more advanced lids. However, they are also cheap and easy to make. This is therefore the cheapest option.
Fibreglass Shells
Fibreglass shells are more expensive than plastic, mainly because they’re a pain in the butt to make. You have to place fibre cloth inside a mold, add a resin, and then heat everything to a billion degrees. Then, you repeat this process over and over to achieve a weave pattern.
At the end of all this, you have a shell that is harder and more lightweight than plastic. It also has a good impact flexibility, which spreads the force across a wider area of EPS foam. Fibreglass is quite brittle and prone to cracking. So, you’ll have to be extra careful not to drop one of these helmets. However, a crackling helmet is great in a crash, because the shell will absorb much of the force before the foam layer even comes into play. So fibreglass helmets don’t require as much foam padding, making them smaller and lighter than plastic lids.
Advanced Fibreglass Shells
These are similar to fibreglass shells. However, they are made from advanced fibres that have already been mixed with a resin or some other substance. So, there’s often no need for multiple layering steps. The manufacturing process is therefore simplified, which results in a more perfect material balance. The result is a helmet with all the same safety features as regular fibreglass, at about 80% the weight.
Kevlar and Carbon Composite Shells
Even if you don’t plan on getting shot in the head, a Kevlar shell has some distinct benefits. The process is the same as with fibreglass but, instead of using fibre cloth or Aramid, manufacturers use Kevlar. And since Kevlar is hella strong, you don’t need to use as many fibres to achieve the same result. So, Kevlar composite shells can be about 20% lighter than fibreglass shells.
Notice the word composite ? It’s there because Kevlar – which has an awesome tensile strength – kind of blows in terms of compression strength. So, helmets always combine Kevlar with some other material to make up for its shortcomings. Most often, you’ll see carbon fibre filling this role. Carbon fibre is super strong and lightweight. It's also very expensive, but since Kevlar isn't cheap to begin with, who cares? The result is a top-of-the-line shell, which achieves the safety standards of fibreglass and plastic helmets with much less size and weight.
The important thing to remember is that advanced materials do not imply advanced safety; a plastic lid can be just as safe as some space-aged helmet. The difference is that the plastic model uses more volume and weight to achieve the same results. If you’re going down, there won’t be much difference between the two. When you’re riding, however, your neck will thank you for a lighter helmet.