PROJECT OVERVIEW
Our goal for this project is to design and 3D print an airless bicycle tire. The honeycomb-like construction surrounding the tire provides the ability for the tire to restore to its memorized permanent geometry after receiving a physical change. This eliminates the need for air in bicycle tires, reducing the risk of air punctures during rides. The additive manufacturing process chosen is fused deposition modeling (FDM), and the printer chosen for printing airless tires is Stratasys F370. Additionally, the material that will be used to print the airless tire is thermoplastic polyurethane (TPU). TPU is a thermoplastic elastomer with high elasticity and durability, making it an ideal material for airless tires. TPU has properties of both plastic and rubber, which include high elongation and tensile strength, and abrasion resistance.
INITIAL TEST PRINT
Due to the limited print bed size (355mm x 254mm x 355mm) on the Stratasys F370, our first print was a scaled down model of a standard bicycle tire. Our print model was scaled down by a factor of four, with an outer diameter of 110mm and an inner diameter of 60mm. The width of the tire was 5mm. The materials used in this printing were Stratasys TPU and Stratasys QSR Support. Stratasys TPU was used mainly on the printing of the tire, while the Stratasys QSR Support was used mainly for printing the supports between the tire and the print bed.
The printing process took five hours to print the scaled down model of the airless bicycle tire. After the print was completed, Makerspace staff assisted with post processing of the print. Post processing completed includes dissolving the QSR supports in the Stratasys NaOH solution bath for eight to twelve hours, or until the support is dissolved. Upon soaking in the bath, the solution had been noticed to seep into the printed part. Therefore, the printed part was then soaked in clean water for twelve hours to remove the remaining solution.
FUTURE WORK
3.1 Printing Testing Samples Plan
After our initial print, our team faced difficulties performing tests on the printed model due to its small scale. Over the next week, we will be mainly working on designing and printing on a scale that is close to a standard bicycle tire (700mm in diameter, 23mm in width). We plan to continue using the Stratasys F370 printer with the TPU material.
Since the infill density and infill shape of the print were previously set by the printer and cannot be manipulated, our team plans to design another model with a different density of honeycomb surrounding the tire. The density of the honeycomb surrounding the tire can be manipulated by changing the radius of the honeycomb shape.
3.2 Test Plan
Two of the most important factors in determining if this non-pneumatic bike tire will be functional for real world application would be to determine the strain on the tire under a load that a bike tire may experience as well as the ability for the tire to recover its shape after loading. In order to ensure the structure that is selected for print will not collapse under the expected load, simulations of the parts will be run in SolidWorks to find several appropriate inner structure (honeycomb) sizes. To address the first factor, the plan is to perform a compression test on the test print, which will be a section of the tire. Using a universal testing machine, a compressive load of up to roughly 900 Newtons will be applied to the test print, and the strain of the part will be measured. To simulate the movement of a bike tire on an uneven surface, this load will be applied cyclically and will fluctuate between 400 and 900 Newtons. The strain will be measured and compared to the strain on a pneumatic bike tire. The second test that we expect to perform will be a recovery time test, by applying a deformation to the tire and then measuring the time it takes to return to its original shape once the load is removed. This will ensure that expected loads will not cause long term significant deformations to the tire structure that may impact the rotational ability of the tire. This test will also be done on an universal testing machine.