Volume 31, Issue 2 (March 2003)
Methods for Fatigue Testing Off-Road Bicycle Handlebars Based on Assembly Effects Using Two Different Stem Designs
Assembly of off-road bicycle handlebars with a stem that clamps the handlebar around its circumference would be expected to affect fatigue performance by introducing both assembly stresses and stress concentration. Because the effect of clamping on fatigue performance is unknown and because of the need to insure structural reliability in the stem-handlebar assembly to prevent serious injury, the objectives of the work reported by the present article were fourfold. One was to determine the stresses due to assembly and the stress concentration induced in a handlebar for two different clamp designs (i.e. 1-bolt and 2-bolt), a second was to determine experimentally the high cycle constant amplitude load fatigue lives of the two stem-handlebar assemblies, a third was to determine experimentally the variable amplitude load fatigue lives, and the fourth was to predict the variable amplitude load fatigue life with constant amplitude load fatigue test results. The handlebar was instrumented with strain gages and the assembly strains were measured when the stem clamps were tightened. The handlebar was also loaded as a cantilever beam while the applied strains were measured for each assembly. Stresses were computed and the maximum stresses induced by clamping exceeded 200 MPa for both assemblies. A method unique to this study was devised to determine the stress concentration at an arbitrary angular location around the circumference of the handlebar and for an arbitrary loading direction in the plane of the bicycle. For a load directed along an angle of -38° (clockwise rotation from horizontal viewed from the right), both stems created similar stress concentration; the location of maximum applied stress was shifted by 30° from the point that would be expected in the absence of assembly and the stress was increased by 40% at this location. The measured fatigue lifetimes for constant amplitude loading were similar for the two stem designs but the variable amplitude load fatigue lifetime for the 1-bolt stem assembly was shorter than that for the 2-bolt stem assembly by 19%. The fatigue lifetimes for variable amplitude loading based on constant amplitude load fatigue test results were predicted to within 3% and 33% for the 1-bolt and 2-bolt stems, respectively. Thus, constant amplitude load fatigue test results can be used to approximate the variable amplitude load fatigue life. However, the ranking of different assemblies may not be accurately indicated by constant amplitude load fatigue data.