Helmet foam:

Styrofoam is and has been the reigning energy absorption material used in helmets. It is cheap, easy to work with, controllable and lasts 20 years or more.

Styrofoam’s effectiveness was established around 1960 at USC (University of Southern California) by Professor Lombard and his successor Professor Hurt.
Their research was sponsored by the US Navy which was looking to reduce the number of severe head injuries to its pilots when they ‘crashed’ onto carrier decks.

Lombard and Hurt were testing differing materials when the Germans accidentally gave them the clue to the answer. They had ordered a quality 35mm camera from Germany (I do not remember the manufacturer). When the camera arrived Professor Hurt, then a grad student, opened the box and found that the camera was cradled in Styrofoam. They then tested Styrofoam and found the answer to their quest.

Since that time, more than a half-century ago, Styrofoam (Expanded Poly Styrene) remains the best proven material to attenuate impact loadings to helmet wearers’ heads. You’ll also find it cradling your new TV, computer, etcetera.

There is no ‘one’ Styrofoam used to make helmet liners. Hemet liner material varies in density. The density, back when I researched the matter, was expressed in pounds-per-cubic-foot. Motorcycle helmet liner foam varied from 1.5 to 3.

The higher the density, the greater the force required to start the brain-saving crushing. In order to meet performance standards, lower density foam had to be thicker so that it would not ‘bottom’ during testing.

The best performance USC’s Head Protection Laboratory found while testing helmets for my article published in Motorcyclist magazine in 1981 was also the cheapest. That helmet used 1.5 pound foam which was about 1.5” thick. It was huge. It cost $40 at K-Mart. Neither company is now in business as far as I know. The Snell helmets gave the highest deceleration numbers while still barely passing the DOT loading and dwell specifications.

Current bicycle helmets use much higher density foam than any motorcycle helmet I am aware off, as high as 6#/cubic foot. This higher density is needed to make-up for the lack of complete coverage dictated by the need for ventilation cutouts. Professor Hurt expressed concern to me about this hardness and wondered, he was by then retired, if threshold (the load that would begin crushing) “g” numbers might be rather high for the then current bicycle helmets (circa 2005). This is a very good question and one that is not easy to answer because helmet testing is now done almost exclusively by manufacturers who, for various reasons, do not share their testing data.
Bicycle helmet liners break and crush. Motorcycle liners just crush. Breaking is not as effective as crushing.

Joe