PSI - Issue 76

Filip Likavčan et al. / Procedia Structural Integrity 76 (2026) 145– 150

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3. Load on the bicycle frame during operation The operating modes of a bicycle depend on its intended use – e.g., city bike, mountain bike, off-road bike, etc. Pedaling on city roads represents quasi-static stress on the frame with small variable components that do not compromise the fatigue strength of the bicycle frame. The first significant stress amplitudes are caused by riding onto the curb, riding off the curb, riding over potholes on the road, and the like. In mountain and off-road bicycle operation, jumping on and off uneven terrain is added. These operating situations are associated with a significant dynamic effect that causes high stress amplitudes in the most stressed cross-sections of the bicycle frame. Determining the magnitude of these stresses directly in operation is experimentally difficult, and measuring stresses in the most stressed cross sections is not always possible. Critical frame elements are often welds or more complex shaped elements with high stress gradients, where the application of stress sensors is often not possible. One way todetermine the stress distribution in the most stressed cross-sections of a bicycle during operation is to measure the overload factor using suitably positioned accelerometers. Repeated crossings over selected obstacles causing decisive stress amplitudes for fatigue strength and frame life (climbing stairs, descending stairs, passing over potholes, jumping from higher terrain irregularities, ...) it is possible to obtain a spectrum of dynamic load multipliers using accelerometric sensors, as documented in Fig. 6. The corresponding bicycle frame overload multipliers are also shown in Fig. 6. From these overload records in the form of accelerations measured by highly sensitive piezo sensors in all three axes, it is necessary to identify the part of the dynamic acceleration that causes relevant stress amplitudes in the most stressed cross-sections of the frame [7,1]. These "effective" components of the dynamic load factor are then applied in computer simulations on a frame model. The simulation results for each load condition then allow the identification of stress components in individual frame cross-sections that are key to assessing its fatigue strength.

Fig. 6. Measurement of bicycle frame overload multiples during selected riding situations.

The final assessment of the fatigue strength and service life of the frame requires taking into account additional stress components arising from geometric shape deviations (e.g., in the form of shear stresses from the torsional moment shown in Fig. 5). Furthermore, it is necessary to assign the corresponding cyclic properties of the material to the assessed frame node in the form of the relevant S-N curve (Fig. 3), which is most reliably determined by cyclic tests on samples manufactured using the usual technological process for bicycle frame production[8]. Due to the significance of the multiaxial stress state, it is necessary to use a suitable multiaxial fatigue approach. In the case of non-proportionality of individual stress components, it is necessary to identify and pair the individual cycle amplitudes using a suitable algorithm and take into account their mean values [9-11]. This area of multiaxial stresses, their non proportionality, and the consideration of these complex influences in the computational assessment of fatigue strength is a separate topic requiring a separate paper.