Chassis Redesign

Since 2013, my team has made a carbon fiber monocoque chassis. Due to declining involvement since covid, the monocoque has been more and more unreasonable for our team. With this being my last year on the team (2024-2025), I want to leave my team in a spot where they are able to still manufacture a car, as well as get more test time. Because of the decline in involvement we only get test time at the beginning of the year with the prior years car, and get maybe one or two test days with the new car before we go to competition at the end of the season. For these reasons a space frame chassis is a much smarter choice for our team.

Time Study

In order to determine how much time would be saved for a space frame chassis vs monocoque chassis times were estimated from prior tool and monocoque manufacturing and other FSAE teams were asked how long their manufacturing time takes on average.

Monocoque Time Study Results

Tube Chassis Time Study Results

Overall Time Saved

Seating Position

To meet the rulebook, the car must be able to accommodate a 95th percentile male and 5th percentile female. Skitter Yaeger has a great article over anthropometry on designjudges.com which I referenced a lot during the design of the seating position. Based on his model of a 95th percentile male and NASA anthropometry and biometric data, I made a 6' tall model which represents the average height of our drivers and designed the seating position around this model.

6' tall driver model

Seating position

To verify the seating position we made a simple seating jig and adjusted everything to a comfortable position. The final seating position was set, and then the rest of the chassis was designed around this position.

Managing CG location

For optimal handling, 50/50 weight distribution is the goal. The design of the chassis depicts the CG location by changing the longitudinal placement of suspension in relation to all other components, as well as driver position in relation to the suspension since the driver is roughly 35% of the total mass of the vehicle.

Side view of CG

With components of the vehicle in an assembly, we can adjust the position of the CG by tweaking suspension placement or driver placement.

SES

Structural Equivalency Spreadsheet (SES) is a tool SAE uses to confirm that the chassis we construct is safe in the event of a crash. SES has a number of saftey requirements for tube size, placement, what to do and what not to do.

Tube size

FSAE ruleset on tube size by application

SES depicts what size tube you can use in certain regions of the vehicle, and for many regions the tube does not need to be any stronger and can not be any lighter. However, for size B tubing 1.25" OD 0.049" wall can be used instead of the standard 1"OD 0.065" wall. The larger 1.25" OD 0.049" tube is actually lighter than the 1" OD 0.065" wall and stiffer as well.

Tube comparison spreadsheet

Tube Configuration

Front bulkhead support, and Front hoop bracing configuration example per SES

Front bulkhead, front bulkhead support, front hoop, and front hoop bracing

Side impact structure configuration example per SES

Side impact structure

Main hoop, main hoop brace, and main hoop brace support example configuration per SES

Main hoop, main hoop brace, and main hoop brace support

Note the reclined angle of the main hoop, this frees up the rear rollover bounding box, as well as provides the driver more visibility to the side of the vehicle.

Torsional rigidity

Torsional rigidity is arguably the most important parameter of the chassis. If the torsional rigidity is low, the car will be slow to respond to driver inputs and suspension tuning will be inadequate. If the torsional rigidity is too high, it has unnecessary weight. Based on my vehicle dynamics tool, to achieve adequate LLTD tunability a torsional stiffness of at least 1200 ft*lb/deg is needed. After a baseline chassis configuration was designed, I put the model into ANSYS to determine the optimal tube sizing and placement to achieve a desired torsional stiffness and suspension hardpoint deflections. Any tube not included in SES is free game.

ANSYS plot of deflection

Torsional stiffness equations. The point of rotation is however at the front left tire contact point, not the center of the vehicle for how I have defined ANSYS.