Design sprint for new members for the Fall 2024 term!

(Acronyms):

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 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*

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).

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 assembly

Open

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).

Open

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.

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?

Why FEA?

What is yield stress?

What is safety factor?

What forces will the car experience?

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

Technical Resources

Software to Download

SOLIDWORKS

You can get your SOLIDWORKS 2024 license from #me-announcements in Discord:
Discord - Group Chat That’s All Fun & Games

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

If you guys aren’t on github yet, a pack-and-go zip file can be found here: MSXV-RSU.zip

FSU pack-and-go zip file: FSU Assem.zip

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

Design Resources

Bearings

SKF is a good supplier of bearings: SKF

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