Frame Design Early Research

1) Early Design Considerations of the Frame

Ultimately, all design choices are to be driven by ASC regulations. So prior to starting the project, one should familiarize themselves with the competition regulations and everything that impacts the FRAME/ERGONOMICS in any way
https://www.americansolarchallenge.org/ASC/wp-content/uploads/2023/11/ASC2024-Regs-EXTERNAL-RELEASE-C.pdf (ASC Regs)
Examples may include
- Maximum vehicle dimensions
- Occupant space required clearences
- Roll cage construction regulations
- Required load cases
- Wheelbase to track width

2) Define main goals for the competition and create a chassis around that. i.e., solar car competitions are endurance-driven

Light weight to reduce work on the battery and rolling resistance
High stability to prevent roll overs (i.e., a low center of gravity is more stable)
Ergonomic comfort (i.e.,roll cage and occupant space must have enough clearence so driver can seat comfortably for 1,000s of miles

3) Generate and decide on high-level design choices that effect the team as a whole. (Body shape, wheel configuration, and frame type)

Body shape - important for aerodynamic performance and locations of driver/battery box. Ensure there are enough clearences for these major systems
- Monohull
  - More suitable for cross-country races
  - Tend to be very rear-biassed in weight (due to the solar array and battery box)
  - Long and narrow, arrow-like shape
  - Suspension geometry must be narrow enough to fit the track width and within the aeroshell package
  - Challenging to keep stable because of its shape
- Catamaran
  - Wider and bulkier
  - Weight is distrubuted front to back
  - More interior room
  - More stable and can’t roll over as easily
Wheel configuration - important for not only energy efficiency but vehicle stability
- 4-wheeled
  - Slightly less energy efficient than 3 wheels
  - More dynamically stable
  - Asymmetrical design with motor biassing left or right rear wheel
  - Simple suspension design
  - Wheels are consistently in contact with the ground, which permits even tire wear
  - Simple, stable handling
  - Traditional design with lots of design resources
- 3-Wheeled
  - More energy efficient, with less drag and rolling resistance
  - Less dynamically stable, especially at corners because of the lateral load transfer and rear wheel slippage
  - Symmetric design with simple rear wheel alignment
  - Complex suspension design for rear wheel (trailng/swing arm)
  - Aerodynamic package could be better with a narrower tear-drop profile
  - Need a wider track width and wheel base for stability
  - Need to balance COG and the position of driver/battery so that 60-70% of the weight shifts to front wheels during braking. I.e., we need to make sure most weight nearer to front axle
  - Vehicle handling is difficult
  - More innovative with less design resources
Frame Configuration - Important for weight-savings and strength, but must also take into account complexity, design time, manufacturing time, etc.
- Tubular Space Frame
  - Quick design and development
  - Cheap and less time-consuming to manufacture
  - Heavy with more rolling resistance
  - Design is adaptable and can be reiterated relatively easily
  - Mounting is less complex
  - Simulation is less complex (or should be) because of the consistent material properties of tubes
  - More jigging during manufacturing
- Composite Monocoque
  - Extensive time designing and developing
  - More expensive and time-consuming to manufacture
  - More lightweight and energy-efficient
  - Design must be completely finalized before manufacturing
  - Mounting suspension is complex
  - Simulation is complex because of the inconsitent material properties of composites
  - Less jigging during manufacturing
- Hybrid Monocoque
  - Relatively simple to design, but composite panelling adds complexity which adds research, adds time, etc
  - Can switch structural areas of heavy tubing with lightweight and rigid panels, decreasing weight
  - Complicates validation of load cases (i.e., empirical testing, FEA, both…?)
  - Kickstarts development of composite monocoque which drives the team forward from failed past designs

4) Define main goals for the team and create a chassis around that i.e., the chassis exists to meet the design and packaging requirements for all the major systems

Design to integrate with suspension
- Form the chassis around the hardpoints
- Settle on the desired chassis width and height based on what vehicle kinematics the team wants
Design to integrate with driver area/ergonomics
- Leave enough interior room for driver area and roll cage clearences, while still fully enveloping the driver safely (free movement of head)
- Enough room for the legs of the driver to fit in the car (maybe do a 3 level frame to seperate design contraints of suspension from the design constraints of ergonomics so frame height is not fixed
- Enough room for ergo components, including pedal box, dashboard, steering, ballasts, driver seat
Design to integrate with battery and motor controller
- Leave adequate space for the motor controller and battery box, including insertion and removal of battery, modularity of electrical components, etc.
Design to integrate with aeroshell
- Ensure the canopy and roll cage are designed in tandem so that the car is balanced and there is enough room for the driver
- Work with aeroshell when determinig the thickness of the chassis in various locations i.e., where will it be more narrow and where will it be wider

5) Define performance targets

Locate major systems to optimize vehicle stability
- Use weight estimates to determine a target center of gravity based on a predetermined track width and wheel base
  - For 3-wheeled, COG should be longitudinally located nearer to front axle so as to balance 60-70% of weight on front wheels during braking
  - Find ways to lower the COG height, i.e., suspension components lower to the ground, a lower frame height, type of driver we are aiming to design around, etc
  - Where along the chassis should the driver and battery/array area be so that the car is the most dynamically stable during driving, cornering, and braking conditions.

Resources:

Foundational book on everything solar car related, Chapters 9 and 10 are very chassis-centered sections but I would recommend reading more if you can

The Winning Solar Car Book.pdf

Documentation of solar car design

KU Solar Car History Part 2.5 – Astra’s Car Design

A brief rundown from another team of how to build a solar car

So You Want to Build a Solar Car...

Article of team who built a 3-wheeled chassis

Engineering a 3 Wheel Vehicle Chassis

Wikipedia on three-wheeled cars

Three-wheeler

Interesting article about a solar car team who pursued a composite chassis: