A fracture of a bicycle frame close to the steering column or a fracture of the front fork leads unavoidable to complete loss of driveability and to accidents with high risk of serious injuries. Fork and frame of bicycles inevitably suffer from a dynamic problem due to their design similar to a tuning fork and due to a permanent excitation of a bending mode high numbers of load cycle result. The problem arrises not only on leightweight frames, especially single tube designs and Pedelecs with front hub motors intensify this problem due to low section modulus or high masses at the front wheel with preloads from the electric motor. A very dangerous failure of a bicycle frame is a fracture of the top tube and down tube close to the steering column due to fatigue. By just looking at the maximum loads this is not obvious, but a closer look on the strain history shows periodically fluctuation at certain frequencies. By performing a modal analysis on the front fork it can be found that the first bending mode of the fork is responsible for the cyclic mechanical stress in top and down tube of the frame leading to high number of stress cycles. The dynamics of the front fork and the stiffness of the frame have significant influence of the lifetime and therefore have to be taken into account for bicycle design to reduce fatigue, especially because resonance effects on rough roads are unavoidable. Test procedures should contain a fork-frame fatigue test or a complete bicycle test for all kind of bicycles to ensure safe operation.
The difference between tuning forks and bicycle forks - and why bicycle frames suffer from that
2009
10 Seiten, 17 Bilder, 8 Quellen
Conference paper
English