A single loading direction for fatigue life prediction and testing of handlebars for off-road bicycles

Components for off-road bicycles including handlebars continue to be recalled with regularity because of problems with structural failure as a result of high cycle fatigue in the off-road environment. The objectives of this study were to 1) devise a method for determining the point on the handlebar cross section that experiences the maximum cumulative damage when the handlebar is subjected to loads applied by the rider’s hands that vary randomly in both magnitude and direction, 2) use this method with an existing database of handlebar loads (DeLorenzo and Hull, J Biomech Eng, 1999) to determine a single loading direction to be used in design and testing of the handlebar, and 3) determine the sensitivity of the point of maximum cumulative damage to structural and material properties of the handlebar. The load database was generated by seven subjects who rode a rough downhill course in the standing posture and provided a total of 28 trials for analysis. For each of the 28 trials, the stress histories at 1-degree increments around the handlebar circumference were determined. The cumulative damage at each of the 360 points for each of the 28 trials was computed using rainflow counting in conjunction with Walker’s equation to represent the S-N diagram for the handlebar material. The maximum cumulative damage varied by more than six orders of magnitude between trials and the location of the point of maximum damage ranged from 110° to 343° (angle measured from horizontal axis pointing forward with positive counterclockwise rotation viewed from the right side of the bicycle). The median location was 142°. To create a tensile stress in bending at 142°, a load would have to be applied at 322° (322° = 142°+180°). Thus, 322° was found to be the single loading direction representative of the variable-direction load database. This direction did not change for a handlebar with different structural and material properties and coincided approximately with a vector drawn along the line of the arms of the rider. This loading direction can be used in conjunction with information on the effects of assembly of the handlebar with a stem to analytically predict the high cycle fatigue life of a particular stem/handlebar assembly. Furthermore, this loading direction can also be used to experimentally determine the expected in-service fatigue life of a particular stem/handlebar assembly.