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Design Sprint Goals
This design sprint is intended to be a good starting point for new team members to learn about engineering concepts and how to develop a design from (nearly) the ground up. Other goals include:
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And the potentially more nuanced goal of this sprint is to design a functional trailing arm! Note that we’re going to be designing a trailing arm that will go on THE LEFT SIDE of the car. It isn’t important in the actual design, but makes the loading conditions easier to describe later on.
Topics Covered
Design Concepting
Computer-Aided Design
Design for Manufacturing (DFM)
*Design for Assembly (DFA)
Finite Element Analysis (FEA)
*Not guaranteed, but the opportunity is there!
Background Information
Let’s start at the basics, what is a trailing arm suspension? It’s commonly seen in the rear suspension of bikes as seen below.
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Now, back to the technical stuff. To answer a lot of these questions we need to set some design constraints.
Design Constraints
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You’ll probably learn some different terminology on this, but I’m using it as the values we know that we should design around. A big part of designing parts is finding number that you need to work around, but it’s very time consuming, so I’ve done the tedious stuff so you don’t! So, I’ll list them out here, and I’ll add in sections that show how I determined these values. You don’t need to know it, but I want to feed people’s curiosity where I can, and it stands to show a good example of documenting your design decisions.
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Loading Cases From the Wheel (must pass all four loading cases)
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Direction
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Case 1
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Case 2
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Case 3
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Case 4
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X
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2177.82 N
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2177.82 N
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-1569.6 N
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-1569.6 N
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Y
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-2177.82 N
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2177.82 N
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-2177.82 N
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2177.82 N
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Z
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4355.64 N
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4355.64 N
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4355.64 N
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4355.64 N
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CCW X
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-327.28 N*m
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885.76 N*m
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-327.28 N*m
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cases, 1-4 are important to sim, 0 is for reference)
*All cases have 2x safety factor, and have 2G bump applied as well!
# | RSU Load Case | Force X (N) | Force Y (N) | Force Z (N) | Moment X (Nm) | Moment Y (Nm) | Moment Z (Nm) |
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0 | Rest | 0.00 | 0.00 | 3789.43 | 242.94 | -11.44 | 0.00 |
1 | Accel + Right Turn | 1149.94 | -2299.88 | 4599.76 | -345.63 | -334.15 | -80.67 |
2 | Accel + Left Turn | 808.84 | 1617.67 | 3235.34 | 657.94 | -235.03 | -46.97 |
3 | Braking + Right Turn | -2074.22 | -2074.22 | 4148.45 | -311.71 | 565.14 | 126.71 |
4 | Braking + Left Turn | -1458.95 | 1458.95 | 2917.91 | 593.39 | 397.51 | 97.94 |
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RSU Coordinate system:
Positive x is rear to front
Positive y is right to left
Positive z is up
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left
Positive z is up
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** Force values in explanation may not be 100% accurate. See table above or spreadsheet linked at bottom of expanded section for up-to-date values. ** The regulations define the loading condition in terms of acceleration; 1G Steering, 1G Braking, 2G Bump (https://www.americansolarchallenge.org/ASC/wp-content/uploads/2021/12/ASC2024-Regs-EXTERNAL-RELEASE-A.pdf, Appendix F, F.2). We can see how load transfer effects the loading conditions on the wheels. Since the trailing arms are used on the rear of the car we’ll specifically look at those values.  The above screenshot is from a spreadsheet I developed that will calculate the the load distribution under a 1G brake and 1G steer. In a braking scenario there is more weight loaded to the front however. So I made some adjustments to the calculator and we can see that the rear should expect around 111 kg of mass.  So by the 2G bump case, we should expect an upward of force of around 2177.82 N How the steering and braking cases impact our loading conditions are through the friction between the tire and the ground. If the car was turning with a centripetal acceleration of 1G, it would require a force equal to it’s weight. This force would be supplied by the friction from the tire, which is calculated by the coefficient of static friction and the normal force. Based on generally accepted values, the coefficient has a value less than 1, which means it cannot produce a force equal to the the weight of the car. So now we need to make a decision. The car needs to be safe, but we don’t want unrealistic loading conditions either. Based on our priority to have a race-worthy car rather then a high performing car, we’ll assume the higher loading which means that the expected loading is:
There two additional to consider on top of this; the application of a safety factor, and the location of the loading. At the time of writing this, the highest safety factor that needed to be applied in the previous car was 2 and therefore will be applied to this sprint. However, if this changes I’ll add it to the end of this section. So the new loading conditions are:
By the regulations, the loading condition is applied to the contact path of the tire (where the tire makes contact with the road). Because of this separation between the application point and the wheel mounting point, there will also be moments generated around the wheel loading point. The unloaded diameter of the wheel is 557 mm, so, there will be a moment from the acceleration/brake condition and the steering conditions. The moment arm for the 2G bump case comes from the geometry of the wheel assembly which is measured to be 64.1105 mm. There is also pneumatic trail which would create more moments around the mounting point, which should not be neglected. Based on the tire specifications under Sources and some comparison to other rolling resistance coefficients, the rolling resistance coefficient given is likely not a unitless coefficient, but a measurement of the pneumatic trail as rolling resistance is the force required to overcome the moment created due to a non-uniform loading at the tire. Units are not provided, and thus will be assumed as millimeters. Meaning the pneumatic trail is 3.02 mm at it’s worst (larger moment arm, larger moment).  Basically from here calculate all the moments created, and you’ll see them at the top of the dock! Also, if you read this far please verify my calculations (I’m doing them at 2:30am), first person who does I’ll give a really cool sticker. See updated calculation spreadsheet below:
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Lastly, we’ll design with the following shock in mind:https://www.royaldistributing.com/bronco-rear-gas-shock-for-yamaha-au-04409.html
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Feature | Dimension |
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Travel (distance between full compression and extension) | 2 15/16 inches 74.6125 mm |
Length at Max Compression | 12 3/4 inches 323.85 mm |
Eye Diameter (hole to bolt the shock) | 12 mm |
Design Criteria
Earlier we looked at constraints, specific numbers that limit what solutions are possible. Criteria are a bit more continuous rather than restricted. The best way to explain it is to apply them to this sprint.
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We’ll be using subtractive manufacturing methods (think sculpting, removing stuff to leave the part you want) on the parts of this car (additive we’ll need sponsorships, and we don’t have any secured). You’ll need to have more material than the part you want when you make a part with subtractive manufacturing. So finding a way to enclose your design, or a piece of your design, into the smallest volume possible. When you buy the blocks of stock (material that hasn’t been shaped) you’ll find that smaller blocks tend to be cheaper than larger blocks of metal (if you chose to go with metal). So, as a very rough starting point, you can “block out” the volumes that your design would need and roughly estimate cost by exclusively looking at material. |
Timeline
Week 1 - Concepting
Sept 11 - Sept 17
The idea’s pretty simple, come up with ideas! In concepting we want to come up with as many possible solutions as possible that can solve the problem we have. In our case, the problem is how do we support the wheel given all the information in Design Constraints.
What also might be useful to consider is how to make the part. If you take the best design possible that can’t be manufactured to a machinist, they still can’t make it. We won’t have the training/info session on manufacturing methods until later in the sprint, so only consider it at this stage if you have the time.
Deliverables
2 - 3 Concepts with Sketches (Digital or on Paper)
Week 2 - Computer-Aided Design (CAD)
Sept 18 - Sept 24
Now that we have some ideas on how to solve our problem, we need to pick one to develop further. Developing multiple ideas in parallel is very time consuming and not advised, but we won’t judge if you want the extra practice.
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An extension to this part of the design sprint would be to make your design parametric. A parametric design means that the dimensions of the part are defined “externally” from the file so it can automatically update if something needs to be changed. In essence, it makes it easier to tweak your design by not needing to dig through your feature tree to change a dimension.
Deliverables
1 Concept in SolidWorks Fully Defined
Week 3 - Manufacturing Design (DFM)
Sept 26 - Oct 1
With a design digitized, it time to start thinking about how it’s going to be made. Like I mentioned before, a great design is actually pretty bad if it can’t be made. Now that you’ve learned about different manufacturing techniques, it’s time to start specifying material and manufacturing techniques.
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A hint/idea I’ll throw out there is to see if your design can be made using multiple parts. I’ll try to bring example trailing arms during the presentation, but hopefully you can consider your design not as a single, solid piece of metal, but smaller more manageable chunks. Notices how this reduces costs as well, smaller blocks of metal are cheaper per unit volume than larger pieces of metal.
Deliverables
Bill of Materials with a manufacturing plan along with any changes to your design in CAD.
Week 4 - Static Structural Simulations (FEA)
Oct 2 - Oct 8
We have a design we can make, but will it hold up to the forces we need it to? Again, the idea is simple, but the execution is a lot harder. Hopefully you’ll understand the basics of SolidWorks FEA to run the simulation, but if you’re having any trouble with it, feel free to reach out!
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If through a thickness the part is failing, make the section thicker to increase the cross-sectional area.
Deliverables
A passing simulation with a stress distribution that makes sense along with any changes to CAD.
Final Review - Oct 22nd
This is where we get to put all your work together and see what others came up with! Everyone’s solution to the problem will be different, and their approach is something you can learn from. What we’ll do for the final review is combine the small groups on each time slot to see more solutions!
The date is not a typo, but it’s considering reading week and midterms for most of you. Which also means that you don’t need to finish the FEA for the 8th. But, I imagine during reading week and midterm week you have better things to do than figure out why a simulation isn’t working, but I let that be up to you to figure out.
Info Session Schedule
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Sept 17th | *SolidWorks CAD | Rm 4417 |
Sept 24th | Manufacturing Methods | Rm 3052 |
Oct 1st | SolidWorks FEA | Rm 2004 |
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All will be happening from after Mech General (~1:15pm) to 2:30pm (hopefully). After the training session we’ll start doing reviews!
Review Schedule
We’ll be making small groups of around 3 people in which you will be paired up with either myself or Shangheethan. We’ll try to spend about 15 minutes on each of your designs, but we’re hoping it becomes a bit of a discussion on what the strengths were of designs and where there’s room for improvement.
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Sources
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Credits
This design sprint was heavily inspired by the one created by Aidan Lehal, Min Qian Lu, Kevin Bui, and Emily Guo! Big shoutout to them for the hard work they put in!
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