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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 pressure and by extension 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)
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- 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)
Torsional Forces
- Torsional (or twisting) forces occur when torque is applied about the longitudinal axis of a part
- A good way to visualize this would be when the driver takes a turn, where one wheel must travel a farther distance than the other, meaning there is a differential in the forces on either side, causing a torque
Supports will come in later when you take Statics/are introduced to Ansys (FEA).
- Supports assume infinite stiffness in specific degrees of freedom. Some supports (like fixed) have reactionary forces and moments in all DOF, some like (roller and pinned) have reactionary forces and moments in only specific DOF
- 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
Two Force Members
- A two force member Supports are integral to setting up a simulation right. If you have too many constraints, your sim will be too stiff and you will have inaccurate results. Too little constraints, same problem, inaccurate results, that can cause you to falsely presume your part is safe/effective.
Two Force Members
- A two force member in our case would be a tube with forces only acting in two locations, with the forces being equal opposite and colinear. Two forces are
Truss Structures
- In our case, a truss structure is structure made to take high loads while utilizing 2 force members, thus taking high axial loads, while minimizing bending loads.
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How to start a simulation
- After opening Ansys Mechanical, create a Static Structural project, and import your parasolid (.x_t) file into the geometry slot.
- Next, open the model and wait for Ansys Mechanical to launch.
- When you get into Ansys Mechanical, ensure that your geometry has the proper material applied (this affects yield and ultimate strength, force propagation, and essentially all your results)
Meshing
- Now when it comes to meshing in Ansys, this is really a key feature where Ansys shines above standard SolidWorks FEA, in terms of the vast amount of settings and tweakable options available to you for quality mesh refinement. However, having all these options means that if you don’t know how to use them properly, you can run into a lot of headaches.
- That being said, typically using a 3mm automatically generated mesh can give you decent results, although if you know how to/want to get the best, most accurate results, please look deeper into sweep meshing, tetrahedral meshing, etc.
- IK this section is pretty lacking, that is solely because as of writing this, I do not have enough experience to confidently lead any reader in the right direction myself. However, anything is possible with enough research and practice, so I encourage you to get out there and start meshing :D
- This is an excellent resource made by the now LHR Combustion Body Lead, Ryan Gretta
Supports
- Supports are pretty easy to grasp IMO, basically what they do is properly hold your geometry so that when you simulate a load case, it doesn’t just move and take 0 force. Supports make your simulations realistic.
- That being said, we typically opt for a type of fixed support, specifically located in 4 areas of the car that mimic the tire contact patches
In my experience, I’ve found the supports used in the bottom picture (remote displacement) as opposed to the top (fixed supports) to be more accurate and useful for results because:
- A fixed support makes it so the selected bodies do not deform or move. This is not optimal because the actual frame WILL deform, which makes these results partially inaccurate. Hence, a displacement or remote displacement will accurately represent the deformation of the frame tubing, as well as properly propagate the forces throughout the selected tube.
- Additionally, a remote displacement allows you to utilize a specific coordinate to represent where the forces/deformation should propagate. In this case, though we selected the bottom suspension box tubing, the remote displacement mimics the location of the actual tire contact patch, which is more accurate to where the actual load case of a crash would propagate.
Otherwise, supports are pretty simple, just support the four tire contact patch locations, and you are good to go.
Forces/Load Cases
- Again this is fairly simple, all you need to do is apply a force at the specific place the ASC guidelines want it applied. The only “hard” part of this process is understanding the actual amounts of force you should be applying.
- For instance, a “5g” load case does not just mean five times the force of gravity on the frame, but rather five times the force of gravity relative to the weight of the entire car. This means you need a mass in kg, multiply by 9.81 (gravitational constant), and then multiply by 5.
- Additionally, when a load case has multiple components, make sure that your force in Ansys is “defined by” components, not vectors.
- Lastly, if a load case is at an angle, for example, 30 degrees from the horizontal, all this requires is the use of simple cos and sin functions. Pretty straight forward
Solution and getting your sim results
- Now all that’s left to do is add your solution result plots and run the simulation.
- Per ASC guidelines, you need to have the following solutions when you screenshot/document your passing sim results:
- Equivalent (von-mises) stress
- Maximum principal stress
- Displacement
- Factor of Safety (this does not need to be documented but it makes analyzing your results a lot easier)
- Now when viewing your results, keep these in mind when checking if your design is successful/validated and passing:
- When viewing the occupant cell sims, does the frame displace more than 25mm? Is the equivalent or maximum principal stress over the ultimate tensile strength of the material? Is the factor of safety ever below 1? If yes to any, you have failed these sims.
- When viewing roll cage sims, does the frame have eq or max principal stress over the yield strength (in the roll cage, anywhere above the rest of the chassis) or the ultimate strength (anywhere below the roll cage)? Is the factor of safety ever below 1.1 (in the roll cage) or 1.0 (below the roll cage)? If yes to any, you have failed these sims.
- Otherwise, if you pass the sims for every load case, your frame is finished, simulated, and validated. 👍
What to do if you fail
If you end up failing a load case, it’s back to the drawing board with your design.
When you end up changing your design to try again, it is best to analyze ALL LOAD CASES, and not just the one you are failing as changes in design AFFECT. EVERY. LOAD CASE.
- This sucks but it is the harsh reality, design changes can critically affect every load case and component of the car, so change designs strategically and cautiously
- This is why I gave you materials, solids, statics, and structural knowledge beforehand, so your changes are more educated decisions and not guesses
Making changes to the car can be as easy as changing the outer diameter or wall thickness of a tube
- Deciding between the two should be based on the stiffness (inertia) to weight ratio of the tubing. The overall goal of the car is to be as stiff as possible with the least amount of weight, therefore you want the greatest gain in stiffness with the smallest change in weight
- Additionally, a change like this should only be made if the load pathing of the structure is wrong or inoptimal.
On the other hand, if your load pathing needs to be changed, this is where cross bracing should be added/moved/removed altogether. Again, keep weight in mind, sometimes adding the biggest and strongest cross bracing can be overkill and just add unneeded weight.
Just keep in mind that frame design is an iterative process. Creating a successful design is completely different from creating an optimized design, as you can have a successful, passing design, without it being optimized for weight, stiffness, etc. This means you may have to run tens of hundreds of sims, you may have to stare at your design and try many different remedies to solve your problem. It all depends on the goal of the lead, the subsystem, and the team as a whole.
VR3 Ordering
- As mentioned before, VR3 is our place for mitering and acquiring all tubing. That being said, I won’t go into much detail, as they have everything documented themselves. Follow their guides and you will be fine, they are very accommodating and friendly.
Frame Jigging
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To start, we utilize a static structural analysis to validate our frame design, as we want to understand how the chassis will react to a load case as a static structure (so we are essentially saying the frame is not allowed to move, so that we can analyze the response as purely structural, and how specifically our design affects stress and deformation distributions).
For a static structural analysis, there are a few different parts that go into the analysis. First, the engineering data holds all the numerical stats of each material that can be utilized, which play a part in calculating the simulation results (think about all they constants and data you need for the equations you learned in Solids). Next, geometry holds the design file that you want to perform the simulation on. Additionally, you can prepare the design (make the design into beams, shells, etc.) by using Ansys SpaceClaim (I will go over this later). Then you have the model tab, that opens Ansys Mechanical, and is the main area we will use in Ansys.
Meshing
- Now when it comes to meshing in Ansys, this is really a key feature where Ansys shines above standard SolidWorks FEA, in terms of the vast amount of settings and tweakable options available to you for quality mesh refinement. However, having all these options means that if you don’t know how to use them properly, you can run into a lot of headaches.
- That being said, typically using a 3mm automatically generated mesh can give you decent results, although if you know how to/want to get the best, most accurate results, please look deeper into sweep meshing, tetrahedral meshing, etc.
- IK this section is pretty lacking, that is solely because as of writing this, I do not have enough experience to confidently lead any reader in the right direction myself. However, anything is possible with enough research and practice, so I encourage you to get out there and start meshing :D
- This is an excellent resource made by the now LHR Combustion Body Lead, Ryan Gretta
Supports
- Supports are pretty easy to grasp IMO, basically what they do is properly hold your geometry so that when you simulate a load case, it doesn’t just move and take 0 force. Supports make your simulations realistic.
- That being said, we typically opt for a type of fixed support, specifically located in 4 areas of the car that mimic the tire contact patches
In my experience, I’ve found the supports used in the bottom picture (remote displacement) as opposed to the top (fixed supports) to be more accurate and useful for results because:
- A fixed support makes it so the selected bodies do not deform or move. This is not optimal because the actual frame WILL deform, which makes these results partially inaccurate. Hence, a displacement or remote displacement will accurately represent the deformation of the frame tubing, as well as properly propagate the forces throughout the selected tube.
- Additionally, a remote displacement allows you to utilize a specific coordinate to represent where the forces/deformation should propagate. In this case, though we selected the bottom suspension box tubing, the remote displacement mimics the location of the actual tire contact patch, which is more accurate to where the actual load case of a crash would propagate.
Otherwise, supports are pretty simple, just support the four tire contact patch locations, and you are good to go.
Forces/Load Cases
- Again this is fairly simple, all you need to do is apply a force at the specific place the ASC guidelines want it applied. The only “hard” part of this process is understanding the actual amounts of force you should be applying.
- For instance, a “5g” load case does not just mean five times the force of gravity on the frame, but rather five times the force of gravity relative to the weight of the entire car. This means you need a mass in kg, multiply by 9.81 (gravitational constant), and then multiply by 5.
- Additionally, when a load case has multiple components, make sure that your force in Ansys is “defined by” components, not vectors.
- Lastly, if a load case is at an angle, for example, 30 degrees from the horizontal, all this requires is the use of simple cos and sin functions. Pretty straight forward
Solution and getting your sim results
- Now all that’s left to do is add your solution result plots and run the simulation.
- Per ASC guidelines, you need to have the following solutions when you screenshot/document your passing sim results:
- Equivalent (von-mises) stress
- Maximum principal stress
- Displacement
- Factor of Safety (this does not need to be documented but it makes analyzing your results a lot easier)
- Now when viewing your results, keep these in mind when checking if your design is successful/validated and passing:
- When viewing the occupant cell sims, does the frame displace more than 25mm? Is the equivalent or maximum principal stress over the ultimate tensile strength of the material? Is the factor of safety ever below 1? If yes to any, you have failed these sims.
- When viewing roll cage sims, does the frame have eq or max principal stress over the yield strength (in the roll cage, anywhere above the rest of the chassis) or the ultimate strength (anywhere below the roll cage)? Is the factor of safety ever below 1.1 (in the roll cage) or 1.0 (below the roll cage)? If yes to any, you have failed these sims.
- Otherwise, if you pass the sims for every load case, your frame is finished, simulated, and validated. 👍
What to do if you fail
If you end up failing a load case, it’s back to the drawing board with your design.
When you end up changing your design to try again, it is best to analyze ALL LOAD CASES, and not just the one you are failing as changes in design AFFECT. EVERY. LOAD CASE.
- This sucks but it is the harsh reality, design changes can critically affect every load case and component of the car, so change designs strategically and cautiously
- This is why I gave you materials, solids, statics, and structural knowledge beforehand, so your changes are more educated decisions and not guesses
Making changes to the car can be as easy as changing the outer diameter or wall thickness of a tube
- Deciding between the two should be based on the stiffness (inertia) to weight ratio of the tubing. The overall goal of the car is to be as stiff as possible with the least amount of weight, therefore you want the greatest gain in stiffness with the smallest change in weight
- Additionally, a change like this should only be made if the load pathing of the structure is wrong or inoptimal.
On the other hand, if your load pathing needs to be changed, this is where cross bracing should be added/moved/removed altogether. Again, keep weight in mind, sometimes adding the biggest and strongest cross bracing can be overkill and just add unneeded weight.
Just keep in mind that frame design is an iterative process. Creating a successful design is completely different from creating an optimized design, as you can have a successful, passing design, without it being optimized for weight, stiffness, etc. This means you may have to run tens of hundreds of sims, you may have to stare at your design and try many different remedies to solve your problem. It all depends on the goal of the lead, the subsystem, and the team as a whole.
...
VR3 Ordering
- As mentioned before, VR3 is our place for mitering and acquiring all tubing. That being said, I won’t go into much detail, as they have everything documented themselves. Follow their guides and you will be fine, they are very accommodating and friendly.
...
Frame Jigging
- In short, a frame jig is essentially an assembly of parts that properly support the tubing/suspension tabs/any welded part during the welding process.
- A lot of the writing composed in this section is from some of the lovely Combustion and Electric members who have helped guide and educate me and my members on the jigging process.
Making your frame is fundamentally an exercise in precision locating and weldment construction, your main considerations should be datums (a fixed starting point) and stiffness. The optical table is your only reference for both geometric and positional tolerances, ergo, every critical jig you make should reference something about the optical table or something about a tube that is located off of the optical table. There are three main frame categories, critical external, critical internal, and non-critical.
After we fully weld the frame, we do the hardpoints (suspension tabs, ergo tabs, etc.) to mitigate warping from the welding. When welding/jigging, you need to have a planned order -> table flush (bottom layer) to the 2nd and potentially 3rd level, roll hoops, everything else (cross-braces) building up.
How to design a frame jig
“Table flush jigs" or “frame hockey pucks” are cylindrical stock that we turn down and then have a hole in the middle to go into the optical table.
- These support the entire bottom level of the frame, which is the most critical part in welding the frame (i.e. if you start badly from the base, the entire frame will be bad, so support the bottom well and you will have better results overall)
After we fully weld the frame, we do the hardpoints (suspension tabs, ergo tabs, etc.) to mitigate warping from the welding. When welding/jigging, you need to have a planned order -> table flush (bottom layer) to the 2nd and potentially 3rd level, roll hoops, everything else (cross-braces) building up
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- badly from the base, the entire frame will be bad, so support the bottom well and you will have better results overall)
After we fully weld the frame, we do the hardpoints (suspension tabs, ergo tabs, etc.) to mitigate warping from the welding. When welding/jigging, you need to have a planned order -> table flush (bottom layer) to the 2nd and potentially 3rd level, roll hoops, everything else (cross-braces) building up
- Critical external features are parts of your frame that drive car parameters decoupled from the rest of the frame (hardpoints, roll hoops, headrest supports, etc.
- Critical internal features are parts of your frame that only rely on internal geometry (steering column, rear box, etc.) These features of course will matter externally, but it’s more important to locate them to critical mating components, which if done correctly will satisfy external constraints.
- Non-critical features are tubes that don’t matter and are only there to provide structure to your frame, these are your buffers that take up tolerance stacks as they propagate through your frame and are there to support your critical features when the jigs are gone. These are the only tubes in your frame where you should be filling gaps, in an ideal world.
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