It has been suggested that many injuries to the brain are caused by rotational acceleration (RA) which in turn is caused by head impacts. Further, RA of the brain would be accentuated if the blows to the head approached tangential. Since a large number of accidents involving two wheeled vehicles involve the rider being thrown from their vehicle in a manner which results in sliding, or tangential blows to the head, the measurement of RA under such conditions becomes important. No performance test of protective helmets measures RA, and indeed most tests, with the exception of the British Standards Institute test (Standard 2495), use only linear impacts in their protocols. In order to measure RA during sliding or tangential impacts, a helmeted (fibreglass shell with a polystyrene liner) or unhelmeted head form, instrumented with linear and rotational accelerometers, was impacted at angles approaching tangential. The head form was suspended by nylon cord in a manner such that it could be considered a free floating mass. A hard covered ball, propelled by air to high velocities, was used as the impacting mass in some cases; on others a pendulum was used as the impacting mass. Results of a number of impacts suggest that even when the angle of impact approaches tangential, the rotational acceleration is small compared to the linear acceleration. This occurred even when the surface of the helmet was abraded to increase its coefficient of friction, or when the surface of the helmet was covered with fine sandpaper. It may be concluded that under the conditions described only low levels of RA are transmitted to the head, compared to linear acceleration, and thus RA is not as major a factor in the injury causing accidents as was thought.
Samenvatting