Mechanical Engineering Department
BS in Mechanical Engineering
The Cal Poly Formula Electric team has been in need of a chassis redesign, making that a logical choice for a senior project. The original goal of the project was to build the lightest possible frame while still maintaining adequate strength and stiffness. The existing frame made a good starting point. The steel tube spaceframe weighed a colossal 105 lb. There was ample room for improvement, even with a similar spaceframe design. The greatest potential to reduce weight lay in submitting designs under the Alternative Frame (AF) rule set. This could avoid the added weight of many required tubes. The new 2014-15 rules introduced more strict requirements for tube sizes used in alternative frames. These new requirements effectively eliminated any advantage in building a steel tube frame under AF rules, making the developing chassis design no longer viable. In response to the rule changes, possibilities were reevaluated and a cut-and-fold carbon composite monocoque was chosen as a good alternative to the steel tube design. This new design direction promised significant weight savings while maintaining the strength required to pass the tests set out in the AF rules. The primary body of the frame began as flat panels of 3/4” 3-ply carbon composite that was then cut to the desired outline and folded into shape. These folds were reinforced with a wet layup using carbon tape and the resin, and filled with glass micro-spheres in order to reduce the weight of the resin used. This folded panel was bolted to the front and main roll hoops. The roll hoops were still steel tubes, as this was required by any rule set. Various properties were tested along the way in order to properly document chassis construction and justify FEA analysis to the FSAE officials. Most of these tests were destructive material tests on the composite panels themselves. All major subsystems except the battery box were carried over from the existing car to the new one. The suspension, drivetrain, and space for the new battery box were all part of the design from the beginning for a seamless transition from one chassis to the next. Once the monocoque was completed, the other systems were simply assembled into it. Once the entire car was assembled, the final tests for the chassis were to be passing technical inspection and performance at competition. Since the team was unable to get into the competition from the waitlist, this was not possible. Final design validation, instead, came from a technical inspection performed by Professor Fabijanic before the car was driven and from driver feedback.