Frame Research Overview

Owned by Grace McAloon
Dec 03, 2024
7 min read

Frame 101: Guide to the Frame Subsystem

  • Goals

    • Take hardpoint and design specs and design a tubular frame

    • 1/1.1+ FOS, minimize stress and displacement of tubes

    • Validate using ANSYS

    • Design frame job

    • Send design to VR3

    • Weld using TIG welding principles

  • Statics

    • Forces & Moments

      • Axial Force

        • Parallel to the member

        • Cause tension or compression

        • Plastic deformation (necking) or buckling

        • Tubes most resistant to this

        • Short, stiffer tubing -> less chance of buckling

      • Shear Force

        • Perpendicular to the member

        • “Cutting force”

        • Tubes aren’t that resistent

      • Bending Moment

        • Rigid body bends

        • Rotation about a fixed point causes bending stress

      • Torsional Forces

        • Twisting forces result of torque applied

    • Supports

      • Assume infinite stiffness in specific degrees of freedom

      • Resist different forces and moments based on type

  • Materials

    • Stress

      • F/A

    • Strain

      • Change in length/Original Length

    • Elastic Modulus

      • Strain/Stress

      • Elastic region - linear

    • Yield Strength

      • Maximum stress a material can have before plastic deformation

      • FOS measured by this

    • Ultimate Strength

      • Maxiumum stress a material can have before fracture

  • Solids

    • Stresses

      • Normal

        • Perpendicular to cross-section (compression/tension)

      • Shear

        • Parallel to cross-section (bolt shearing)

      • Torsional

        • Shear stress caused by moment

        • Twisting force

      • Bending

        • BAD ONE

        • Caused by moment with tension and compressive stress

        • Calculated by bending moment, moment of inertia, and vertical distance

  • Triangles

    • Most stable

    • Create lateral support

  • Bends

    • Avoid if can

    • Results in reduction of overall strength

    • Add bracing to compensate for bends

  • T-Junctions

    • Avoided if can

    • When one tube dead ends another

    • Could cause bending

  • Design Tips

    • Separate different parts of the frame

    • Create the load cases contact points

    • Use VR3 design reqs immediately

    • Weldments

      • Trim Tube

        • Ensure both “Allow Extension” and “Bodies” is turned off

        • “Face/place” only utilized when one side of tube is being cut

    • Interference Detecion

      • To ensure there are no tubes intersecting\

      • Zero Thickness Geometry

        • Means there is a gap between tubes

        • Fix by ensure “Allow Extension” is turned off

    • Combine frame with combine tool and export as parasolid (.x_t)

  • How to FEA

    • How to start a simulation

      • Static Structural project and import parasolid file

      • Open model, wait to launch

      • Make sure proper material is applied

      • Meshing

        • More nodes the better/more accurate

      • Supports

        • Hold geometry to simulate a load case

        • Use a displacement or remote displacement to accurately represent deformation

        • Support four tire contact patch locations

      • Forces/Load Cases

        • 5g

        • Apply force to specific place ASC guidelines want, and document/screenshot

          • Equivalent (von-mises) stress

          • Maximum principal stress

          • Displacement

          • FOS

    • Solution

      • Frame can’t displace more than 25 mm

      • FOS can’t be below 1 (for roll-cage it’s 1.1)

      • What to do if fail

        • Analyze ALL load cases

  • Frame Jigging

    • Assembly of parts properly supporting the frame

    • How to design a frame jig

      • Table flush jobs support entire bottom level of frame

      • Build from ground up

      • Optical table used for constraining DOFs

      • Consider dynamics of joining process

        • Parts cool at fast rate in open air = martensitics formation (deformed shape) and weld distortion

          • Martensite = extremely hard and brittle phase

          • Reduce by increasing time takes for weld to cool

      • Most likely to see distortion when weld cools, consider heat transfer

      • Critical external features then critical internal

      • Simple stiffest jigs that restrict all 6 DOFs and prioritize accessibility

  • Welding

    • TIG welding means Tungsten Inert Gas Welding

      • Involves tungsten electrode generate arc to melt metal

      • Welder pedal -> arc generated by bridging gap between mtal and torch’s electrode

      • Puddle of molten metal joins it

    • AC - Aluminum, DC - Steel

    • Pre and Post-Flow

      • Pre-Flow refers to flow before arc

      • Post-Flow refers to flow after arc is stopped

    • Keep torch at 15-30 degree angle

 

Frame Design Early Research

  1. Early Design Considerations of the Frame

    1. Look at design regulations

      1. Maximum vehicle dimensions

      2. Occupant space required clearances

      3. Roll cage construction regulations

      4. Required load cases

      5. Wheelbase to track width

  2. Define main goals

    1. Light weight

    2. Low center of gravity stab;e

    3. Ergonomic comfort

  3. High-level design choices

    1. Body Shape

      1. Monohull

        1. More suitable for CX races

        2. Rear-biased in weight

        3. Long and narrow

        4. Shape hard to keep

      2. Catamatan

        1. Wider and bulkier

        2. Distributed weight

        3. Interior room

        4. Can’t run over as easy

    2. Wheel Configuration

      1. 4 wheeled

        1. Less energy efficient

        2. Dynaniacall stable

        3. Motor on left/right rear wheel

        4. Simple suspension design

        5. Tire wear

        6. Simple, stable handling

        7. Lots of design resources

      2. 3 wheeled

        1. More energy efficient

        2. Less dynamically stable (esp at corners)

        3. Symmetric design

        4. Complex suspension design

        5. Look better with narrower tear-drop profile

        6. Need wider track width and wheel base

        7. Balance COG and position of driver/battery so that 60-70 percent of weight shifts while braking

        8. Handling is difficult

        9. More innovative

    3. Fram configuration

      1. Tubular Space Frame

        1. Quick design

        2. Cheap and less time consuming

        3. Heavy

        4. Adaptable

        5. Mounting is complex

        6. Simulation less complex

        7. More jigging during manufacturing

      2. Compositive monocoque

        1. Extensive time

        2. Expensive and time-consuming

        3. Lightweight and energy efficient

        4. Mounting suspernion  complex

        5. Simulation complex

        6. Less jigging

      3. Hybrid

        1. Mixture

        2. Heavy structure turns light weight

        3. Complicates validation

        4. Starts development of composite monocoque

  4. Main goals

    1. Design for suspension

      1. Form around hardpoints

      2. Choose height and width

    2. Design for ergonomics

      1. Interior room and enveloping driver trade off

      2. Room for driver’s legs

      3. Room for ergo

    3. Design for battery and motor controller

      1. Leave space

    4. Design for aeroshell

      1. Canopy and roll cage are designed in tandem

      2. Thickness or chassis

  5. Performance targets

    1. Locate major systems to optimize stability

      1. Determine target center of gravity based on predetermined track width and wheel base

        1. 3 wheeled, COG longitudinally located nearer to front axle

        2. Ways to lower COG height

        3. Where driver and battery location to dynamically stable max

 

ASC 2024 Regulations

  • 10.3A: Occupant Cell

    • Roll cage

      • Structural cage encompassing driver from shoulders up

      • Can’t be made of composites, have to be metal

      • Needs to deflect body/array panels of car up and away from occupant

      • Front roll cage angled backward

    • Structural Chassis

      • Tubular frame/monocoque composite chassis/hybird encompassing occupant’s body and where suspension sys is connected

    • Occupant Cell

      • Combo of roll cage and structural chassis

      • Must provide documentation stating which part of car is occupant cell

      • Encompasses driver in all directions and chassis can’t interfere with cell at all

      • Protection must be documented

      • Preliminary sketch and description need to be submitted to ASC HQ

      • Helmet comes in contact then padding that absorbs energy and must be bonded and secured

      • Must be 50 mm of clearance in all directions between any member in occupant cell and helmets of occupants seated in norm driver position

      • Must be 30mm to clearance btw occupants helmet and padding

      • Any carbon fiber within 500 mm of center of occupants head and above occupants shoulders need shatter resistent fabric

    • Can’t deform more than 25mm and will not fail UTS

    • Must be a head restraint behind occupant’s head without use of cable ties, fabric straps, or temp attachments, must support head

  • 10.3C: Occupant Space

    • Occupant space for each upper torso is arc with 835 mm radius measured from hip point and projects forward 45 degrees from vertical, 25 degrees rearwards, and 7 degrees side-to-side from centerpoint of driver

    • Structure must lie outside of occupant space, only steering wheel, mirrors, seat backs, and head restraints can be inside

    • Driver’s head must be above and behind driver’s feet, seat must be approx constructed with solid base and back rest

  • Appendix F

    • Report Presentation

      • Submit report following format

    • Loading conditions

      • Suspension and steering systems

        • 1G turn, 2G bump, and 1G braking

        • Loads applied to wheel patch

    • Vehicle Impact Analysis

      • Specs

        • Describe vehicle frame, construction techniques, incl materials used, important dimensions and properties

        • List specific impact criteria adn sample calculations and computer output

      • Drawings

        • Structural drawings from top, front, side, rear, isometric

          • Driver location and orientation

          • All members considered “structural”

          • Locations of ballast and batteries

          • Locations of chassis hard points (points of attachment)

          • Calculated center of mass

        • Driver’s compartment from top, front, side

          • Driver location

          • Roll cage design and location

          • Location of structural members

          • Driver’s harness attachment points

        • Must contain isometric drawing of body and solar collector

        • Analysis

          • FEA

            • 3D elements should be used for all joints

            • Shell elements require a FOS of 1.4 or greater

        • Occupant Cell Impact

          • Bumper height 100mm, width of 600mm, and elevation 350mm

          • Can’t deform more than 25 mm and can’t exceed UTS

          • Load cases (5g)

            • Front

            • Rear

            • 3 side impact locations

        • Roll Cage Impact

          • Loading patch no more than 150mm diameter

          • Can’t exceed yield strength

          • Load cases

            • Combined loading (5g down, 4g backward, 1.5g lateral)

            • Sideways angled loading (5g at 30 degrees downward from horizontal)

            • Sideways angled loading (5g at 60 degrees downward from horizontal)

            • Sideways horizontal loading (5g at the top of the hoop)

            • Rearward horizontal loading (5g at the top of the hoop)

 

Roll Cage Notes

  • Bars are placed to share the load

  • Diagonal tubes prevent longitudinal loads, made it more rigid and stronger

  • Try to attach the horizontal and diagonal members as high up as possible

  • Asymmetrical roll cages aren’t recommended

  • Front and rear members of roll cages needs slope to deflect

  • Slope of roll cage bars of 15 degrees

 

Figure 1: Daybreak Car LHRS

Figure 2: Michigan Car ASC 2018

Figure 3: Michigan Car 2024

Figure 5: Meet University of Michigan Student Engineer Garrett Simard - Corporate Blog

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