# Experimental Development of Composite Bicycle Frame
## 1 Introduction  
Bicycle frames made from carbon fiber reinforced polymers (CFRP, see Figure 1) are now widely commercially available. Besides prepreg molding technology, composite wound tube technology is the most common method for the manufacture of bicycle frames. In general, the most problematic points of a bicycle frame, which consists of wound structural tubes, are the joints of these structural tubes. One common method of joining the tubes, by using glued sleeve joints, has both performance and visual limitations. These are caused by the technology of the composite manufacturing itself [1], which differs from the traditional manufacturing technology for steel or alloy frames, especially with regard to the service life of the adhesive joints of composite tubes [2]. Therefore, research has focused on joint improvement, by trying to improve the strength of the hand lay-up laminated joints. Structural stress-strain analysis is an integral part of such research and experimental verification is necessary. In order to achieve the optimal design of a bicycle frame, as well as other components such as forks, handlebars, etc., it is essential to have the best possible knowledge of the static and dynamic properties of the frame and other individual components. These can now be routinely tested in the laboratory. To identify the dynamic parameters of the structures, input-output methods are used [3]. Newer methods combine, for example, the wavelet-transformed representation of system responses with an independent component analysis [4; 5]. The static, as well as fatigue tests, need to demonstrate that the bike's structure has sufficient strength. This must be demonstrated from the stiffness point of view; for example, [6] describes the analysis of the frame stiffness response, and from the fatigue life point of view; for example, [7] describes the fatigue testing of the front carbon fork. The increasing demand to improve the performance of bicycle frames, especially from a safety point of view, necessitates an accurate description of the operational load of the frame. Due to the complexity and cost of such a task, which results from the requirement for small, lightweight and durable experimental equipment, operational measurements are performed only by some bicycle frame manufacturers. The origins of these operational experiments date back to 1968, when the first strain gauge (SG) analysis of pedal-and-crank strain was performed [8]. Today, even operational fatigue analyses are common, where the load operating spectra are measured by strain gauges calibrated to measure force, both for metal frames [9] and for composite frames [10]. However, due to the complexity and cost of operational measurements, bicycle manufacturers mainly focus on numerical simulations. These are created from basic models using beam elements to determine stiffness behavior [11], through the use of shell elements [12], to optimization tasks performed on the composite frame lay-up configuration [13]. The common solutions for laboratory and operational measurements use strain gauge analysis. In addition to bicycle frame testing, they are focused on testing other critical components such as handlebars, pedals and forks. However, experiments which commonly use the electrical strain gauge method for the stress-strain verification of the frame have some crucial disadvantages. A strain gauge cannot be embedded inside the composite lay-up or inside an adhesive joint. Therefore, only the surface strain can be measured. This could be insufficient in a case where a strain analysis of a geometrically complex shape is needed. A structural analysis of the specific layers of composite lay-ups or joints can be performed using Fiber Bragg Grating (FBG) sensor measurement technology, because FBGs are small enough to be placed between the particular filaments [14; 15]. Tests of bicycles, including their components, are described in ISO 4210 [16]. The standard is constantly evolving and adapting to emerging cycling trends.