As team flying squirrel, Philip Keller and I at the very outset challenged the established gospel truths and decided to start ground-up. We made an early commitment to having 4 wheels instead of 3 that have been the norm for the past few years. The power produced by the rubber band is quite large, but useless if it cannot be transferred to the ground with very little loss. Traction was identified as the key criteria to make a successful car. Also, early on, we committed to Minimum Functional Prototypes – making a prototype as soon as a theory was developed with the least effort to demonstrate it rather than continue to theorize about it. In this effort, we went through 7 fully functional cars, with the 7th being the one that we raced on the final day. Using materials easily available and easiest to work with. We settled on plywood, tubes as material for the frame and zip-ties and string for fastening. No traditional bonding agents were used allowing us a turnaround time of about 90 mins from fully disassembled to fully assembled.

Here’s a video documenting our journey!

And here’s a breakdown of our material and design choices for the car:

1. Body as suspension – This idea was tested out early in the term by using different materials such as plywood, plexiglas, sheet aluminum, MDF and masonite to achieve theeffect of suspension by using a flexible upper layer and a rigid lower layer. After the experiments, 1/8th” birch plywood was arrived at as the material for the body. Plywood had the right density to ensure light weight and leant itself to iteration.
2. Fastening system- During the experiments for the body as suspension, we came across zip-ties as a fastening system. It could be used to laminate multiple pieces of plywood without a traditional bonding agent. After validating it’s rigidity with our second prototype, we settled on zip-ties as a preferred fastening system. It also created an interesting color dynamic from an aesthetic point of view.
3. Frame material – As described earlier, plywood was decided on a body material to implement the suspension idea. Although, the whole body made of plywood was increasing the weight substantially and hence we decided to experiment with a space frame structure to reduce the weight, keeping the strength the same. Square cross section tubes were preferred because of easy fastening with slots in a laser cut plywood part. Round tubes would have been difficult to secure. The first experiments were with hollow aluminum square tubes. After some testing during the midterm prototype race, we discovered the force that was need to make them fail was quite low. To reinforce them, we sheathed a smaller tube inside to increase the thickness. This performed better although failed after a couple of test crashes. We tried brass tubes next, and although heavier, the strength advantage was hard to ignore.
   
4. Tire material- Theoretically, we were of the opinion that a thin tire with only a single point contact with the ground would lead to loss of traction from the motor to the ground. Hence we required a wheel which a larger surface area in contact with the ground. Initial experiments were with silicon tubes, but we had issues with bonding them to the wheel material. We tried wide elastic bands mounted on laminated plywood wheels and faced similar problems. The challenge again was to affix the tire material to the wheel reliably. We then experimented with pipe insulating material – this came in a hollow circular cross section and easily mounted around the wheel. The adhesive that came on the foam was strong enough to use as adhesive on the wheel. This performed much better than our expectations and also added a component of suspension in the rear wheels which we were not able to achieve through the body. The major issues with this material were (i) wear after testing on the track was too much – we would have had to change it out very often to ensure optimum performance and (ii) since this was an extruded material, we needed to join the ends in order to make a circular tire which was proving to be a challenge. Finally, we found some RC in-wheels made of a similar foam material but were cast and they were circular with approximately the right diameter for our requirements. We settled on that material for our final tires.

    

5. Motor- A key innovation was the two rear wheel direct drive system. A single tube and two wheels on the ground afforded us much more stability around turns than some of the other competing three wheeled racers. Aluminum was chosen as a material for good strength to weight ratio.

    

6. Brake pads and brake disc- Silicone tube and bare aluminum were used as the materials for the first braking system. The silicone tube was tensioned around the aluminum drive shaft and when pulled, it provided a adequate braking system. Once the stronger motor was introduced, it was not able to hold the high initial torque of the rubber band, so we went back to the drawing board to figure out a better braking system. A scissor brake system was settled on as the optimum design in the space available. Materials that were tested as brake pads include foam, plywood, rubber, elastic band and pen grips. Pen grips were settled on due to easy affix-ability to the brake shoes. Elastic band wrapped around the aluminum tube was then used as the brake disc.
7. Frame tensioning – The triangle shape of the frame was arrived on as the optimal arrangement of elements to provide best crash resistance. The force was more evenly divided around the body than with two linear elements running down the center of the frame. String was used inside the hollow tubes to add tension amongst the various elements of the frame and served to hold everything together.