When a car hits head or tail first, the bumper and the front (or rear) clip helps dissipate the energy of the car by deforming. The energy is takes to deform the metal is energy that isn't transmitted to the driver. Energy-absorbing foam is used in both doors to fulfill the same function: In the case of a side hit, the foam dissipates energy, which means that there is less energy that is transmitted to the driver. The NASCAR R&D Center tested over 200 foams before settling on IMPAXX^TM foam, which is made by Dow. IMPAXX^TM won the Safety Innovation of the Year award from the Professional MotorSport World Expo in November 2007.

Energy-absorbing foams convert the motion (or kinetic) energy of the car into other forms of energy. The motion energy of the car crushes the foam the same way a cushioned mat protects you when you fall. IMPAXX^TM is an extruded polystyrene foam, which makes it a close relative of STYROFOAM Brand Foam and the foam in which electronics are packaged. Extruded foams are made by melting the foam materials, then blowing the foam through a die using a gas. The gas bubbles make the pores. IMPAXX^TM is a closed-cell foam, which means that the pores in the foam are surrounded by polystyrene on all sides. There are also open-cell foams, which are more like sponges in which pores are connected to each other. It is much easier to crush open-cell foams than it is to crush closed-cell foams because you have to force out all the gas from the pores before the foam starts to crush.

IMPAXX^TM dissipates energy differently than most foams. IMPAXX^TM deforms in three steps: first, it compresses, then it buckles, and–if there is energy left–it breaks into pieces. Each of these steps requires energy, energy that comes from the motion energy of the car. The stress-strain curves reflect the unique way IMPAXX^TMdeforms and compares it to other types of foams. The key to keeping a driver safe is dissipating the energy before it reaches the driver.

In addition to the foam, there is a 0.090" steel plate in the driver's side door and a plate made of Tegris^TM in the passenger-side door. (Tegris is the same material from which the splitter is made.) These plates keep objects from penetrating the door and getting into the driver's cockpit.

More information

• A video introducing IMPAXX^TM foam.
• A technical paper on IMPAXX^TM that discusses using the LS-DYNA computer program to simulate energy absorbing foam processes.
• BSCI Energy Impact Systems supplies IMPAXX ^TM foam pieces for the doors of the new car to NASCAR teams.