A hailstone (5) is modeled as a spherical object with an impact speed Vo and mass m. As shown in the left figure, the indentation /impact force P of a spherical indenter (hail radius R) is a function of the indentation depth δ and the contact stiffness C based on Hertz’s contact law (right figure).

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The protection mechanism is based on the energy balance, i.e., the strain energy of the protective material (31) is equal to the kinetic energy of the hailstone when the maximum indentation depth δmax is achieved. As long as the thickness of the specific protective material (Tpm) is greater than δmax, no kinetic energy can be transferred to the car structures (41). Therefore, a thick protective panel is needed for a large hailstone. However, most hail protection products do not state the hailstone sizes for protection.

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For some cheap products designed and made in China, the protective material layer is too thin, and the protective material is very soft.  According to the energy principle, a very soft material requires a large layer thickness to absorb the kinetic energy from the hailstone. Only the hail shield is a perfect design to satisfy (1) the energy principle, and (2) balanced protective material volume, weight, and cost.  Please DO NOT complain about the weight of the hail shield---because we have to stop the damage from LARGE hailstones at a reasonable price!