Bicycle Helmet Safety Institute

What Happens When I Hit?

Summary: Here are the steps as you proceed through the crash sequence.

You have just lost it on your bicycle. It's not important how you did it, since there are jillions of ways to fall. Just think of your body headed - fast - toward some pavement. You are just a big chunk of energy hurtling through space.

You don't have time to react, because it's too quick. Whoops - BAM! Just like that. No time to throw an arm out or tuck and roll. Just BAM!

The asphalt you hit is incredibly hard. Hard, hard, hard. It doesn't give at all. It's just completely unyielding, completely rigid. Hard.

So your head hits that hard pavement. Now what?

The helmet you are wearing cushions your blow. The foam in it crushes, and does not bounce back. It just gets thinner and your head slows down gradually. The plastic on the outside keeps the foam from breaking up and makes sure you will slide on the pavement as you hit, not "stick" and jerk your neck. The rounder and smoother your helmet is, the better it will slide.

All of that just happened in less than a second. The helmet actually brought your head to a stop in about 6 milliseconds (thousandths a second). But if you had not had a helmet on, you would have stopped in less than 1 millisecond. Pavement is incredibly hard.

Your brain inside your head is sloshing around, helmet or no helmet. With a helmet between head and pavement the force your brain is subjected to is not likely to reach the injury threshold. Without a helmet, it takes less than a three foot drop to injure your brain permanently. With a helmet, the energy spike is stretched out over that 6 milliseconds. Without a helmet it all goes to your brain in less than one millisecond. In the two graphs below you can see how much sharper the energy spike is. The first graph is the smooth curve extending over 6ms for the helmet but below it is the graph for the bare head showing a huge spike in energy in the first millisecond.
Somewhere about half way up that spike is where permanent brain damage begins.

So you have crashed. Your helmet is shot. Trash it. Be glad. Nobody ever complains about the cost of the second helmet.