How Your Bicycle Helmet Works

February 10, 2014 Updated: February 10, 2014

In the event of an accident when cycling, a good helmet can mean the difference between life and death. Like wearing a seatbelt while driving (even on the shortest of trips), wearing a helmet should become a habit to the extent that you feel uncomfortable cycling without it! It is essential, too, that children get into this habit early, and much of their learning process is taken from the example set by the adults in their lives.

While muscle and bone are relatively tough (meaning they are both strong and elastic), neural tissue does not have much intrinsic strength—it’s very soft. Indeed, our brains are about 75 per cent water. On impact, the bones of the skull stop instantly, but the softer brain tissue tries to follow through, which can lead to concussion or worse. In serious impacts, the skull fractures, and the blood vessels and neural tissues in the brain may be more seriously injured.

You can visualise the human head as having a structure like a partially-boiled egg: a relatively hard outer shell, and a semi-solid core. Drop the egg on a hard floor and…you get the picture. However, drop the egg on a soft cushion, and it doesn’t break. A bicycle helmet is this cushion—it allows the egg to decelerate over a period of milliseconds, as opposed to the entire deceleration occurring in an instant.

A bicycle helmet is composed of a number of components: a hard outer shell, a special high-density foam-rubber lining, and a number of low-density foam strips and patches next to the head. Some may incorporate a spin-dial headband to tighten the helmet, and all need a chin strap to keep it firmly in place.

While the soft, low-density foam is there for comfort and to ensure a snug fit, the key to impact resistance is the high-density foam. On impact, the soft foam is crushed instantly, providing little or no impact protection. 

The harder high-density foam, however, is more difficult to crush. On impact it spreads the deceleration over a period of about 6 thousandths of a second, allowing the brain to slow down gradually before stopping. Thicker and/or higher-density foam is better, though there is then often a trade-off against bulk, weight and appearance. 

When the expanded polystyrene foam is crushed, however, it cannot recover its original form or dimensions, and will be ineffective in the event of another impact at that point. This is why manufacturers insist that, if the helmet is involved in an impact, it should not be worn again.
So when you get out that bike, mind your head!