A note on scope
This is a history of how helmet designs and rules changed. It is not safety advice, and it does not say how much protection any helmet offers. The effects of helmets and helmet laws on injuries and on cycling numbers have been studied and debated, and this article does not try to settle that debate.
The foam idea
The heavy hard shell helmets of the 1970s were followed by a design that relied on a different material. Expanded polystyrene, a lightweight foam made of tiny air-filled beads, is crushable. In a helmet, the foam liner is intended to compress during an impact and so spread the force over a longer time. It is also light and can be molded into almost any shape.
The foam came from developments in industrial chemistry, which had made plastics and foams cheap to produce in large volumes. The same material was used in packaging and in some other sports helmets. For a helmet, it offered a combination of lightness and low cost that earlier shells could not match.
The first mass-market foam helmets
In the mid-1970s, a California company introduced a lightweight bicycle helmet with a foam liner. It is often cited as the first widely sold modern bicycle helmet, though other makers worked on similar ideas at about the same time and the “first” label is not universally agreed. Early foam helmets usually had a thin plastic shell over the foam, and straps and a basic fit system to hold them in place.
Over the following years, foam helmets improved. Makers developed ventilation channels, so the rider’s head stayed cooler, and refined the shape. By the 1980s, helmet designs from several companies were popular among keen cyclists and racing riders, and the image of the helmet had begun to change from awkward equipment to ordinary kit.
Testing and standards
As helmets grew more common, standardization became important. A standard is a published set of tests that a helmet must pass, usually involving dropping it onto an anvil from a set height and measuring the forces. Several organizations in the United States and elsewhere developed such standards in the 1970s and 1980s. A voluntary American standard for bicycle helmets appeared in the mid-1980s, and the Snell Memorial Foundation published its own. A mandatory federal standard from the US Consumer Product Safety Commission took effect in 1999. Other countries and regions developed their own standards, including Europe and Australia.
Standards matter in the history because they changed how helmets were made. Instead of each company choosing its own approach, designers had a common target. Standards also gave regulators and consumers a way to compare. They do not mean that all helmets behave the same way, and different standards test different things.
In-mold construction
In the 1990s, many makers adopted in-mold construction. Rather than gluing a shell to the foam, the thin outer shell is placed in the mold and the foam expands against it, bonding the two. The result was lighter and could include deeper vents. This is one example of manufacturing methods shaping a design, and helmets of this type became the usual form. More recently, some designs have added layers or sliding parts intended to deal with rotational forces, and some companies have explored custom shapes using additive manufacturing. These are recent developments, and their long-term role is still being assessed.
Laws
From the late twentieth century, governments began to pass laws about helmet use. The state of Victoria in Australia introduced a mandatory helmet law for cyclists in 1990, and other Australian states followed, so that national coverage was reached in the early 1990s. New Zealand introduced a law in the mid-1990s. In the United States, laws are made state by state or locally, and many of them apply only to children. Other countries have chosen to recommend helmets without requiring them.
These laws were controversial. Supporters pointed to the aim of reducing head injuries. Critics argued that required helmets might discourage some people from riding, and researchers have published a range of findings on the effects. It is fair to say that the evidence is mixed and the interpretation is disputed. The point for this history is that helmets moved from a personal choice to a matter of public policy in many places.
Helmets and changing bicycles
The spread of helmets happened at the same time as other changes in cycling, including mountain biking, faster road bicycles, and electric bicycles that can reach higher speeds. Helmet designs now include models for different uses, such as road riding, off-road riding and urban commuting, with different coverage and ventilation. Standards for some faster electric bicycles are separate again.
Where this leaves the story
Early helmets began with little protection and no rules. Foam liners made them light enough to wear, and standards gave them a common baseline. Laws turned them into policy. The history is still moving, with new materials and testing methods, and it is likely to be rewritten as more evidence comes in.