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
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
https://drive.google.com/file/d/1LSd4Qrnsn590R89NNFfRHFjNbKEwbnv1/view?usp=sharing
Documentation of solar car design
https://michaelsri.wordpress.com/2021/06/28/ku-solar-car-history-part-2-5-astras-car-design/
A brief rundown from another team of how to build a solar car
https://www.instructables.com/So-You-Want-to-Build-a-Solar-Car/
Article of team who built a 3-wheeled chassis
https://engineerdog.com/2015/09/09/engineering-a-3-wheel-vehicle-chassis/
Wikipedia on three-wheeled cars
Interesting article about a solar car team who pursued a composite chassis:
https://solarcar.fandom.com/wiki/UMNSVP_Composite_Chassis_Design#Introduction
Exit document from a former frame lead:
https://drive.google.com/drive/u/1/folders/1VDzbmwXF6Og9S13k-U8J58xsFS2Q89PF
Foundational design knowledge on structures, in the context of racing:
https://www.designjudges.com/articles/strong-stiff-light-structures
https://www.designjudges.com/articles/engineering-a-lighter-chassis-part-1
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