What happens in a bike crash

Summary: Here are the steps as you proceed through the normal sequence of a bicycle crash.

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 much. 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, increasing rotational force to your brain. The rounder and smoother your helmet is, the better it will slide. And your head will rotate some inside the helmet, since they are not that closely coupled.

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 cause serious injury, but there is always some. 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 for a bare head. The first graph is the graph for the bare head showing a huge spike in energy in the first millisecond. The second is the smooth curve extending over 6ms for the helmeted head showing how the helmet manages impact energy.

Somewhere about half way up that first 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.

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