3D Printed Bike Strap

10/25/2020

As part of a fall 2020 Rapid Prototyping course’s focus on compliant systems, I was tasked with designing and printing a device to save the pant-leg which inevitably becomes trapped in the chainring while riding casually. Though I personally would lean towards a fabric strap which allows the pant-leg to be rolled up away from the ankle, I tried to create an equally effective solution from the thermoplastic, PLA.

Below are my initial sketches. Note how the design evolves to allow unimpeded movement of the ankle and achilles during a complete pedal stroke. The outward flare is meant to capture the pant-leg hem which bunches and extends from beneath traditional bike straps. From here I decided to take an iterative approach and start my CAD model using dimensions based on my own ankle.

Next, the CAD modeling. I chose Fusion360 for this project over SolidWorks, frankly because the time it takes to access my Windows virtual machine on my Mac is too long to warrant the more accurate modeling capabilities.

Below is a quick evolution from sketch, to extrusion, to revolved cut in order to create the angled flare that was unique to my design concept.

From here, it was necessary to perform FEA analysis on the exact geometry. Rough hand calculations may be possible, and could be accomplished by approximating the strap as a ring of constant thickness and radius. Next, using the known stiffness ratio based on the simplified ring’s geometry, deflections could be calculated given known forces. The open nature of the ring, however, would likely complicate this process as the deformation of an open ring may be highly non-linear.

Given this reality, I turned to AutoCad Fusion360 FEA static simulation to understand the load forces necessary to create enough deformation at the ring-ends to be able to wrap it around an ankle. I iterated the ring thickness from 0.25″ to 0.175″ and finally down to 0.130″ in order to create the proper compliance given a force of ~6N with sufficient factor of safety values.

I added fillets at the edges to ensure a pleasant user experience and can’t wait to test it out! This will hopefully be a great addition to the weekly commute to the 3D Printing CoLab workspace.