Bike helmets are built from two primary components: a hard outer shell made of plastic or composite material, and a foam liner that absorbs the energy of an impact. Every other part of the helmet, from the straps to the ventilation design to rotational safety systems, builds on that basic two-layer structure. Understanding what each layer does helps explain why helmets feel, weigh, and cost what they do.
The Foam Liner: Where Protection Happens
The most important material in any bike helmet is expanded polystyrene, or EPS. It’s the white, lightweight foam that makes up the bulk of the helmet’s interior. EPS works by crushing on impact. When your head hits something, the foam compresses and absorbs the energy that would otherwise reach your skull and brain. It’s cheap, extremely light, and has excellent crush characteristics with very little rebound, which is exactly what you want in a crash. The tradeoff is that once it crushes, it doesn’t recover its protective ability. A helmet that’s taken a hard hit needs to be replaced, even if the damage isn’t visible from the outside.
Not all EPS is created equal. The density of the foam varies between helmet models and even within a single helmet. Research from Virginia Tech found that EPS density can vary by up to 18 kg/m³ within one helmet, and those variations directly affect how well different zones of the helmet perform during impact. Manufacturers intentionally use different densities in different regions, placing denser foam where protection matters most.
Some helmets, particularly those designed for mountain biking or BMX, use expanded polypropylene (EPP) instead. EPP looks similar to EPS but has a slightly rubbery feel. Its advantage is that it recovers slowly after a blow and can handle multiple impacts. Nobody can say exactly how many additional hits an EPP liner will survive, but it’s meaningfully better than EPS for riders who expect repeated lower-speed knocks. The downside is that EPP is heavier and doesn’t absorb energy quite as efficiently in a single catastrophic impact.
The Outer Shell: Polycarbonate vs. ABS
The hard outer shell serves two jobs: it distributes the force of an impact across a wider area of foam, and it keeps sharp objects from penetrating the liner. Most bike helmets use one of two plastics for this shell.
Polycarbonate is the more common choice in mid-range and higher-end helmets. It’s lighter, more flexible, and has roughly twice the impact strength of ABS. Because of that strength-to-weight advantage, polycarbonate shells can be made thinner while offering the same level of protection. That’s why a $150 road helmet often feels noticeably lighter than a $50 one.
ABS (acrylonitrile butadiene styrene) is the standard for budget helmets. It’s cheaper to produce, harder on the surface, and more resistant to scratches. It’s also heavier and more rigid. Where polycarbonate flexes under stress, ABS tends to stay stiff. Both materials provide high impact resistance compared to other plastics, but their priorities differ: polycarbonate protects through flexibility and energy absorption, while ABS protects through sheer hardness.
In practical terms, the shell material matters most for weight and comfort over long rides. Both types meet the same safety standards. If you’re commuting a few miles, ABS is perfectly fine. If you’re logging long days and care about every ounce, polycarbonate is worth the price difference.
Carbon Fiber and Composite Shells
At the top end of the market, some helmets use carbon fiber shells. Carbon fiber offers among the highest strength-to-weight ratios of any helmet material. A carbon fiber helmet typically weighs between 38 and 49 ounces, compared to 53 to 64 ounces for polycarbonate and 56 to 67 ounces for ABS. That’s roughly 15 to 25% lighter than a fiberglass composite and even more compared to standard plastics.
Carbon fiber shells are also thinner, ranging from about 0.10 to 0.14 inches thick versus 0.16 to 0.24 inches for polycarbonate. The stiff, layered weave spreads impact force across a wider area before the EPS liner beneath it compresses. This combination of thinness, stiffness, and light weight makes carbon helmets popular with competitive road cyclists, though prices range from $500 to $2,500. For most recreational riders, the performance gains are marginal relative to the cost.
Rotational Impact Layers
Many modern helmets include an additional internal layer designed to reduce rotational forces during angled impacts. The most well-known system, MIPS, adds a thin sheet of polycarbonate plastic inside the helmet liner that can slide slightly relative to the outer shell. Early versions used a Teflon-coated interface between two concentric layers, connected by a pin designed to break on impact and allow about 15 mm of slip. Current versions are simpler: a low-friction polycarbonate insert with small fabric pads at contact points, or slippery plates encased in stretch fabric.
The idea is that when your head hits the ground at an angle, the slip plane redirects some of the rotational energy instead of transferring it directly to your brain. Other brands use their own variations on this concept, but the core material is almost always a thin plastic layer with a low-friction surface.
Straps and Retention Systems
The chin straps on most helmets are made from nylon webbing, chosen for its combination of strength, light weight, and resistance to stretching. Federal safety standards require the retention system to remain intact without stretching more than 30 mm (about 1.2 inches) under forces equivalent to a crash impact. The buckles are typically molded plastic, and the adjustment dial at the back of most helmets uses a nylon or polycarbonate cradle that tightens around the base of your skull.
How Safety Testing Shapes Material Choices
Every bike helmet sold in the United States must meet the CPSC safety standard, which tests helmets by dropping them onto three different steel anvils: a flat surface, a hemisphere, and a curbstone shape. The helmet is dropped at 6.2 meters per second onto the flat anvil and 4.8 meters per second onto the other two. Peak acceleration measured at the headform inside the helmet cannot exceed 300 g during any of those impacts.
These tests are conducted under multiple conditions, and the standard requires that any external projection extending more than 7 mm from the surface must break away or collapse under impact forces. This is why bike helmets have smooth, rounded profiles rather than decorative ridges or sharp edges. The materials have to perform under what regulators call “worst case” combinations of impact site, anvil type, and environmental conditioning. Every material choice, from the density of the EPS to the flexibility of the shell, is ultimately constrained by these requirements.
Sustainable and Recycled Materials
A growing number of manufacturers are rethinking where their raw materials come from. The French brand URGE now makes its entire range from more than 90% recycled or bio-sourced materials. Their EPS foam is reprocessed from the automobile industry, and in 2024 they introduced a helmet with a 100% recycled polycarbonate shell. The company reports no compromise in performance or durability compared to virgin materials.
Recycled EPS is functionally identical to new EPS once reprocessed to the correct density. The bigger challenge has been the shell: recycled polycarbonate needed years of testing before it met the same impact standards as conventional plastic. As more brands follow this path, the core materials in helmets are unlikely to change dramatically. What’s changing is where those materials come from.