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This will be an overview of everything I worked on during my time as suspension lead from 2021-2023 in descending order. 2023-2024 is a separate section as it would be too much to put all on one page. If you'd like more information, please contact me!
Tire Analysis
The tire is the most important part of the suspension as this is the only surface controlling the vehicle. Coming into my second year as suspension lead I knew that I wanted to change from the 18x7.5 R25B tire currently used for many reasons which I will go over below. My goal with the performance of the suspension is to be responsive, controllable, and maintain high lateral acceleration.
Hoosier is about the only supplier of FSAE tires however they do offer a wide range of sizes and a few different compounds. One of which that interested me is the 16x7.5-10 size due to its low mass, low moment of inertia, and theoretically should be a more responsive tire due to shorter sidewall although the tire data will prove this.
To start, a simple comparison of some of the choices from Hoosier offers is done. The 16x7.5-10 appears to be the better tire over the current 18x7.5-10. Therefore I further analyzed the performance of each tire using FSAE Tire Test Consortium (TTC) data.
Slip angle vs lateral force raw tire data of the LC0 and R20 compound for the 16x7.5-10 tire
Slip angle vs lateral force fitted using Magic Formula 5.2 tire model (16x7.5-10 in red, 18x7.5-10 in blue)
Slip angle vs instantaneous cornering stiffness (16x7.5-10 in red, 18x7.5-10 in blue)
First I compared the two compounds offered for the 16x7.5-10. To reduce time, I did not fit a curve to the raw data as I can see what I need to from the graph. The R20 achieves a higher lateral force as well as a steeper slope which is the instantaneous cornering stiffness, this means the R20 is a more responsive tire while achieving a higher lateral force at higher normal loads. Since our car weights 450ish lbs, high normal loads are seen. Therefore the R20 compound is what I further analyzed.
Slip angle and lateral force are then fitted using the Magic Formula 5.2 tire model for the 16x7.5-10 R20 and the 18x7.5-10 R25B. The 18x7.5-10 does achieve a higher lateral force everywhere, although this is something I am willing to give up for responsiveness and lower mass.
The other major graph to consider is slip angle vs instantaneous cornering stiffness. This tells the story of the tires behavior greatly. A higher cornering stiffness means it is more responsive, and a shallow slope means a controllable tire. The 16x7.5-10 is more responsive at low normal loads, but less responsive at high normal loads. Meaning it will turn in better, but will have less turn in mid corner. The 16x7.5-10 is also a more controllable tire everywhere.
From the graphs, the 16x7.5-10 R20 aligns with the goals of the suspension better than the 18x7.5-10. Not to mention the 8lb mass decrease for an unsprung rotating part.
Kinematics
Kinematics of the suspension control the movements of the tire through the travel of the suspension. The goal here is to maximize tire contact patch. All of these parameters were set based on TTC data, packaging constraints, and driver feel. All kinematics were modeled using Lotus SHARK. I wont share the exact graphs of everything here but I would be happy to talk about them!
Reducing Compliance
One thing that can plague a good suspension design is compliance. If your suspensions points are moving around as you take a corner then all the work you have done to optimize kinematics is worthless. The biggest thing that can contribute to compliance is toe base. Toe base refers to the length of moment arm the tie rod has to keep the wheel and tire at the static/dynamic toe setting.
Here you can see a good example of rear suspension compliance in an FSAE car. (Not my picture)
Old (21-22) rear upright design
The prior year (21-22) uprights had a very small toe base, causing unwanted toe change during cornering. The rear outside suspension would deflect, toe out causing oversteer, driver would correct, and then the cycle would start over again. As you can imagine, this feels very weird in car and is not confidence inspiring, nor fast.
New (22-23) rear upright design
The new design has as large of a toe base as possible to package inside the wheel. Although, I discovered a flaw in this while mounting the inner tab for the tie rod onto the chassis. More on this in the suspension (23-24) project page.
Adding ARB's
An anti roll bar (ARB) is used to tune the roll mode of the front or rear axle separate from the heave mode. The 2020-2021 car did not have anti roll bars (although there were provisions for a front ARB), therefore to achieve an adequate roll stiffness, very heavy weight springs were used which increases the ride stiffness. Increasing the ride stiffness does not allow the suspension to absorb irregularities in the road causing a harsh ride, and less grip due to huge normal load variations.
2021-2022 rear ARB setup
Adding a front ARB is easy as everything was there to add one. However, there was no foresight of adding a rear ARB. As this was my first year as suspension lead, I did not want to take on a full redesign. So, I packaged a rear ARB into the existing jack bar.
Lifting FEA
ARB force FEA
Final Product
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