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Being apart of a bigger project while taking on the responsibility of an aspect of it.

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)

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In the top view show above the system becomes a bit more complex. We can see an additional branch/arm the connects to the pivot axis. This is to help distribute forces which you’ll see soon enough.

At this point, start thinking about what can be changed in the concept. For instance:

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I’ll quickly note here that only one of the rear wheels will be powered (ie. have the motor in it). I want to create two trailing arms which are mirrors of one another, and for the side that will not have the motor we will create a spacer to make up the distance and mounting face to the

At this point, start thinking about what can be changed in the concept. For instance:

• How far does the wheel need to be from the pivot point?

• How far does the shock mount need to be from the pivot point?

• Do all the points need to lie in the same line? (Can the shock be moved vertically?)

• Does trailing arm need to be horizontal? (At what angle is it optimized?)

If you’re getting a bit overwhelmed, that’s normal! This is likely the first time you’re approaching a design problem in this way, so it takes some time to get used to, and also feel free to ask questions! Don’t think you’re bothering us, when you ask questions, you show you’re interested and we want to help build that interest!

Now, back to the technical stuff. To answer a lot of these questions we need to set some design constraintconstraints.

Design Constraints

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Design-Constraints Design-Constraints

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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Maximum Angular Displacement for the Trailing Arm: 10 degrees from the rest position

Angle between the Pivot and the Wheel Mount: Greater than Zero at Max Extension

Loading From the Wheel

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title Reasoning
The regulations define the loading condition in terms of acceleration; 1G Steering, 1G Braking, 2G Bump. 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. Image Removed 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. Image Removed 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. NEEDS A DECISIONthere is a backwards force applied to the wheel mounting point. This will create a clockwise moment around the pivot point, pushing the wheel down. This will contribute to an increase in downward force on the wheel. This will ideally distribute the load transfer under braking better than if it was below the horizontal. Realistically, the moment generated will be negligible (assuming small angles) to the rest of the load transfer seen by the loading condition, and therefore we will not adjust the mass proportion present on the back.

Loading Cases From the Wheel (must pass all four loading cases)

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title Reasoning
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. Image Added 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. Image Added 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: Loading Condition Load Notes 2G Bump 2177.82 N Upward Will be applied in all simulations 1G Acceleration 1088.91 N Forward Will create higher compressive stresses in the part 1G Brake 784.8 N Backward Will create tensile stresses in the part 1G Steer Left 1088.91 N Inward 1G Steer Right 1088.91 N Outward 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: Loading Condition Load 2G Bump 4355.64 N Upward 1G Acceleration 2177.82 N Forward 1G Brake 1569.6 N Backward 1G Steer Left 2177.82 N Inward 1G Steer Right 2177.82 N Outward 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 NEEDS TBD. 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). NEEDS A DECISION

At max compression the shock should be tangent to a circle concentric to the pivot point.

GET THE MOUNTING PATTERN FOR THE MOTOR

Timeline

Week 1 - Concepting

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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 INSERT SECTION HERE 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.

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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 feature 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.

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I hope at this stage you are looking at your design to make changes to make it easier to manufacture. We can can’t purely rely on “if there’s a will, there’s a way” when we want to make something, and this means something need to give, either our bank account, or our design. We rather our design whenever possible.

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If you have a preference on who you’re paired up with for your group be sure to let us know! We’ll probably create a form or a Lettuce Meet link to schedule the sessions. TBA.

*On Oct 22nd I’ll be at a wedding for most of the day, but as it is a final review, Shangheethan can lead the discussion on the different designs so you can see what everyone created.

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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