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FSGP = Formula Sun Grand Prix (our ‘competition’ that occurs every year, along with): MSXV = “M-S-Fifteen”, the car currently in the bay that we raced for FSGP 2024 RSU = Rear Suspension Unit DFM = Design for Manufacturing (ensure someone can actually create/machine what you design) FEA = Finite Element Analysis (a tool we use to simulate how parts are stressed under loading) |
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Project Information
Motivation
MSXV was an SOV (Single Occupancy Vehicle) that had dual rear-wheel drive; that is, both wheels and RSUs had their own motor, that the wheel was mounted to. However, after attending FSGP 2024, we discovered that most SOVs had only one of their rear wheels powered by a motor, with the other one being ‘dead’ or ‘passive’. This is because driving 1 motor at 2p power is more efficient than driving 2 motors at once with 1p power each (source: trust me bro). Motors are also heavy, so replacing 1 motor with a passive wheel would theoretically improve the efficiency, and thus performance, of our car!
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In the image below, GREEN represents unconstrained degrees of freedom (how it’s ‘allowed’ to move), and YELLOW represents restricted degrees of freedom (how it ‘isn’t' allowed to move), all relative to the trailing arm. Basically, wheel should spin in the correct way, and be rigid in all other ways.
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Trailing Arm Attachment
The new motor hub must be able to mount to the trailing arm with NO MODIFICATIONS to the trailing arm itself (i.e. no drilling new holes into the trailing arm)
The trailing arms are annoying to remove and even more annoying to machine now that they’ve been made. We also don’t want to modify the arm in any way, such that it remains as strong as it was designed to be.
(optional) Re-using the MSXV motor mount would be ideal
Less machining involved, because the motor mount is a non-trivial piece to manufacture. Worse-case, you can make your own variation of the motor mount (you’ll have to sim it though!)
Brakes
The hub shall be able to mount a rotor of equal size and location as that of the motored wheel
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The most significant output of FEA is the maximum stress the part experiences, and where this maximum stress occurs. If the maximum stress is less than the yield stress, then theoretically, the part should never deform, and life is good. However, we wouldn’t be comfortable if the part just barely passes sims (e.g. yield stress = 240 MPa, max stress from FEA = 220 MPa). Thus, we introduce safety factor. Simply put, a safety factor of 2 means that the maximum stress from FEA must be at least 2x lower than the yield stress (e.g. in order for an aluminum part with yield stress = 240 MPa to ‘pass sims’, the max stress must be 120 MPa or less |
What forces will the car experience?
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this is complicated we will cover this in a future week 😇 As an estimate, a single rear wheel of the car can experience 1-2kN of force, so that gives you a ballpark of how much force we’re talking about. Anyway, don’t worry about this for now, you need do do concept design and then CAD before we even talk about running sims. |
Performance
The hub shall exhibit minimal rotational frictional
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Manufacturing this part in 1 piece is painful (requires lots of milling & CNC)  Instead, we can break it into two, much simplier pieces   And then weld the two pieces together |
Technical Resources
MSXV CAD
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Software to Download
SOLIDWORKS
You can get your SOLIDWORKS 2024 license from #me-announcements in Discord:
https://discord.com/channels/711290483317669961/1277404188690874401/1281497201754771497
Github
All our CAD is stored on Github (for now…).
MSXV CAD
In github, the folder that stores all relevent files this project is: GitHub\MSXV\Development\DYN\RSU
Please do NOT modify any of the MSXV CAD, and if you do, pls revert your changes and do NOT git push to the server 🙏 . We will share details later about how to add your work to github, but for now, if you wish to CAD, s save it somewhere else on your own computer (not in the github repository)
CAD Images & Descriptions
Name + Description | Picture | Details / Exploded View |
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‘Top level assembly’ that contains the trailing arm in the context of the car, including the chassis and aerbody (hidden). |
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Assembly that includes only the trailing arm, motor mount, motor, and brake caliper + mount. This is the assembly that will be most useful for analyzing the geometry and mounting of the previous design (less things going on compared to the top level RSU assm) |
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Assembly that contains everything from the motor mount to the tire (motor mount, rotor, motor, rim and tire) Probably the most important assembly, because your final product, the rear hub, is essentially going to be a variation of this! |
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Design Resources
Bearings
SKF is a good supplier of bearings: https://www.skf.com/us/products/rolling-bearings/principles-of-rolling-bearing-selection/bearing-selection-process
FSU
If you have access, look at the FSU