Shell. Not just for turtles. The shell is the first line of defense, designed to absorb and spread impact energy. Two basic constructions are used: fiberglass and polycarbonate. More specifically, fiberglass shells can be made from any fiber-reinforced material, including fiberglass, carbon fiber, or aramid. Most use a cross section of materials, strategically placed to exploit their strengths. The different materials are laid down in a certain order, resulting in a shell that's thin yet strong. Areas more or less likely to sustain impacts in a crash are made thicker or thinner by adding or subtracting layers in those spots.