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What is Solstice?
Solstice is a 2-seater solar-electric vehicle designed by the MIT Solar Electric Vehicle Team for the American Solar Challenge's MOV class (akin to cruisers in the World Solar Challenge). As a part of the competition, the vehicle must be able to complete 200 or 300 miles around a track and a 1,500-2,000 endurance rally across North America.
Timeline
Design work on Solstice started at the end of the Summer of 2024 and is set to conclude with manufacturing beginning in the Spring of 2025. The competition is scheduled for the Summer of 2026.
Previous System
We started with a review of the suspension system built for our previous vehicle Gemini. The consensus is that our previous "Lambda-Arm" system performed well but was overbuilt and heavy. The design also necessitated a very stiff spring and an expensive damper.
Suspension system from our previous vehicle Gemini.
Functional Requirements and Constraints
All actual numbers have been removed per team policy on sharing car specifications before race.
Initial Geometry
The decision over the type of suspension design to go with was made at a mechanical team level. Ultimately the collective decision was made to create leading/trailing arm designs for the front and rear due to design simplicity. The handling improvements of the double wishbone (the other main contender) were found to be negligible due to the vehicle's incredibly short suspension travel and the competition's speed limits.
We then moved on to designing and initial geometry and using hand-calcs to optimize it. Based on early calculations we found that the "lambda-arm" design used in Gemini induced higher loads into the chassis than a comparable "L-arm". However due to the high torque on the system during turning it would require a heavier system. Being that several past cars had issues with chassis loading the decision was made to go with an "L-arm design".
Due to the nature of trailing arm suspension, any spring used will become non-linear as it compresses. This behavior also had to be modeled. Once we had all the equations we used a MATLAB script to quickly optimize the remaining parameters. We also produced a table of reaction forces at the various mounting points for the chassis team.
Further Design
Design work continued on two competing designs due to different members of the project wanting to try different things. One is primarily made from aluminum with threaded fasteners connecting components and one from welded steel. I helped out with both of these and provided feedback as well as FEA data.
Analysis
FEA was done in Ansys Mechanical primarily using static structural. Contacts were used to simulate welds, revolute joints simulated bearings, and the chassis mounts were fixed to the ground where they met the chassis. Forces were applied to the road contact patch of a rigid wheel. von-Misses and principle stresses were then examined as well as reaction forces at the chassis.
The data and discussion with Alumni ultimately led us to conclude that it was a better idea to go with the steel design due to fatigue concerns and weight
FEA showed that the walls of the x-tube in an early steel design would buckle during cornering.
Increasing tube thickness fixed the issue.
Continued work
In the near future we plan to do the following:
Validate FEA results and find the approximate strength of our welds using Instron testing
Further fatigue and modal analysis in Ansys
Lightweighting and structural optimization
Start planning out the system's manufacturing
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