Emily Cazes- 4 Bar Bicycling Mechanism

Introduction and Background

The purpose of my mechanism was to represent how different recommended crank angle arm lengths can impact knee angle, as well as user comfort, during an extended bicycle ride. There are several different methods to obtain the appropriate crank arm length, depending on the philosophy of a bike fitter. Some optimize for maximum torque of the drivetrain system with the longest possible crank arm length, while others prioritize the angle formed at the back of the knee to prevent injury. My mechanism used dimensions taken from myself and two of my bicycle configurations to illustrate the difference between optimizing for torque and optimizing for comfort. 

Presentation:

Design Process

Early on, I knew that the dimensions for my mechanism would from my personal measurements. I chose to scale the model 1:10, which would allow me to carry my mechanism inside my backpack. I also wanted to illustrate both the crank arm length for maximum torque and maximum comfort on the same ground. I also wanted to ensure that the cranks would always be offset by 180 degrees from one other, which involved using a D-shaft.

I also wanted to represent my bicycle as accurately as possible. To do this, I modeled my mechanism in SolidWorks to ensure that my components would fit well together and also animated my mechanism in PMKS: RMD Project Animation .

Kinematic Analysis and Synthesis

Below are the hand calculations and graphs for position, velocity, and acceleration of the knee.

Based on my MATLAB kinematic analysis, there is a noticeable increase in angular acceleration of the knee by 2 in^2/s using the longer crank arm length (172mm). 

Manufacturing and Assembly

Early designs of my mechanism involved the usage of 4-40 bolts and nuts. I sourced my bolts from Texas Inventionworks, but these were too long to properly fit my mechanism. To attempt a custom fit, I rotated a nut to the desired length of the bolt, then clipped off the end of bolt with bolt cutters. This created a very crude lock nut, since the end of the bolt was warped enough to prevent the nut from sliding off during movement of the mechanism. However, the bolt cutter was not capable of cutting flush to the surface of the nut, creating inference issues with the rest of my mechanism.

While I was testing the limitations of TIW's FDM printers on my shift, I realized that these printers would be capable of printing pins and nuts to my desired diameter of 3mm. This would also allow for me to have a perfect length fit for my mechanism and prevent the mechanism from dragging on the surface of the static body. To seal the nuts to the pins, I used a soldering iron with a broken tip, then heated the unit to 230C to melt the PLA filament between the gap of the pin and nut. This only melted the upper layers of the pin and nut together and allowed for easy removal of the pins with a soldering iron as I replaced linkages. 

In the early iteration shown below, both sets of 4 bar mechanisms use black acrylic and very crude iterations of my final pin and nut assemblies. The base is of 1/4" acrylic, which proved to be flimsy when the mechanism was moving.

In my final design, the linkages and frame were laser cut from 1/8" acrylic and the wheels from 1/4" plywood. The base and gears were 3D printed and designed in SolidWorks. Each set of 4 bar mechanisms is contrasted, either using black acrylic for the shorter crank arm or clear acrylic for the longer crank arm. I increased the diameter of the nuts to provide a cleaner and smoother assembly process with the soldering iron. 

Electronics and Software

For my electronics, I selected an L298N motor controller, an Arduino Uno, and a 12V motor. 

Software:

Conclusions and Future Work

My design was successful, as my mechanism moved as it was intended to. As shown in my video, the cranks stay offset by 180 degrees from each other and each 4 bar mechanism flows smoothly. 

In the future, I would like to build the mechanism with CNC'd components for improved durability, as well as include bearings for the pins. 

Appendix