Wiggle Car
Objective
I worked with three other students to design--from scratch--a wiggle car that won a class competition (race). We were given the following requirements:
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The wiggle car's body must be fabricated from sheet metal
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Manufacturing the wiggle car must involve CAM, plasma-cutting, welding, milling and lathing.
Technical Details
We started by using SolidWork's sheet metal tool to build the chassis and seat of the wiggle car, ensuring the dimensions were comfortable for everyone on the team (our heights ranged from 5'9" to 6'3"). We then ran FEA analysis on these parts to ensure they could withstand the weight of the the heaviest on the team (~260 lbs).
The chassis and seat of the wiggle car was made by bending plasma-cut pieces of sheet metal and riveting them together. I was personally responsible for designing and fabricating the diving/propulsion system for the wiggle car. I started by scanning through the available literature on wiggle cars, before sketching out ideas. Early iterations of the design involved converting continuous motion into wiggling, because I felt it was a good way to achieve speed with less effort. I designed a propulsion system that involved using bicycle pedaling to achieve wiggle motion. However upon further evaluation, it became clear that it would be too time consuming to fully design and build in the allocated time of 5 weeks.
I eventually settled on a design inspired by the EzyRoller. My design however included a twist: the front wheel was not only a negative caster, but also a swivel wheel. This made riding it more "compliant" as the wheel was always oriented tangential to the path of wiggle, thus reducing friction. I chose to set the caster angle to 60 degrees, which is the same as RipStiks.
The propulsion system includes a clamp block, that allowed us to adjust the angle of the footrest. This allowed us to achieve the most comfortable angle for the footrest. We determined that the longer the distance between the front of the car and the front wheel distance was, the faster the car moved. However there was a downside, increasing this distance increased the deflections in the assembly which could increase friction in the system. To navigate around this problem, we included side plates that allowed us to adjust the distance (see image above).
After creating a CAD model of the entire assembly, I ran FEA simulations to determine most likely points of failure and maximum stress/deflection. I then machined all the parts. I machined both of the clamp blocks on a mill using CAM. The side plates were plasma-cut from aluminum. I made the footrest by turning aluminum rods on a lathe, and then welding them together.
Results
Our wiggle car was incredibly smooth to ride. It was very comfortable for every one on the team, and we were able to find the right balance between effort and speed. It was the fastest of all 7 teams, and after 10 laps of racing, we won the class trophy.
You can find our final report (which contains more technical details, results analysis, as well as drawings for all the parts we fabricated) here.