NAHBS 2018

Summary

NAHBS 2018 was Cal Poly Bike Builder's first foray into carbon fiber. For the show, we wanted to take on an incredibly ambitious project using a medium few of us had experience with. Best of all, this was an incredible learning opportunity for myself and many students to learn the basics of composite design and manufacturing techniques.

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Under the support of one of our professors, several students conducted research to try something never done successfully on campus before: produce prototype carbon fiber tubing. After several months of iteration and testing, the method was successful enough to hand off to the Cal Poly Bike Builders to produce the frame. 

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Simply put, the method started by manufacturing a mandrel--a piece of round aluminum machined to have a very slight taper over the entire length. For instance, over a 36" long tube, one end would be ~.005" larger than the other end. The surface finish of the tube was imperative; a mirror polish was the key to success. Once a mandrel was made, a carbon fiber layup schedule was planned out. This would consist of several layers of unidirectional prepreg carbon fiber at +45 degrees, -45 degrees, 0 degrees, and 90 degrees. With unidirectional carbon fiber, you get the added benefit of customizing the tube's stiffness. For a road bike, you want the tube to be incredibly torsionally stiff, so more +45/-45 fibers were added. Once the carbon fiber was laid up over the mandrel, heat shrink wrap would be applied over the outside of the carbon fiber, then the mandrels with the carbon fiber were placed into the on-campus composites oven. Several hours later, once the prepreg was cured, you would remove the mandrel from the oven, remove the shrink wrap, then allow the mandrel to cool down. Once cool, the aluminum shrunk a tiny amount and then the slight taper on the mandrel allowed us to simply remove the mandrel and voila! A carbon fiber tube was born! We machined mandrels for every tube size on the bike, then we started making tubes!

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For the headset, the bottom bracket, and the seatstays, we used the trapped rubber method to make the extremely complicated lugs. A mold was machined for every lug. First, a layer of wax was added to the inside of the mold. Then high temperature silicone was poured into the mold. Once cured, we removed the silicone, removed the wax, then started to lay the carbon fiber over the silicone mold. After the layup was complete, the lugs were put into the machined mold and closed up. The machined mold was then compressed between two very hot plates until the resin cured. Then you simply removed the lugs, extracted the silicone, and voila! A very complicated carbon fiber part!

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These procedures were repeated for each lug and each tube several times until we got perfect parts. Then, all the tubes and lugs were bonded together into a bike. In organizing the manufacturing for this project, it was fun to see students who had never touched carbon fiber get to have an extremely hands-on experience and learn an incredible amount about carbon fiber.