Purpose: This page is intended to give you an idea on things to look out for when starting the design process for a new vehicle chassis.

Understanding Frame Constraints

The goal of the structural frame is to create a structure to tie all subsystems of the vehicle together while creating an enclosure to protect the driver in the emergent case of a collision or roll over.

Occupant Safety

The Number 1 Priority of the chassis is to protect the driver. For as much as everyone hopes the protection systems of the chassis aren’t used, there have been several instances in which solar vehicles flip over or loose control due to their nimble nature. Earlier this year at ASC 2024, 3-5 vehicles rolled over or had a minor tumble during the FSGP track event. With the most notable case being Michigan State’s car doing a double roll over during their first track day. SAFETY IS NO JOKE!

That being said, with chassis design, no corners are to be cut. For every stage of the design, thoroughly validate your designs, sims, etc… Optimizing the design is great, but if it means loosing out on structural integrity, that’s no bueno.

Subsystem Constraints

In terms of actual design, the frame is consistent of 3 major segments. First is the occupant cell protection systems, second is the dynamics and battery subframes, and third is the aero-chassis integration systems

1. Occupant Cell Protection

This section describes the frame surrounding the driver. That is, not only the roll cage, but also the tubes/frame protecting the remainder of the driver’s limbs. In addition to the surrounding tubes, the necessary tabs/mounting points required for the seat, seatbelt, and other driver interfacing systems is to be taken into account.

2. Dynamics and Battery Subrames

Dynamics and battery box are some of the subsystems that will require the most care in the early design stages of this vehicle. Ensuring that the chassis has package protected for these systems will be crucial for the development of these attributes in the vehicle design stage.

3. Aero-chassis Integration

The final attribute to be considered is how the internal structures tie to the outside of the vehicle. Although number 3, this section is not to be neglected. A well done aero-chassis integration determines the success of the remaining vehicle integration

Spaceframe/Tube Frame Design Approach

Structural Planes Approach

The planes approach is the design strategy that was used to execute the design for MSXV. Note that this is what worked for me as a chassis designer, but doesn’t have to be the only method you used to design the future frame.

Before designing any of the frame, the occupant cell and vehicle criteria file is created and shown below. Note that this file will help parametrize your design and help your design auto update if the driver position ever changes relative to the rest of the vehicle.

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Figure 1: Vehicle Top Level Constraints File

The main premise of this approach is to define a sideview profile of the chassis that will outline your structural planes. The most critical aspect of this design is to define your occupant cell and the roll hoop positioning. Try to visualize the egress path of the driver, use bounding boxes from the dynamics team, battery team, etc… to establish where tubes will need to go.

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Figure 2: MSXV Chassis Side Profile Definition

Similarly, the bottom profile for the chassis is established using the driver thickness, wheel positions, etc…

Notice how only half of the vehicle frame was established. It’s easier to design half of the frame then mirror than to do double the work. Additionally, notice how the roll hoops are tilted at a minimum of 15 degrees. This tilt was a regulatory requirement to deflect any panels that may travel towards the driver in the case of an emergency or accident.

The next step is to better establish the structural wireframe as follows. Position your sketches to account for current and future subsystems.

After developing your main planes, create mounting points, add structural reinforcements, etc… around the entire vehicle. Reinforcements are one of the systems that will take a fair bit of optimizing, but will not be a difficult design process once you get the hang of creating 2D and 3D sketches.

Once the full vehicle wireframe, the more meticulous step is going through the weldments generation process. This process will require a fair bit of attention and DFM/DFA focus as the way each tube is trimmed will dictate how it gets assembles.

Note that in SolidWorks there is a very beneficial tool called the “Interference Detection”. This tool allows you to take a final sweep of your part to check if any of your bodies are still overlapping/untrimmed.

Future Design Approach and Takeaways

The planes approach is a robust approach to designing the vehicle structural frame. However, more emphasis should be put on establishing vehicle hard points prior to designing the tube frame. One of the downfalls of the MSXV design is that the battery box dimensions was constrained by the chassis design. Occupant cell, dynamics systems, and battery box take precedence in setting the constraint on the design of the chassis. Focus on establishing a proper package protected space from each subsystem (aero is last to take priority, don’t neglect it, but don’t set it first).

The other aspect of future design is truly honing in on where the dynamics systems are going to reside and the possibility of developing tabs instead of designing for the usage of clevises.

That being said, everyone has a slightly different approach. Feel free to experiment with ways that can make the car’s design better.