Traditional motorcycle helmet performance standards provide a test for the strength and stiffness of the retention system. While such tests assure adequate strength, they do not assure that the helmet will be retained in place on the motorcyclist’s head, even when securely fastened. The reason is that the geometry of the retention system can allow the helmet to roll off when contact or inertial forces are generated in a collision. Different types and styles of motorcycle helmets were tested to determine the susceptibility to roll off, i.e. “positional stability” (Thorn et. al., 1997).

Tests were performed using two commonly used, adult sizes of headforms corresponding to standards of the U.S. Department of Transportation (DOT) and the International Standards Organization (ISO). The test results were validated by comparison with essentially identical tests on a large number of human subjects. The results of the human subject tests show a meaningful relationship to the laboratory test which employs a 1Okg mass dropped 60cm to jerk the helmet forward to roll off.

The geometry of the retention system has a powerful effect on the ability of the helmet to resist forward roll off, in both laboratory and human subject tests. Also, there is considerable difference in the retention characteristics between DOT and ISO headforms, with the DOT headform more closely correlating with human subject data.