CHassis Design

Each year Shell hosts an international competition where teams demonstrate an energy efficient vehicle. The goal is to achieve the lowest energy consumption while traveling a distance of 10 miles.

For my capstone project, I worked in a small team to design the chassis system for the Queen’s Supermileage (QSM) team’s 2023 vehicle. This vehicle was designed as the team’s first entry into the Urban Concept class, thus requiring a complete re-design of their previous vehicle.

Preliminary Design

Space frames are an effective chassis solution as they allow for minimal material by addressing directional loads. Since the vehicle will travel at a top speed of approximately 15 miles per hour, the net weight of the vehicle has a significantly greater impact on the energy efficiency compared to aerodynamics. To minimize the chassis weight, the space frame will be manufactured with carbon fiber as it has a high strength to weight ratio.

Since the QSM team was still developing many of their mechanical systems, the preliminary chassis design has been kept simple to allow for future adaptation and refinement later in the project. As a result, the preliminary chassis design is largely based on the dimensional rules outlined by Shell.

Key Features

A ladder frame base reinforced with a honeycomb carbon fiber panel
A roll panel designed to withstand roll over forces in excessive of 700 N (competition requirement)
Front wheel panels to accommodate a hinge based front steering mechanism
Slotted rear panels that accommodate a solid rear axel
Front tube bumper with integrated crumple zone

While a monocoque frame may be structurally advantageous, it did not adhere to the team’s budgetary constraints. Instead, the design uses carbon fiber tubing attached at joints using connectors. This approach was more affordable as the QSM team had an existing inventory of 1-inch (inner diameter) carbon fiber tubing.

Stress and deformation analyses were conducted on the design to determine the system response when subjected to the weight of the passenger and the vehicle components. A static load analysis was deemed suitable as the vehicle operates at a low and relatively constant speed and does not experience significant forces during turning or acceleration. Based on the applied loads, a Solidworks simulation analysis resulted in low magnitudes of stress and deformation. A high safety factor was anticipated with this design due to the limited load that the chassis experiences during operation. While the orthotropic properties of the carbon fiber structure were included in the analysis, the accuracy of the results are still considered low due to the difficulty of modelling these properties with student accessible software.

Final Design

Since the QSM team had not established their hardpoints during the first iteration phase of the project, the refined design largely reflects the implementation of the hardpoints and other component mounting locations.

In the front end of the vehicle, additional vertical members were added to provide mounting locations for front wheel mounts, and horizontal top tubes to accommodate a steering column mount. To help support the doors, vertical members were added to each side. While the door hinges and latches mounts will attach to the exterior shell, the chassis must be able to provide additional support to the shell in these regions.

The rear of the vehicle contains majority of EV components for the vehicle as well as the vehicle’s trunk, a necessary feature for the competition. Since this region of the vehicle experiences significant loading, additional vertical and horizontal members were added to stiffen the chassis. These members create a shelf like structure that also helps separate the trunk from batteries and powertrain below.

The tubes are joined using aluminum modular connectors that allow for incremental angle changes of 45 degrees. While this restricts the possible shapes for the chassis, the aluminum connectors can be easily purchased in bulk from multiple vendors. While aluminum is a softer material that steel, based on a structural loading analysis it was deemed that aluminum connectors would provide a sufficient safety factor for this application due to the low magnitude of loading that each connector would experience. Carbon fiber connectors were briefly explored as they would greatly reduce the weight of chassis. However, off the shelf carbon connectors would have exceeded the chassis budget and building them in-house was considered too time consuming based on the team’s timeline. In the future, manufacturing custom carbon connectors should be considered in earlier stages of the design process.

Manufacturing

The roll panel and base plate are composed of a ½ inch Nida core honeycomb plate sandwiched between layers of carbon fiber. This allows the carbon fiber to be farther from the neutral axis of bending and provide greater resistance to bending as well as torsional rigidity. Both panels were manufactured in-house using a wet layup process. This process was completed on a large glass sheet to help ensure a smooth finish on the exposed carbon fiber surfaces. The photo to the right demonstrates how we used squeegees to ensure even distribution of the MGS epoxy resin within the carbon fiber sheets. This process was completed for each layer of fabric that was added. Once each layer had been added, the Nida core was placed on top, and the entire assembly was sealed using a vacuum bag. By placing it in a vacuumed state it helped remove air bubbles from between the layers and held the fabric still while the resin cured. After 24 hours of curing, the panel was removed from the vacuum bag, and the process was completed for the other side of the panel.

The completed panels are shown below.

To assemble the space frame, a spreadsheet was generated to detail the necessary tube lengths and cuts required, as well as the configuration angles for each joint. Since not all tubes provided by the team were stock length, a cutting length optimization approach was used to minimize material waste by finding cut combinations that best aligned with the tube lengths in the team’s inventory. The tubes and joints were then connected using epoxy resin.

Seen below is the frame in its assembled state prior to the installation of the roll panel and base plate.