Design sprint for new members for the Fall 2024 term!
Background Info
(Acronyms):
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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) |
Project Information
Motivation
MSXV was an SOV (Single Occupancy Vehicle) that had dual rear-wheel drive. That is, each
Why
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; 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!
Therefore, for MS15.5, we wish to replace one of our rear motors with a ‘rear hub' (or, as a more fun term, ‘dummy motor’). Even if we test this configuration and find that we want to go back to a 2-motor drive, that’s entirely okay! Regardless, 1-wheel drive is something we definitely want to test and collect data from, as this will help drive decisions for MS16 and onwards.
Seeing that FSGP 2025 is less than a year away, and with so many talented new members joining, there’s no better project to introduce as a design sprint for this Fall 2025 term!
Project Description
The Rear Hub Design Sprint is a project that ALL incoming/new dynamics members for the Fall 2024 term will work on, in groups! Each group will develop their own Rear Hub design, including the conceptual design, CAD, FEA and maybe even assembly.
The goal is that, by the end of the term (December), there is at least one viable design that we can start manufacturing in the new year to put onto MS15.5! (maybe we can even start manufacturing this term if you guys are really fast!)
There will be weekly in-person check-ins (Thursday 7:30pm in the bay), and each team will have their own thread in Discord in the #rear-hub-design-sprint on Discord to work on this project and ask for help from leads. The first meeting on Thurs Sept 12th will be a longer session to introduce this design sprint in detail and welcome all new members to dynamics 😇
Timeline
*WIP*
Date | Goal / Milestone |
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Thurs, Sept 12th | Introduce design sprint in detail, form groups, preliminary concept designs |
Thurs, Sept 19th | Review concept designs, finalize 1 idea, begin CAD |
Late September | Working on detailed CAD, component sourcing |
Late October | FEA |
Mid November | Finalize design |
Late November | Technical drawings + manufacturing sendout |
Requirements
Wheel Position
The wheel mounted on the Rear Hub must be in the exact same position (relative to the trailing arm) as the original design (using the motor)
(i.e. the location of the wheel in the car should not change when going from motor → rear hub)
This should be self-explanatory. We want MS15.5’s wheel locations to be the exact same as MSXV’s, with the only difference being that one of the motors was converted into this rear hub (a ‘dead’ motor).
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Basically, it just really messes with vehicle dynamics. The behaviour of the car and stress that components will take due to a weirdly-configured wheel will be weird and unaccounted for during MSXV’s original design. We are not trying to reinvent the wheel (placement) here, so keep it the exact same) |
Unfortunately, due to the geometry of the parts in CAD, there aren’t super beginner-friendly ways to find this distance. I have put one way to design around this below, but find whatever reference point you want to define the position of the wheel
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The point shown below is the origin of the XV-RSU021-Motor Wheel Assembly, also the same as the origin of the XV-WHE001-Rim in the assembly  Also, I hope this sketch helps, but from a side view, the circular wheel is ‘centered’ between the top and bottom face of the trailing arm (green), and also centered between the two bolts (red).  Either way, just make sure in your new CAD, the wheel perfectly overlaps with the wheel in the old CAD |
Weight Limitation
The rear hub + trailing arm assembly, in its entirety, should not weigh more than the original assembly’s weight with the electric motor.
The final trailing arm assembly (after your rear hub design has been added to the assembly) should not weigh more than the original assembly with a motor, because then it wouldn’t make sense to use it.
You should keep this in mind during your initial design but do not make it a priority. A lot of mass optimization usually happens during the FEA stage, where you can identify areas of low stress (not worried about it breaking there) and thus you can get rid of some material.
Symmetry
Ideally, the design of the rear hub should allow it to be mounted on either the left or right side trailing arm without any issues.
We are aiming to collect data to help guide our design for MS16. Being able to swap the motor between wheels can help us better understand how our lap times and efficiency are affected by having a motor on different sides of the car (when considering a track with more left or right-hand turns). The original motor mount is already symmetrical for both sides. Take a look at how the motor mount attachment to the trailing arm looks right now and try to replicate it.
Degrees of Freedom
The wheel shall only be allowed to spin about the lateral axis, and must be fixed in all other degrees of freedom, relative to the trailing arm
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, the 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 is involved, because the motor mount is a non-trivial piece to manufacture. Worst 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
… to be explained later
A brake caliper must be mounted sufficiently around the rotor
In an ideal world, we can re-use the previous rear brake calliper mount. If you’re feeling fancy, you can design your own mount too! Just make sure the caliper is mounted the appropriate radial distance from the center of the rotor/ axis of rotation.
(optional) the hub shall provide sufficient space to mount a new parking caliper and its associated mount
TBD, but nice to have extra mounting space
Strength
All new or affected components must be simulated to not yield under worst-case loading calculations with a minimum safety factor of 2X
This is what we mean by ‘FEA’, or ‘passes sims’ (simulations). There’s a lot to unpack here
What is FEA?
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FEA (Finite Element Analysis) is a type of analysis that solves the amount of stress that a part is experiencing at a given location.  See in the above example, the maximum stress occurs in the red section. |
Why FEA?
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We use FEA to ‘simulate’ and determine whether our parts are strong enough to withstand the harsh conditions and forces our car experiences. If the part is not strong enough, we have to change the design/geometry or change the material to make it stronger. If the part is way too strong, we can mass-optimize by reducing material. By regulations, all dynamics components have to be simulated and shown to pass worst-case loading! (explained later in this section) |
What is yield stress?
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you will learn more about this in your materials courses, but essentially, yield stress is the point at which a material will begin to permanently deform (think about bending a paper clip into a different shape). Yield stress (and all stress) is in units of Pascals, Pa = N/m^2, and for metals is in the MPa range. Every material has its own yield stress, with our common values being: 6061-T6 (Aluminum) = 240 MPa 4130 (Steel) = 460 MPa |
What is safety factor?
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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 should exhibit minimal rotational friction
There should be minimal friction losses that occur just due to the wheel spinning. This is a given, no one likes squeaky wheels.
The hub shall be as light as possible (while still passing FEA!)
Less weight is better for vehicle performance optimization, cost, and even for DFM/DFA.
Also, minimizing the weight of the rotating part reduces to polar mass moment of inertia (basically, wheel is easier to spin, meaning braking and acceleration are easier)
Fastener Regulations
All fasteners must comply with ASC2024 fastener regulations
See Section 10.4 (page 46) of: https://www.americansolarchallenge.org/ASC/wp-content/uploads/2023/11/ASC2024-Regs-EXTERNAL-RELEASE-C.pdf
The entire RSU, including this Rear Hub, is a ‘critical component’. Therefore, all fastened connections (usually bolt and nut) must comply with Section 10.4.B. Notable points are:
All bolted connections must be secured either with a flex-loc nut or castle nut
All bolts must have at least two full threads extending past the nut
All bolts used in ‘blind-hole’ applications must have safety wire
(these are mostly things that can be considered later in the design cycle, but I just put it here for completeness. We will check these for you when the time comes)
DFM/DFA
DFM: The rear hub should be designed with as simple (to machine) geometry as possible
Ideally, all new parts for the rear hub should able to be machined with only a mill and lathe (no CNC required). There is a good chance that we will have members machine these parts, so keep it simple!
DFA: The rear hub should be designed with as few components as possible
More parts in dynamics is generally worse, as it adds extra complexity during FEA, more parts to machine, more pain during assembly, and introduces more failure mechanisms and room for manufacturing tolerance stack-up.
The ONLY time where more components = better is when DFM drastically improves. (I.e. I could make the 1 part using CNC milling, or I could break it into 2 sections, each of which can be waterjet, and then welded together
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Manufacturing this part in 1 piece is painful and expensive (requires lots of milling & CNC)  Instead, we can break it into two, much simplier pieces   And then weld the two pieces together |
Technical Resources
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
If you guys aren’t on github yet, a pack-and-go zip file can be found here: https://drive.google.com/file/d/10Ao7uPcyRexNKk0FjUKKmdlSccdW6mjR/view?usp=drive_link
FSU pack-and-go zip file: https://drive.google.com/file/d/1TL4PUaPFDZDX2CsvD91Hg5oNJac4Yr6Q/view?usp=sharing
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
Bearing selection relies heavily on the expected force on the bearing itself, unfortunately for now we can’t give an exact value for this BUT when roughing out a design a good estimate would be to use bearings similar to what was chosen on the FSU (front suspension). One bearing had an OD of 62mm and ID of 30mm and the other had an OD of 55mm and ID of 30mm. Since we are expecting higher forces in the rear it would be wise to use these dimensions as the minimum for the rear hub design (e.g a OD 55mm bearing as the lowest to implement into your design).
FSU
If you have access, look at the FSU