###### Abstract  
Oblique impacts of the helmet against the ground are the most frequent scenarios in real-world motorcycle crashes. The combination of two factors that largely affect the results of oblique impact tests are discussed in this work. This study aims to quantify the effect of the friction at the interface between the headform and the interior of a motorcycle helmet at different magnitudes of tangential velocity. The helmeted headform, with low friction and high friction surface of the headform, was dropped against three oblique anvils at different impact velocities resulting in three different magnitudes of the tangential velocity (3.27 m/s, 5.66 m/s, 8.08 m/s) with the same normal component of the impact velocity (5.66 m/s). Three impact directions (front, left-side and right-side) and three repetitions per impact condition were tested resulting in 54 impacts. Tangential velocity variation showed little effect on the linear acceleration results. On the contrary, the rotational results showed that the effect of the headform's surface depends on the magnitude of the tangential velocity and on the impact direction. These results indicate that a combination of low friction with low tangential velocities may result into underprediction of the rotational headform variables that would not be representative of real-world conditions.  
motorcycle helmet; oblique impact; tangential velocity; friction 28 September 2021 25 November 2021 29 November 2021 29 November 2021  
Oblique impacts between the helmet and the ground are the most frequent collision scenarios in real world [11; 12]. Consequently, different laboratory testing protocols measure the helmet energy absorption capabilities and the resulting kinematics of a headform that represents the human head in oblique impacts [13; 14; 15; 16].  
[PERSON] et al. identified the impact velocity component normal to the road, the friction coefficient between the shell and road, and the impact direction as the factors that control the peak headform rotational acceleration in oblique impacts [17; 18; 19]. [PERSON] et al. showed that the impact velocity component normal to the road needs to be of a certain magnitude to induce the headform angular motion [20]. A follow-up study revealed that, from lower to higher tangential impact velocities, the motion of the helmet transitions from rolling to sliding, which affected head kinematics and its injury outcome for different friction values between the helmet and the ground [21]. However, the effective friction coefficient of the helmet as a whole is a function of the friction coefficient at the road/shell and at the head/liner interfaces [22]. Therefore, the tangential component of the contact force causes not only the rolling/sliding motion of the helmet on the ground but, due to similar underlying mechanics, it can also cause a relative motion between the head and the interior surface of the helmet [23].  
To our knowledge, neither experimental nor computational studies have quantified the interaction of friction and tangential velocity in the resulting kinematics of the headform. Thus, the objective of this study is to quantify the influence of the friction between the interior surface of the helmet and the headform in the resulting kinematics of the headform at different magnitudes of the tangential impact velocity.