## 4 Discussion
While the above statement is correct for helmets in which only the shell thickness or helmet stiffness is increased [21], it does not hold for actual helmets in which both the shell and the protective padding can be varied jointly. Indeed, the impact performance of a motorcycle helmet depends both on the material and dimensions of the shell and on the characteristics of the protective padding or liner, and then there is a combination of the characteristics of the shell and liner that makes it possible to improve the helmet impact performance [22]. During an impact, a stiffer shell distributes the impact load over a greater area of the helmet, reducing the crushed volume of the liner and, therefore, decreasing the energy absorption, which may result in an increase of the linear acceleration. However, this effect can be compensated using a lower density of protective padding as long as its thickness is enough to prevent a bottom out effect. This practice is very common in current helmet design to compensate shell stiffness caused by shell geometry. For example, the higher shell stiffness due to the concavity form of the top part of full-face helmets is compensated with lower density or grooved shape liner at the top part [23]. This attempts to make the helmet impact response site-independent; however, other limitations such as liner thickness, especially at the side of the helmet, makes this point site-dependent because higher liner densities must be used at this location in order to prevent a bottom out effect of a liner. The site-dependent impact response could explain the contradictory results observed in the X point impacts (side impact) of this study. Therefore, a stiffer shell does not necessarily mean that the helmet will exhibit a global stiff mechanical behavior, but that the characteristics of the liner will be chosen to balance the effects of the stiffness of the shell, which depends on the material, thickness, and external geometry. Therefore, if a helmet stiffness increase caused by a stiffer shell can be compensated with the characteristics of the liner, the next question is: which type of stiff shell or soft shell improves the protection capabilities of the helmets? Although no general trend was observed in the results of this study to provide a convincing answer to the above question, some particular results such as the significantly higher PLA for the soft shell group for the point P at 8.2 m/s and the extremely high acceleration peak in the impact on the B point of one helmet within the soft shell group also at 8.2 m/s suggest that hard shell helmets would provide better protection at higher impact speeds. These results are in line with a simulation study that stated that the impact speed is an important parameter in helmet design and concluded that for high impact speeds, the helmet should be designed with a stiffer shell and denser protective padding than for low speeds [16]. Furthermore, the importance of the impact speed in helmet impact performance can also be appreciated by comparing the impact absorption test results between both impact speeds for each impact location. If the PLA and HIC values within the hard shell group were lower at 5 m/s for the X point than the values of the soft shell group, then the results of the hard shell group were higher at 8.2 m/s for the X point and vice versa for the B and P points. These results also highlight that shell stiffness has an important influence in the overall dynamic performance of the helmets. While helmets with stiffer shells tend to absorb energy by liner deformation from the inside, where the load distribution is determined by the compatibility of the liner dimensions and headform shape, helmets with softer shells tend to absorb energy predominantly from the outside, where the load distribution is determined by the geometry of the object hit. As a result of the higher load distribution capacity of helmets with stiffer shells, helmets with softer shells tend to bottom out sooner compared to helmets with stiffer shells [17].