Evaluating bicycle frame loads through semi-analytical multibody simulation methods

: The transport industry is currently experiencing a revolution, as a result of which sales of bicycles are increasing worldwide. For bicycle manufacturers, it is necessary to solve the conflict of goals between a time-and cost-efficient design and an inexpensive, yet safe product. At the same time, the weight of the bicycle is playing an increasingly important role, which means that new lightweight designs have to be developed. In modern design processes the operating loads to which the system components are subjected, play an important role. However, determining these is a major challenge, since not all components are accessible to measurement technology and existing test specifications often do not correspond to the real load situations. A popular approach is the use of fully analytical multi body simulation methods to recreate the loads of real world driving. The results form the basis for finite-element

The transport industry is currently experiencing a revolution, as a result of which sales of bicycles are increasing worldwide.For bicycle manufacturers, it is necessary to solve the conflict of goals between a time-and cost-efficient design and an inexpensive, yet safe product.At the same time, the weight of the bicycle is playing an increasingly important role, which means that new lightweight designs have to be developed.In modern design processes the operating loads to which the system components are subjected, play an important role.However, determining these is a major challenge, since not all components are accessible to measurement technology and existing test specifications often do not correspond to the real load situations.
A popular approach is the use of fully analytical multi body simulation methods to recreate the loads of real world driving.The results form the basis for finite-element analyses with which the component stress can be determined (Johannesson, 2014).However, compared to other vehicle industries, bicycle development faces special challenges when it comes to the application of such simulation methods.To perform these fully analytical simulations, a numerical model of the driving route as well as the driver is required.Although there are many approaches on this subject, the models of driver and track are too inaccurate due to unavoidable simplifications in regards to the determination of quantitative loads (Bruni, 2020).This leads to an inadequate design foundation, which is especially true for the (mountain) sports sector, since both the complexity of the load scenarios and the load itself are extremely high.
A high potential to solve existing challenges offers the so-called semi-analytical approach (SAA).This simulation setup applies forces directly on the system in form of measured data (Tebbe, 2006).In this way, challenging modeling tasks such as driver and route can be circumvented.Several studies in the literature explore these methods for moving systems without common constraints to the environment (Tebbe, 2006;Joubert 2020).Whereby these publications only consider vehicles that have a much higher weight ratio between rider and vehicle and lower load variety compared to bicycles.To prevent the system in these moving, unconstraint systems from accelerating uncontrollably in the simulation, passive and active stabilization methods can be used.These can be represented in the form of artificial constraints in the system, with the goal of recreating the applied forces in form of reaction forces or by control loops, stopping the acceleration of the center of gravity (Tebbe, 2006;Joubert 2020).For the purpose of designing bicycles, it is necessary to use the simplest simulation and measurement method possible, while being able to calculate component loads based on loads applied to the system, without distorting them through the simulation process.
This paper aims to show the potential of the semi-analytical approach for the calculation of loads on bicycles through the evaluation of various existing methods.To achieve this goal, synthetic measurement data at load application points are generated through fully analytical multi body simulations using a full suspension bicycle.The simulation is excited by a passive driver model and various road models, shown in Figure 1a.The excitation investigates straight-line rides with periodic excitations and jump excitations by board stone crossings and jumps as they occur in mountain biking.These data are applied to passively and actively stabilized simulation setups in order to calculate component loads of the frame structure.For passive stabilization, the suitability of artificial constraints on various components such as the frame or wheel hubs is investigated, given the lack of clear constraints in the environment of a bicycle, see Figure 1b.Furthermore, control loops are utilized to provide the stability, see Figure 1c.In order to evaluate the suitability of the different SAA, forces are compared at reference points with the forces from the fully analytical measurement run.The results generally show that systems such as bicycles, in which the critical loads are not applied by the system inertia itself but from outside, are particularly suitable for the application.Existing disadvantages such as the neglect of system inertia in some approaches thus only play a subordinate role.Furthermore, a good agreement between the component loads from the full-analytical simulation and the loads calculated from the SAA could be determined for the scenarios considered.

Figure
Figure (a) Generation of synthetic measurement data.(b) passive semi analytical simulation setup.(c) active semianalytical simulation setup.