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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 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 |
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.
Mass - Since it takes more energy to move an object with more mass, we want to minimize the mass of our car to minimize energy consumption. Therefore concepts that use less material will be better than those with more material.
Cost - We don’t run on an infinite budget, money is a limited resource. Since we can only spend money once, design that cost less will be better. Since you may not know much about materials and manufacturing methods, start off with “blocking” out the part you made. Bigger blocks are more expensive and smaller blocks are cheaper. The explanation is in the expansion below.
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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
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