Helmet Lab: Force Pairs When a Head Hits the Ground

Bike Safety Project: Lab Notes

1For their bike safety project, Soren Achebe and Mira Kowalczyk wanted to know how a helmet protects a rider's head in a crash. Most bike helmets have a thin, hard plastic shell on the outside and a thick foam liner inside. Their science teacher, Mr. Faisal Rahimi, helped them build a drop tester like a smaller version of the machines used to test real helmets.

2The tester holds a 5 kg metal headform, a model of a human head, inside a helmet sample. The headform slides down a guide rail from a height of 0.5 m and lands on a flat steel block called an anvil, which is bolted to the floor. A force sensor under the anvil measures how hard the helmet pushes down on the anvil, many times each second, so the team can find the largest force during the crash, called the peak force. Each sample was dropped twice from the same height onto the same spot, and only the liner was changed between samples. The shell was the same every time.

3The team also studied a bump between two riders. On a closed, padded practice track, a 45 kg student coasting at walking speed bumped shoulders with a 70 kg adult who was standing still on a bike. Both wore padded vests with force sensors. The adult's sensor showed a peak push of 900 N from the student's shoulder. The student's sensor was not working that day.

4Last, the team set criteria for a new bike helmet liner. On the first drop, the peak force must stay below 2,500 N. The liner must have a mass of 150 g or less so the helmet is comfortable, and it must be no more than 3 cm thick so the helmet fits under a hood. Their friend Ana, who skateboards, asked about a different helmet: one that takes many small falls. For that helmet the team allowed up to 250 g, but both drops on the same spot had to stay below 2,500 N.