Goals of the Frame Subsystem (subject to change):
Take in hardpoint and design specifications from accompanying mechanical/electrical teams, and with said specifications, design a basic tubular (hopefully monocoque soon) frame to support those needs
To meet the required load cases, design cross-bracing structures to satisfy roll-cage and occupant cell load cases, maintain 1/1.1+ FOS, and minimize stress and displacement of the tubes
Design and fabricate a frame jig, to plan out and maintain the structure during welding and construction
Send out the finalized design to VR3 for mitering/tube-cutting
Weld the frame!
Enjoy life again :D
TLDR: build the damn frame 🫡
Necessary Statics/Solids/Materials/Structural Knowledge:
To build an effective frame, one must be knowledgeable in the realms of primarily Statics, Materials, and Solids, as well as knowing the optimal ways to build structures as a whole.
DISCLAIMER: I never completed any of these classes upon first drafting this work, so here’s the quick and dirty, cut-and-dry, condensed version of what you will need to know 👍
Statics
Pictured here is an explanation of moment
Firstly, tubes in a frame will experience various forces and moments when analyzing the load case requirements. (see the ASC regulations)
- Force → load applied at a point, causes stress (over an area), displacement, work, etc.
- Moment → measure of a force times a perpendicular distance from a point (I.e. tendency of a force to rotate around an axis of an arbitrary point)
Takeaways from this → we build a frame to take on different forces and moments, and furthermore the things that they cause
What happens to a tube when a load case is applied?
Inside each beam, internal forces are present, which can cause different effects (I.e compression → buckling, shear force→ shearing, bending moment → bending stress, etc.)
Axial Forces
- Axial forces (forces parallel to the center axis of the tube) can cause compression or tension in a tube, which can essentially be broken down into squeezing or stretching a tube
- Too much of either tension or compression can lead to either buckling or plastic deformation (see picture below/will talk about later)
- Tubes are the most resistant to axial forces as compared to shear forces or bending moments
- Buckling is more common in longer, less stiff tubes (I.e. shorter, stiffer tubing → less chance of buckling)
Shear Forces
- Shear forces (forces perpendicular to the center axis of the tube) can cause the shearing of a tube
- Think of it as a cutting force, like scissors cutting paper, a force perpendicular to the tube can sort of “cut” the tube in half
- Tubes are not as resistant to shear forces, as all the force has to do is overcome the wall thickness of a tube to shear it (as opposed to the entire length of a tube with Axial forces)
Bending Moments
- Bending moments (forces that cause a rotation around a fixed point, causing bending) can cause bending stress (this is the root of most all problems in frame design)
- Bending moments have a lot to do with load pathing (i.e. if your tubing and therefore your load path is a straight line, no bending moment is caused (forces are only axial), but as soon as you introduce an angled path, bending moments are caused)
Supports will come in later when you take Statics/are introduced to Ansys (FEA).
- All that matters is that different types of supports resist different types of forces/moments, which you can use to solve/isolate forces and understand the reactions happening in a system
Materials
Every material is made up of a crystal structure (crystal defects, a unique organization of atoms, etc.) which leads to materials with unique/different properties (i.e. this is how you get alloys, a mixture of chemical elements with at least one metal)
- You can look more into this on your own, but essentially, we pick a tube material based on its makeup and its properties (weight, density, yield strength, Young’s modulus, etc.)
- We have previously used 6361 aluminum alloy and 4130 chromoly steel.
What happens to a tube when a load case is applied?
This is a stress-strain curve. All you need to know is:
- The graph has stress on the y-axis, strain on the x-axis
- The elastic region is where the material can have stress applied (elastic deformation) and the process is reversible (the material can go back to its original length)
- The elastic line on the graph is linear because the stiffness of material in this region is like a spring (everything in every situation is like a spring from here on out, remember this lmao)
- The plastic region is where the material has stress applied (plastic deformation) and the stress overcomes the energy required to rupture bonds, which is an irreversible process
- Two Important things: Yield Strength and Ultimate Strength
- Yield strength is the maximum stress a material can absorb before plastically deforming
- Ultimate strength is the maximum stress a material can absorb before fracturing (breaking)
- I’ll talk about this later on in the guide when we get to welding, but essentially, just know that we mainly design things around the yield/ultimate strength.
Solids
What happens to a tube when a load case is applied?
There are 4 main types of stresses each tube will encounter. Those include:
- Normal Stress: Stress that acts perpendicular to the cross-section of interest (compression, tension, or rather axial force)]
- Shear Stress: Stress that acts tangential (parallel) to the cross-section of interest (like a cutting force)
- A common application of this is with bearing stress on a bolt, which leads to shearing
- Torsional Stress: A shear stress that is caused by a moment about the longitudinal axis of the tube (think of a twisting force)
- Bending Stress (THIS IS THE BAD ONE): Stress caused by a moment that has tension and compressive stress at each end (think of the cantilever beam problem below
- Also, bending stress is related to moment of inertia (you’ll learn this in Physics I hopefully, but its basically related to a tube’s stiffness/inertia/resistance to movement)
Some Necessary Basic Structural Knowledge
Triangles - Triangles are the most stable simple geometric structure. Without triangulation, squares have no lateral support. Triangulate everything you can. This principle should be applied as much as possible to a chassis or cage design. Every tube should be one leg of a triangle whenever possible. This is especially true with the primary structural tubes.
Bends - Avoid bends. The strength of a roll cage is based on compression and tension within the structure. Bending reduces both no mater where you do it. Some are unavoidable in a design but they should be minimized. A straight line is more rigid than a curved one. However, it's not always practical to have your roll bar tubes run in a straight line. Bending the tube results in a reduction in overall strength. If you have to use a bend, add bracing to compensate for the bends. Bends should never be mid-span, or unsupported. The apex of a bend should be a node point or junction for at least one other tube and gusseted unless several tubes meet at the node.
T-Junctions -Â T-Junctions are when one tube dead ends into another. This should be avoided whenever possible because the dead end tube could apply force to the tube it dead ended into and cause it to bend.
Imagine a force on the middle tube. The axial forces in the middle tubing would cause shear AND bending stresses on the other tube (because the middle tube is perpendicular to the other tube)
Some other useful tidbits
Simple Bending or Pure Bending A beam or a part of it is said to be in a state of pure bending when it bends under the action of uniform/constant bending moment, without any shear force. Alternatively, a portion of a beam is said to be in a state of simple bending or pure bending when the shear force over that portion is zero. In that case, there is no chance of shear stress in the beam. However, the stress that will propagate in the beam as a result will be known as normal stress.
Design Tips:
- Start by utilizing a combination of 2D and 3D sketches (separate the different parts of the frame!!!)
- This helps make the differences in each part of the frame distinct and separate so that errors do not become prevalent if changes need to be made.
Weldments:
This file should be read before any weldments take place, as it showcases various weldment use cases, common mistakes, and the proper way to use weldments if they (VR3) are to manufacture/cut the tubes
- To actually create the tube profiles of the frame, SolidWorks weldments are used
- BEFORE ADDING STRUCTURAL MEMBERS, DOWNLOAD VR3’s CUSTOM WELDMENT PROFILES (VR3 is our main supplier of miter/cut tubing)
Common Mistakes/Misconceptions (specifically with trimming tubes):
The “Trim Tube” tool is used to cut excess tubing off of your weldment (i.e. to ensure there is no intersecting geometry between frame tubes)
- When cutting a tube, ensure both “Allow Extension” are turned off, and the “Bodies” option is selected
- “Face/plane” is only utilized when one side of the tube is being cut (i.e. the right side is untouched but the left side is being trimmed, see VR3 docs for more details)
“Allow Extension” is fairly self-explanatory. For “Bodies to be Trimmed”, the tube that will be trimmed is extended to meet the other tubes that are the trimming boundary (rarely needed)
- For the “Trimming Boundary” section, the tubes that are used to trim the other bodies is infinitely extended, this is almost always useless/bad in our case
I.E. The left is a good case with “Allow Extension” turned off, the right is bad and will cause many issues later on ( I will talk about zero-thickness geometry later )
Interference Detection:
Before moving to Ansys Workbench/SolidWorks simming, MAKE SURE to utilize the interference tool as it ensures that tubes are not intersecting each other (i.e. the tubes are properly trimmed and connected)
- If there are interferences, that most likely means you failed to select a trimming boundary tube
Zero Thickness Geometry:
- Zero thickness geometry occurs when there is a gap between tubes, most likely caused by improper trimming/connecting tubing
To fix this, ensure that “Allow Extension” is turned off for the trimming boundary, which makes sure that the tube that is trimmed is not overcut.
Step-by-step guide
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