Design sprint for new members for the Fall 2024 term!

(Acronyms):

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FSGP = Formula Sun Grand Prix (our ‘competition’ that occurs every year, along with):
ASC = American Solar Challenge (happens every 2 years, on even years, next is 2026)

MSXV = “M-S-Fifteen”, the car currently in the bay that we raced for FSGP 2024
MS15.5 = the ‘upgraded’ version of MSXV that we will bring to FSGP 2025

RSU = Rear Suspension Unit

DFM = Design for Manufacturing (ensure someone can actually create/machine what you design)
DFA = Design for Assembly (ensure someone can actually assembly 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, 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 2-motor drive, that’s entirely okay! Regardless, 1-wheel drive is somethhing 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 talentled new members joining, there’s no better project to introduce as a design sprint for this Fall 2025 terrm!

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 memberes to dynamics 😇

Timeline

WIP

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).

Why is changing the wheel position bad?

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 unnaccounted for during MSXV’s original design. We are literally not trying to re-invent 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

Some useful geometry

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 assmbly

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

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, wheel should spin in the correct way, and be rigid in all other ways.

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

… 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 caliper mount. If you’re feeling fancy, you can design your own mount too! Just make sure the caliper mounted the appropriate radial distance from the center of the rotor/ axis of rotation.

(optional) the hub shall provide suffficent 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 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 1B materials, 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 shall exhibit minimal rotational frictional

There should be minimal frictional 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 intertia (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 stackup.

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

(case where 2 parts is better than 1 part for DFM)

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

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…).

GitHub CAD Management

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

XV-RSU001-RSU Top Level.SLDASM

‘Top level assembly’ that contains the trailing arm in the context of the car, including the chassis and aerbody (hidden).
Since the mounting of the trailing arm is not changing, this assembly is not useful for the design sprint, but is good to reference

XV-RSU004-Trailing Arm Assm.SLDASM

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)

XV-RSU021-Motor Wheel Assm.SLDASM

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!

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