Oh boy, a helmet discussion!

When the Bell first came out a big to-do was made in the various media about why it was better. The physics make perfect sense. It goes something like this:

Your head will be traveling at some velocity downward when it hits the pavement. When it hits it must decelerate from that velocity to zero within the thickness of the helmet's compressible padding. A helmet will be as thick as it can be for a given weight, so you have only so much distance over which to decelerate and lose all that downward velocity.

The difference in helmets is in their compressible material. If the material is springy and rebounding, for example like foam rubber, the deceleration will be minimal for the initial compression but will increase as the compression increases. (A pure Hooke's Law spring would mean the force, and thus the deceleration, is proportional to the amount of compression.) The head's deceleration and thus it stress on the brain therefore reaches a maximum at full compression.

If the material is destructively crushable instead, it will decelerate the head immediately as crushing begins and the amount of deceleration will be constant until it is fully crushed or the head stops. To obtain the same loss of velocity in the same compression distance, its deceleration, which is constant over that distance, will be only half the maximum reached with the springy material.

Of course if the material fully compresses before the head's velocity reaches zero, then the last increment of movement is a really hard bump. But if the thickness if the helmet is great enough, the inelastic-compression material wins easily.

So the helmet standard says assume such and such a velocity in a fall from rider's height to the ground, and choose a material and thickness such that the maximum deceleration never exceeds a particular value. Now, one could argue about whether those numbers are appropriate, conservative or liberal, useless, or whatever. But regardless of the numbers, for equivalent thickness the helmet with a crushable material will be better.

That's why the guts of all good helmets today are a crushable foam, and why they should not be used one they have suffered any crash at all.

We and many other riders began using the Bell as soon as it came out. It was comfortable, looked only slightly dorky, and worked in theory. Better'n nuthin'.