# Bicycle Helmet Basics Freshman Engineering Clinic II Section 5 February 5, 2007.

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Bicycle Helmet Basics Freshman Engineering Clinic II Section 5 February 5, 2007

Statistics There are 85 million bicycle riders in the US. 784 bicyclists died on US roads in 2005. 92% of them died in crashes with motor vehicles (720). Eighty-five percent of bicyclists killed in 2003 reportedly weren't wearing helmets. Data collected and reported by Bicycle Helmet Safety Institute from various sources

Statistics About 540,000 bicyclists visit emergency rooms with injuries every year. Of those, about 67,000 have head injuries, and 27,000 have injuries serious enough to be hospitalized. 1 in 8 of the cyclists with reported injuries has a brain injury. Two-thirds of the deaths here are from traumatic brain injury. A very high percentage of cyclists' brain injuries can be prevented by a helmet, estimated at anywhere from 45 to 88 per cent.

How Helmets Work Brains are injured by impact or quick rotation In a rotation injury the head twists by the brain remains stationary straining the blood vessels and nerves The purpose of the helmet is to extend the stopping time of the head.

Physics Newton’s Second Law F average = m a average = m Dv/Dt Impulse = F average Dt = m Dv This shows impulse is a constant for a given impact and equal to change in momentum.

Physics With impulse being constant increasing the impact time will mean a lower impact force Impulse = F average Dt Purpose of the helmet is to lengthen the impact time.

Crushable Foams Helmets are lined with crushable foam Increases stopping time by approx. six milliseconds. May also reduce amount of twisting U.S. standard <300g. EPS, EPP, EPU, others Balance of foam density and thickness must be achieved.

Parts of the helmet Liner This is the foam layer Higher-end helmets require reinforcement of the foam because of the degree of venting. Shell PET (soda bottles) – glued and taped Polycarbonate – bonded to the liner as the liner beads are expanded.

Parts of the helmet Strap Made of multiple materials and weaves Multiple methods of attachment Occipital Stabilizer Rear of the helmet Straps, rings, cams, sliders, etc Interior Comfort and fit pads

Helmet Ventilation With no vents – putting a picnic cooler on your head Large vents and then paths or channels to allow air out the rear of the helmet Issues More vents change failure modes from compression to beam failures in tension More vents increase need for reinforcement which increases manufacturing costs More vents = denser foam

Helmet design issues Impact management High impact crash Concussion control Straps Keep helmet on during series of impacts (car and ground) Playground issues Easy adjustment vs. stays adjusted.

Helmet design issues Comfort Shape Looking cool!! Snagging Accessories Visibility Durability Instructions Environmental issues with materials

Test Standards CPSC - The Government's Standard has been U.S. Law since March 10, 1999. CPSC ASTM F1447 - Had been the Most-Used Standard, sidelined by CPSC ASTM F1447 ANSI - Replaced by ASTM ANSI Snell - B-95 is a Premium Standard; B-90 is comparable to CPSC Snell The Rest of the World - Some Ahead of the US The Rest of the World

Typical tests Drop/impact test Helmet attached to headform Dropped 1 – 2 meters to hard surface Measurement of peak acceleration Stability test Sharp jerk applied to helmet on headform to make sure it stays in place Strap test Head coverage

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