Air Pressure Vs. Airless Tire Insert
Optimizing Ride Feel.
Benchmark Compression Testing
This testing was performed on a mountain bike tire at different pressures using an Instron machine. The tire was first fixed in the machine using a testing fork. Then using a constant step input of displacement, the tire was deflected two inches (0.05 meters) and the force required to deflect the current distance was recorded. This process was done for pressures of 20, 30, 35, and 40 psi. The goal of this testing was to obtain comparable data to the analytical model and prototype testing data.
Analytical Model
As a proof of concept, an analytical model of a simplified multi-spring model was made. The simplified model turned the rings into springs projected radially out from the center of the wheel. To obtain a model, the group first found the spring constant of an individual ring using Castigiliano’s Deflection Theorem and Hooke’s Law. Then using angle relations, the group created a force series with input variables of spring constant and deflection. The resultant equations from the steps above were then coded in MATLAB and using a guess and test method, the group found multiple dimensions of wire and rings which would perform closely to an air pressurized tire system.
Nitinol Tensile Testing
During prototype production, the group noticed plastic deformation of the wire was occurring a lot quicker than what was expected. This created a problem for the group because elastic deformation was an assumption during concept generation. After taking a step back, the group realized the issue was the stress that occurred from bending of the wire from a its production shape, a line, to a ring. To see graphically see where the ring stress was on the stress strain curve just from bending and help the group pivot in prototyping, the group tensile tested a piece of 0.6 mm diameter wire in the Instron and analytically solved for the bending stress as a function of wire diameter and ring radius.
Prototype Testing
After producing a testable prototype, the group completed another compression test in the Instron to compare its performance to the analytical model. The prototype tested utilized the 0.6 mm diameter wire with ring separation of 0.5 degrees and ring radius of 2.0 in. The results of this test were better than the group expected, and although they are not directly aligned with the group’s model, they proof of concept viability.