October 30, 2013
One of our two bike build-off challenge teams is working on building a ridable carbon fiber monowheel. The team has split into two groups to work on the carbon wheel and the steel "carriage" frame that rides inside of the wheel.
Over the last couple of weeks, the wheel team has been working to experiment with various construction techniques. First, the team used the Olin laser cutter to cut thin plywood templates of the wheel. These were installed on flat sheets of pink insulation foam and were used to guide a hot wire cutter along the outside edge of the wheel. The wheel is constructed of three 1 inch thick slices of foam sandwiched together with 1 inch chamfers on the inner edges of the wheel. The chamfers form a track for the inner carriage to ride within the wheel.
Next, the wheel sections were glued together and sanded to form a smooth shape for the monowheel. During the process, we used laser-cut reference sections to ensure a consistent cross section along the wheel's circumference. Carbon fiber was attached to the wheel with attention paid to ensure a smooth running surface on the interior of the wheel. Epoxy was applied to the carbon and the wheel was vaccum bagged overnight to remove excess epoxy. After removing the peel ply and breather cloth material the next day, a monowheel emerged. The wheel rolls nicely and is quite strong. In the coming weeks, we will sand the carbon smooth and add a second layer of carbon.
Meanwhile, the carriage team (mostly) finished the design of the carriage and has began to manufacture it. We are using a steel structure joined by fillet brazing. The carriage rides within the larger wheel on several small skateboard-like wheels along with a larger driven wheel. One set of skateboard wheels is sprung into the edge of the monowheel to keep the system constrained even if the large wheel deviates slightly from round. The driven wheel was salvaged from a small child's bicycle.
Currently, the frame is mostly brazed with the exception of the mounting tabs for the connection points to the main wheel. In the coming weeks, we plan to finish manufacturing the carriage and install it within the wheel. Currently, the carriage team is on hold until the wheel is finished. Once the wheel is done, we will ensure alignment and braze the remaining parts of the frame together.