SENIOR PROJECT

My senior project is the culmination of many of the skills and knowledge I had gained in college. However, like all challenging projects it pushed me to learn more as the year progressed. I would not have been able to deliver such an amazing final product without the hard work of my whole project team. Greg Ritter, Ernesto Huerta, Michael Wu, and myself came together to make the project a success. We had all worked with each other in limited capacity in the past but together we made a killer team. We were all skilled in different aspects of the design and were able to collaborate when needed or independently go and accomplish task after task. Together, our team, Hück's Law became the Senior Project dream team.

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The Cal Poly Bike Builders is an incredibly unique club that guides curious students through the process of designing and fabricating their own bicycle frames. The club already had amazing tools and skills which allowed students to build traditional bicycle frames with relative ease. However, there was the ever elusive full suspension bicycle, which had only been done once in the club's history to a limited degree of success (see: Two Chainz). Our senior project was to develop a suspension system and tooling which a novice bicycle builder would be able to construct simply and cost effectively well before taking any dynamics or FEA courses.

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 The design work required for a full suspension bicycle is several orders of magnitude more intense than a traditional mountain bike. The suspension must be designed in a way which the kinematics improve the performance of the ride. There are generally higher forces seen on such a bike and they are redirected to specific points on the bike. We knew off the bat that in order to make this project a success, we would need to consider the complexity of the dynamic loads as well as the final manufacturing process.

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The initial background research led us to uncover information about suspension systems, loads seen in use, and other design factors for this project. Then we selected a suspension system, a linkage driven single pivot, and started design of the suspension kinematics. We rolled with this design and developed a detailed CAD model off of the kinematics as well as concepts for all the fixtures and tooling necessary to build the bike. Ultimately, after a few months of effort on the linkage driven single pivot we concluded that it would not work. The design had continued to develop and was chock full of compromises ultimately resulting in a sub-par system.

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We decided that it was time to re-evaluate the design and concluded it would be best to revert to a traditional single pivot suspension system. This was not a decision we took lightly. At this point we had about five months of work poured into this project and it was difficult to decide to scrap the effort. We were also concerned that going to a single pivot suspension would lose some of the “cool factor” that our other linkage inherently possessed. Within mountain biking, the single pivot has a somewhat stigmatized name due to a notable amount of poorly executed designs in the past. However, we decided to press on with the single pivot as we knew we could design the bike to have proper kinematics, look cool, and would be much simpler to construct.

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Upon making the decision to redesign we kicked it into high gear. We quickly re-evaluated the kinematics of the single pivot because of how much we had learned from the initial design. We then were able to develop the detailed design, fixture, and tooling concepts once again. With only a few months left until the end of the project, it was time to start building, testing, and breaking things.

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One of the reasons we had strayed away from a single pivot design initially was because with proper kinematics, there is only one spot where the front of the shock can mount--right in the middle of the downtube. This poses the significant issue of the immense forces from the suspension being directed right towards the middle of a thin walled tube. Our solution was to add a doubler plate to the tube to prevent buckling. To verify and test how large the doubler plate needed to be, we tasked ourselves with creating a custom Instron fixture. We recreated the load case in the lab and tested multiple sample down tubes until sufficiently satisfied with the result.

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Another key piece of design and testing for this project was what we affectionately call the "Boomürang." This is the key to the simplicity of our suspension system. This is one machined piece of steel containing the bearings for the suspension pivot, one end of the shock mount and “flip chip”, as well as a dropped chain stay attachment for chain clearance. This was a fairly complex single piece, but since all of the complex machining and manufacturing operations were restricted to this component, the design allowed for easier assembly by a novice student.

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The Boomürang starts life out as a sizeable piece of steel. Specifically, we designed it to be machined out of A36 steel--a common structural steel that costs significantly less than most alloy steels. Additionally, we wanted to reduce the weight as much as possible. This is where some finite element analysis came into play. We were able to take the computer model of this part and simulate the load it would see. From the results of simulation we iterated until we came up with a design which was as light as possible without sacrificing strength or machinability. 

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After our final design of the Boomürang, we needed to verify our computer calculations. We made a fixture to hold the boomerang and hung weights to cause the test sample to deflect. We measured this deflection with a test indicator. After collecting data we plotted the deflection curve against that of the computer model and were able to confirm out results.

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 It was now time to manufacture. We created three additional fixtures that would be used along with the club’s existing Anvil frame fixture. This allowed for the main pivot and seat tube bend to be positioned. Then the front triangle could be completed by locating the front shock eye mounts. Finally, a dummy shock and dummy bearing could be used to fixture the Boomürangs, allowing for the rear triangle to be constructed like normal.

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 The Boomürangs are obviously a very bespoke part of the bicycle. To make these, I designed a custom CNC fixture plate.  After the first machining operation of the Boomürangs is complete, they are simply flipped over and fixtured into this plate. The plate allows for the final operations including the bearing bore and other critical locating features to be machined at the same time. 

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The whole project was completed by building up a test bike to be “tested”. It is so cool to play with something you make especially if it is the result of months of hard work. The Hück’s law Proto 1 was finally complete and we were all ecstatic with how it rode. In fact, we even proved out some of our loading tests due to the fact that I landed a jump incredibly wrong and crashed. Thanks to some extremely generous people we are able to keep the bike built up for future students to test and ride before deciding to take the plunge into making their own.

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I wanted to add an additional note of thanks to my team members: Greg, Michael and Ernesto. As well as the professors who helped manage us and gave advice and input along the way. Prof. Harding, Dr. Kean, and Dr. Mello.