SEP 2018


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NEWS 39 Bicycle Design Integrity Resources exist, and are also readily available. "ASTM, in partic- ular the D30 Composite Materials Committee, has decades of test methods at their disposal for determining (a) composite material design allowables, (b) state of actual cure, and (c) verification of fiber-resin void contents," Beckwith points out. "Yet the majority of bicycle manufacturing companies never seem to use them in their design processes." Beckwith cites this type of corner cutting, done by bike manu- facturers to decrease costs, as the type of major mistake that can lead to part failure and accidents on the road. "e majority of bike manufacturers assume that the materials of construction and the process is forgiving because, almost always, only the outside surfaces are visually observable upon removal from the processing mold, but this is a risky assumption." Most defects in composite structures — in the present case, frames, front forks, handlebars and other bike components — are invisible until the part fails. Beckwith, who also serves as an expert witness in forensic investigations involving composite failures in road-bike struc- tures, observes, "I have not seen a stress analysis, or set of material strength allowables or an applied safety factor for any of roughly 50 carbon fiber high-performance bike failures." at needs to change, he insists: "Manufacturing, structural analysis, quality control, testing and inspection are all important considerations in controlling a very labor-intensive molding and composites manu- facturing process." Here then, CW, with help from Dr. Beckwith, presents some accumulated wisdom from those who have taken seriously the unique challenges presented by the design and manufacture of composite bike components. e starting point, of course, is design. Design factors e first major design factor is the purpose of the bike: Different uses impose different design imperatives. Pierre Legay, structural and CFD analyst for bike manufacturer Argon18 (Montreal, QC, Canada) explains, for example, that "low weight and high pedaling stiffness are required for race bikes, aerodynamics will be more critical for triathlon bikes, and good comfort will be required for endurance road bikes." "Its purpose defines the bike's loads and functionality, defines environmental variables and structural performance and safety targets," emphasizes Bertram Stier, research engineer, advanced composites, for Collier Research Corp. (Newport News, VA, US). e geometry of the bike, mostly dictated by function and style, and the scale of the series (produced units), are also design factors that "dominate the balance between the emphasis on ease (and cost) of manufacturing and structural performance," Stier says. "An enormous amount of data, techniques and constraints all contribute in a major way to restrict and define the design problem, which is the reason why composites optimization software can play a crucial part in design solutions." Design solutions e nearly infinite material design possibilities for a composite structure, however, make traditional design approaches — even those that involve standard computer aids — time-consuming and potentially subject to failure. But there are design solutions FIG. 1 Material placed precisely — and only — where needed The thickness of the composite frame for the Argon18 (Montreal, QC, Canada) Krypton endurance bike — it is as thin as 0.5 mm in some locations — was optimized using the Altair HyperMesh simulation software platform. One result of the simulation process was that hundreds of prepreg patches give the bike a stiff bottom bracket to improve pedaling efficiency but a compliant seatpost to improve comfort. (Graphic color key: dark red areas are thicker, dark blue are thinner). Source | Argon18 FIG. 2 Finding the balance between stiffness and toughness Innegra prepreg weave adds impact resistance in this standard UD prepreg layup for the Alfa all-road bike, built by Allied Cycle Works/HIA-Velo (Little Rock, AR, US). Source | Allied Cycle Works/HIA-Velo

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