The effect of surface friction on oblique bicycle helmet impacts

Philip George Petersen

The friction between the helmet and the impact surface can have a significant effect on the rotational accelerations imparted to the head. The surface roughness of the impact surface is therefore an important consideration when developing future oblique impact standards. The traditionally used 80 grit abrasive paper, used to simulate a road surface, may not accurately represent the conditions seen impacting real road surfaces. The aim of this work was to observe how surface friction affects head kinematics during an impact and if the kinematics of a helmeted headform impacting typical road surfaces can be replicated with a roughened steel anvil for the use in standardized testing. A helmeted NOCSAE headform was dropped using a twin wire guided drop tower at 6.5m/s onto a 45° impact anvil. Helmeted drops were performed onto an anvil designed to accept different surfaces such as; road surfaces, roughened steel, 80 grit abrasive paper and a Teflon coated surface. Linear acceleration, rotational acceleration, rotational velocity, impact force and head injury criteria were used to compare both frontal and side impacts. There were significant changes is many different head kinematic metrics with changing surface roughness. It is shown in side impacts there are opposing moments caused by the frictional and normal forces that control the rotation of the headform. It was found that 80 grit abrasive paper performed similarly to road surfaces in frontal impact scenarios with a maximum difference in the tangential load of only 11.6%. 80 grit overestimated almost all of the metrics in side impacts when compared to our road surfaces, especially in rotation. 80 grit performed best in Y rotational velocity overestimating by 19.9% and worst in overestimating 77.7%, overall 7 of the 10 metrics measured differed by over 20%. Results show that a steel surface with a roughness of Ra=72.6 in behaves more road like in both impact scenarios than 80 grit abrasive paper. Ra=72.6 in performed best in Z linear acceleration with only a 1.37% difference and largest difference in tangential load of 27.8%, however 8 of the 10 metrics measured differed by less than 20%.

The effect of surface friction on oblique bicycle helmet impacts

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