How Does a Helmet Protect a Head?

Step 1 of 5

Force depends on how quickly you stop.

Falling from the same height at the same speed, the thing that decides injury is the time it takes to come to rest. Ten times longer means roughly a tenth of the force.

Step 2 of 5

The foam crushes, and does not bounce.

The thick inner layer is designed to collapse permanently. Crushing absorbs energy; springing back would hand it straight to the head again.

Step 3 of 5

The hard shell spreads the blow.

The outer shell turns a point impact into a wide push, so the foam under a large area does the crushing rather than one spot punching through.

Step 4 of 5

Which is why a used helmet is finished.

Once the foam has crushed it cannot crush again. A helmet that has taken a real impact is spent even if it looks perfect from the outside.

Step 5 of 5

Fit matters as much as the helmet.

A loose helmet moves before it starts working, wasting the millimetres it needed. Most of the difference between a good and bad helmet is whether it was done up properly.

Newer designs add a slippery layer inside so the shell can rotate slightly. Twisting the brain turns out to do damage that a straight-on blow does not.