# Experimental Development of Composite Bicycle Frame
## 4 Conclusions  
Extensive experimental research into the behavior of a composite bicycle frame was performed with a focus on the manually laminated tube-to-tube connection of the frame's down tube with the head tube, and of the top tube to the head tube. The first part of this research focused on the bicycle frame's behavior during the mandatory ISO test load cases, which are necessary in order to approve such frames for commercial use. Using FBG sensors embedded inside the UD load bearing tapes in the HT joint area, the envelope of the required minimal operational strain range was identified. Monitoring the rider's safety, critical strain values were found, especially for loads simulated by sprinting on the ergometer, detecting an increase of up to 66% above the values resulting from the required ISO-defined standard tests. This clearly demonstrates the importance of, at the very least, a basic operational strain analysis of such a structure, which is optimized for a combination of strength, stiffness and weight. The highest absolute strain found by the FBG sensors in this analysis corresponded to 68% of the HT joint limit strain found during the frame structural strength test. The safety of the composite frame is therefore sufficient; however, further development of the frame structure would require a complete field operational strain measurement. A comparison of the responses from  
Figure 27: Maximum principal elastic strain distribution on the surface of the head tube joint area: (a) DT; (b) TT. strain gauges installed on the surfaces of the tubes near the HT joints with the responses of embedded FBG sensors in the HT joint when testing load cases corresponding to standard frame use, such as a pedal forces test or a horizontal forces test, was performed. It showed that, regarding load transfer, HT joints are critical and that further development of the frame must focus on their improvement. Furthermore, it was found that under cyclic loading, there is no redistribution of deformations that could signal the degradation of the HT joints' stiffness. The second follow-up part of this research was carried out on simplified frame specimens without chains and seat stays. Both resistive strain gauges for local analysis and the DIC method for obtaining the strain field were used, supplemented by the AE method for the detection of composite defects and the FEA of individual composite joint layers. The aim was a deeper analysis of the deformation of the HT joint and an increase in the strength of the frame in the HT joints. The performed FEA of the composite lay-up of the HT joint showed a high sensitivity of the methods for local strain analysis, such as strain gauges and FBG sensors, to the position of the particular sensor. From the deformation analysis of the UD tape, it is evident that the FBG sensor can achieve deviations of up to 100% against the position in the neutral axis; moreover, with the opposite sense of strain. However, with a constant sensor position and on one frame specimen, a relative comparison of different types of loads is possible. It can be concluded that the FBG sensors have shown good functionality in the local monitoring of the critical joints of the composite bicycle frame. The same situation would of course occur when using resistive strain gauges installed on the joint surface. The advantage of methods that do not require prior computational analysis for placing the sensor, such as DIC which can cover the entire test area, is clearly demonstrated. Using a combination of the DIC method, the HT strain gauge measurement and the AE method, the improvement of frame specimen 7 in terms of strength compared to the frame specimen 4 sample was confirmed. By adding a combination of fabric layers and UD tape to the HT joints, the point when the load characteristic measured by strain gauges stopped being linear had moved from 35% to 46% of the limit load. This confirms a better use of the linear response operating area of the frame.