## 2 Methods
### Testing Procedure  
The tests were performed at the Impact Laboratory of the University of Zaragoza. A free fall guided impact machine (Model: Quebrantahuesos 6.0, +D, Pozuelo de Alarcon, Spain) was used for the drop tests. In the drop tests, the helmeted headform is placed on a carriage and restrained by pre-cut paper tape to prevent helmet motion during the fall. After releasing the carriage assembly from a specific height, the helmeted headform impacts the top surface of an anvil and the carriage assembly continues to fall onto a cushioned bed plate without interfering with the helmeted headform kinematics. The helmet used was a full-face motorcycle helmet (model: FF104 RAPIDE, MT, Cartagena, Spain) (Figure A1). In total, 18 full face helmets of the M (57-58) size were used (using three impact locations per helmet, although each impact location on each helmet was tested just one time). The outer shell of the tested helmet was made of Fibre Reinforce Composite (FRC). The liner of the helmet was made of expanded polystyrene (EPS). The retention system of the helmet was based on a double D-ring buckle. The helmet applied with the United Nations Economic Commission for Europe (UNECE) regulation and Department of Transportation (DOT) standard. The baseline case was a drop test performed at an impact velocity of 8.0 m/s onto a 45\({}^{\circ}\) oblique anvil, which originated two impact velocity components (normal and tangential) with the same magnitude of 5.66 m/s. The tangential velocity component (V\({}_{\rm T}\)) of this test set-up was altered approximately \(\pm\)40%, while keeping constant the normal velocity component (V\({}_{\rm N}\)). Therefore, six helmets were drop tested at 6.5 m/s onto a 60\({}^{\circ}\) oblique anvil, obtaining a V\({}_{\rm N}\) of 5.66 m/s and a V\({}_{\rm T}\) of 3.27 m/s. Finally, six more helmets were \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Resultant Impact** & **Anvil** & **Headform** & **Impact Point** & **Test ID** \\ \hline \multirow{6}{*}{6.5 m/s} & \multirow{6}{*}{60\({}^{\circ}\)} & & \multicolumn{1}{c}{} & Front (Y-rot) & DT0322-0N-01 \\ \cline{3-5}  & & & Left side (X-rot) & DT0322-0N-02 \\ \cline{3-5}  & & & Right side (Z-rot) & DT0322-0N-04 \\ \cline{3-5}  & & & Front (Y-rot) & DT0321-0N-01 \\ \cline{3-5}  & & & Left side (X-rot) & DT0321-0N-02 \\ \cline{3-5}  & & & Right side (Z-rot) & DT0321-0N-04 \\ \hline \multirow{6}{*}{8.0 m/s} & \multirow{6}{*}{45\({}^{\circ}\)} & & \multicolumn{1}{c}{} & Front (Y-rot) & DT0320-0N-01 \\ \cline{3-5}  & & & Left side (X-rot) & DT0320-0N-02 \\ \cline{3-5}  & & & Right side (Z-rot) & DT0320-0N-04 \\ \cline{3-5}  & & & Front (Y-rot) & DT0319-0N-01 \\ \cline{3-5}  & & & Left side (X-rot) & DT0319-0N-02 \\ \cline{3-5}  & & & Right side (Z-rot) & DT0319-0N-04 \\ \hline \multirow{6}{*}{9.9 m/s} & \multirow{6}{*}{35\({}^{\circ}\)} & & \multicolumn{1}{c}{} & Front (Y-rot) & DT0318-0N-01 \\ \cline{3-5}  & & & Left side (X-rot) & DT0318-0N-02 \\ \cline{3-5}  & & & Right side (Z-rot) & DT0318-0N-04 \\ \cline{3-5}  & & & Front (Y-rot) & DT0317-0N-01 \\ \cline{3-5}  & & & Left side (X-rot) & DT0317-0N-02 \\ \cline{3-5}  & & & Right side (Z-rot) & DT0317-0N-04 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Test matrix. For N from 1 to 3, indicating repeated tests for a particular condition. drop tested at 9.9 m/s onto a 35\({}^{\circ}\) oblique anvil to obtain the same V\({}_{\rm N}\) and a V\({}_{\rm T}\) of 8.08 m/s. These two test conditions are referred as \(-\)40% V\({}_{\rm T}\) and +40% V\({}_{\rm T}\) respectively (Figure 19). The oblique anvils were made from a solid steel cylinder with the diameter of 130 mm. The impact surface of the anvil was covered with a sheet of grade 80 close-coat aluminium oxide abrasive paper, that was replaced after significant damage or after three impacts. There was not variation of the friction between the anvil surface and the exterior helmet surface.