Helmet pad performance

The big debate in helmets seems to be focused on the brand of the helmet.  This is really irrelevant since we don't know what goes into the specific brands.   We need to discuss helmet pad technology to really get at this issue properly.   Most helmets function as a system,  that means the parts all work together.  It makes little sense to talk about the shell since it doesn't do all the work of protecting the person.

First, lets look at what the shell does.  The shell is the hard covering of the helmet, it is its outer most limit.  The shell usually holds any accessories, colors, and masks.  It also contributes to the look and impression of the helmet.  In the case of a ballistic helmet, the shell prevents bullets and fragments from penetrating into the head.   In bump or sports helmets the shell acts to deflect the force of the blow and contain the pad system.

The army gives its soldiers the Advanced Combat Helmet.  The helmet shell is made of a kevlar based material that prevents shrapnel from going through into the person's head.  The army has done extensive testing and has chosen to use a material called Zorbium in pads to line the inside of the helmet.  The polymer pads absorb much of the energy of an impact and are the key to the helmet's ability to prevent brain injury from bumps and blunt trauma.   The manufacturer of the material also makes a liner that absorbs some of the energy from blast waves encountered by the wearer.

The pads perform the important role of absorbing the energy of an impact.  The function of the pads can be understood when we look at how the pads work.  Any pad inside a helmet will absorb energy.  The exact behavior of the pad depends on its materials.  Zorbium is a unique polymer and displays characteristics unique to itself and was specifically engineered for helmet padding.  Many other pad systems are made of foams and polymers that were transferred from other purposes.  In one older helmet pad system, the foam used in the pads came from carpet underlayment cushion.

The helmet pad absorbs energy by changing its shape under the stress of the impact. In order to better understand how pads work, its important to break this down further.   When something impacts the helmet, the force of the blow can be understood as an impulse.  This technical term expresses the amount of energy transferred over the time of transfer.  Most impulses from blunt impacts last less than 15 milliseconds.  The pad absorbs much of the energy through crushing.  The best pads crush evenly and allow the helmet/head to be decelerated evenly and smoothly.  Stiffer pads require more energy to crush while softer pads crush at lower energy levels.  Once a pad is fully crushed it reaches a point where it no longer absorbs energy and only transmits it.

The next feature of a good pad is that it does not rebound quickly.   The best pads will crush and not spring back quickly.  The recoil could actually transmit energy into the head.  We really want a pad that will crush and stay crushed until the energy is removed.  

The last feature is pad thickness.  A thin pad will only be able to absorb a small amount of energy compared to a thicker pad.  Since most helmets are a specific size relative to the sport or activity, the thickness of the pad is often limited by the size of the shell.  If you look at some of the newer NFL helmets, they are bigger, allowing for thicker pads.