The effect of surface roughness on oblique bicycle helmet impact tests

Abstract

The friction between a helmet and impact surface affects the accelerations imparted to the head. The roughness of the impact surface is, therefore, a consideration when developing oblique impact standards. An 80-grit abrasive paper is commonly used in oblique impact tests to simulate a road surface, but has not been validated for bicycle impacts and may not accurately represent real road surfaces. In the following study, a helmeted NOCSAE headform with a Hybrid III neck was dropped onto a 45° anvil at 6.5 m/s using a twin wire guided drop tower. Helmeted drops were performed in two orientations (frontal and side) on road surfaces, roughened steel surfaces, 80-grit abrasive paper and a low friction surface. For each impact, measures of linear and rotational acceleration were obtained. These metrics were compared across impact orientations and surfaces to assess the influence of surface roughness on headform impact response. Frontal impacts were less sensitive to the impact surface roughness than side impacts across metrics. Among metrics, rotational acceleration showed the largest effect due to surface roughness. Compared to the road surface, peak rotational acceleration from impacts on the 80-grit surface were 6.5% less and 48% greater for frontal and side impacts, respectively. Based on consideration of the peak and cumulative impact measures, steel impact surfaces appear to better simulate road impact than the commonly used 80-grit abrasive paper.

A Review of Cyclist Head Injury, Impact Characteristics and the Implications for Helmet Assessment Methods

“…In helmet testing, the coefficient of friction of the anvil has been found to significantly affect the helmet kinematics, in particular rotational motion. 92 Current helmet standard and rating methods which include oblique impacts commonly use oblique anvil covered with 80-grid abrasive paper. This anvil condition represents an upper bound of the coefficient of friction.…”

Modeling the mechanical behavior of a helmeted headform impacted with a laminated windshield with consideration of composite failure

“…Composite materials, such as carbon fibre reinforcement, were evaluated as an alternative material of the helmet shell with good penetration resistance and ability of absorbing impact energy [20,21]. Besides, Finan et al [22] and Petersen et al [23] reported that the friction coefficient between the helmet and the obstacle had some influence on the head rotational acceleration in the oblique impact, but the influence in the normal impact seemed relatively small. Other conditions, such as impact velocity, impact angle and strap tension, are also considered in the impact between a helmet and an obstacle.…”

An Overview of the Effectiveness of Bicycle Helmet Designs in Impact Testing

“…B95) and American Society for Testing and Materials (ASTM F1447) ( Commision, 1998 ; Hansen et al, 2013 ; McIntosh et al, 2013 ). In these tests, helmets are placed on a headform and dropped onto a steel anvil coated with adhesive-backed 80-grit paper ( Commision, 1998 ; Hansen et al, 2013 ; McIntosh et al, 2013 ; Bland, 2019 ; Bliven et al, 2019 ; Petersen et al, 2020 ). The head kinematics during the drop tests are then measured using accelerometers and gyroscopes, which are attached at the center of gravity of the headforms.…”