Solstice Rear Suspension System

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.

Functional Requirements

We started out by going through the competition regulations to collect rules applicable to our system. We then reviewed and sorted them into the project phases they were applicable to. For example, loading conditions matter throughout but fastener securment only comes into play later on. We then reanalyzed the regulation load cases and found that some (specifically the cornering case provided) were likely a significant underestimate of real-world conditions. We chose to reanalyze this by finding the maximum centrifugal force our previous car would need to incur before the tires would start to slip or it would incur a rollover. We know from competition that Gemini began to tip before the tires lost traction but the tires slid before rollover, suggesting both occur at similar levels of centrifugal force. We calculated that level of force to be 2.6 times higher than the competition loading case. Unlike Gemini, Solstice has a lower target center of gravity meaning the force required for a rollover would be higher. However, the tires and rims were set to remain the same meaning this new figure could be used for later analysis. 

The general design requirement for any SEVT system is to be manufacturable and serviceable by students. This means complex designs or intricate manufacturing methods are out of the picture. All systems also have to be lightweight to ensure vehicle efficiency.  We were also met with geometric constraints imposed by other systems. Although we intended to determine these as early as possible we tended to receive these sporadically, requiring frequent design tweaks.  

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 

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