Helmets for Ride Safe Day
1Every May, the bike-safety club at Juniper Hollow Middle School helps the city run Ride Safe Day, when children get free helmets and a short riding lesson. While unpacking boxes of helmets, club members Nayeli Cruz and Lior Ben-David noticed that every helmet had the same three main parts. They asked their science teacher, Mr. Hamid Sadeghi, what each part is made of and where it comes from.
2The thick inner liner is expanded polystyrene, or EPS, a foam made of tiny beads. Making it starts with crude oil or natural gas, natural resources pumped from underground. At a chemical plant, small molecules taken from these resources react and link into long chains, forming a new substance: polystyrene. The polystyrene is made into small, hard beads. Steam puffs the beads up to many times their size, and then they are packed into a helmet-shaped mold and steamed again so that they stick together. In a crash, the foam crushes and spreads out the force of the hit. Crushed EPS does not spring back, and even small knocks can leave dents.
3The thin, hard outer shell is polycarbonate, a plastic made from crude oil by other chemical reactions. The straps are nylon, which also starts as crude oil or natural gas. Some newer helmets use a different foam called expanded polypropylene, or EPP. It is made from natural gas or crude oil in a similar way, but its beads bend and spring back after a hit.
4Before foam liners became common in the 1970s and 1980s, many bike racers wore helmets made of padded leather strips, which gave little protection in a hard fall. Today, studies have found that wearing a helmet greatly lowers the chance of a serious head injury in a crash. Foam helmets are light and cheap enough that cities can give them away. But the oil and gas used to make them cannot be replaced, few recycling centers accept old foam, and a liner thrown away can last for hundreds of years in a landfill.
5Nayeli and Lior wondered whether a natural material could protect as well as the synthetic foams. Mr. Sadeghi cut square pads, all the same size and 3 cm thick, from four materials. For each pad, he dropped a 1 kg steel weight from a height of 25 cm onto the pad, which sat on a force sensor. He then dropped it a second time onto the same spot. The sensor recorded the peak (highest) force of each hit. A lower peak force means the pad spread the hit out better. He tested three pads of each material and averaged the results.