BGR CFRP Monocoque challenge

In the 1980's an international competition was set up by SAE (Society of Automotive Engineers) to let engineering students compete in designing and manufacturing a single seat race car, so as to allow engineering students to gain hands on and practical experience before graduating. In 2011, our university entered the competiton for the first time and has been competing since then.

The Ben-Gurion Racing BGR team is organized into three main engineering branches: Mechanical, Electrical, and Autonomous Systems. Within the Mechanical branch, key sub-teams include Chassis, Suspension and Steering, Aerodynamics, Thermal Management, Drivetrain, and Battery Packaging. The Electrical branch comprises Low Voltage, High Voltage, and Communications teams. The Autonomous branch includes integrated teams focused on perception, mapping, and control, many of which interface more directly with the electrical system but also influence mechanical design—especially the Chassis team, through requirements related to sensor placement and mounting (e.g., LiDAR, cameras).

At the core of the vehicle lies the chassis, serving as the structural backbone to which all other systems are mounted. It plays a critical role in both mechanical integration and driver safety, and its design is subject to strict compliance with the FSAE rulebook, including structural equivalency and providing a Structural Equivalency Spreadsheet, and a tightly managed BOM (Bill of Materials).

The current chassis is based on a Chromoly tubular space frame. However, to reduce weight and enhance performance, the team is pursuing a transition to a Carbon Fiber Reinforced Polymer (CFRP) monocoque structure.

The move to a CFRP monocoque introduces several critical engineering and management challenges, some of which have emerged during early prototyping and past project cycles:

1. Early Design Freeze: CFRP requires the final geometry to be locked down much earlier than with steel frames. This poses a synchronization challenge with other sub-teams (such as suspension geometry, battery size, aerodynamic surfaces, and autonomous sensor locations), many of which iterate longer into the development cycle.
2. Complex Manufacturing Process: The chassis must be produced via a multi-stage process—first creating a plug typically from MDF, then producing a multi-part mold, and finally executing the carbon layup. Each of these stages involves material trade-offs. For example, plug material needs to be low-cost yet dimensionally stable, and mold material must match the thermal expansion of CFRP if prepreg is used, or be smooth enough to support vacuum-assisted resin transfer molding (VARTM) if that route is taken.
3. Integration Without Core Crushing: All connections such as suspension mounts, pedal box, and seat, must be carefully designed to avoid crushing the CFRP’s core material. This requires potted inserts or specially designed load spreaders, the implementation of which is highly sensitive to both design accuracy and assembly quality.
4. Rulebook Compliance & Equivalency: Unlike metal frames, CFRP structures require rigorous justification through FEA, physical testing, and documentation to prove safety equivalence to mild steel as mandated by the FSAE rules.
5. Material Anisotropy and Lack of Experience: CFRP’s directional strength characteristics add complexity to design and FEA, particularly given the team’s limited prior experience with composite structures.

1. Budget Constraints & Resource Availability: CFRP development is cost-intensive. The team lacks access to an autoclave, advanced CNC machining, and relies on mostly inexperienced labor. This severely impacts production method choices and timeline reliability.

From prior seasons, delays in finalizing suspension geometry and battery packaging have caused chassis redesigns late in the season, which would be unacceptable with a CFRP monocoque due to the high cost and time associated with remaking molds and plugs. Additionally, an early experiment with wet layup over a foam core revealed significant inconsistencies in wall thickness and resin saturation, further emphasizing the need for controlled processes like prepreg or well-managed VARTM.

Feedback gathered from:

1. Suspension and Aerodynamics leads indicates a need for adjustable mounting points and higher placement accuracy to reduce shimming or realignment.
2. Electrical and Autonomous teams stress the importance of predefined sensor mounting zones and wiring channels within the monocoque.
3. External advisors from the composites industry recommend materials with matched coefficients of thermal expansion (CTE) for molds and suggest early trials with smaller CFRP components to build experience.

By: Itay Shamir, Omer Vered, Ran Dravkin