Tube Frame
A tube frame or space frame is a chassis constructed from rigid truss members attached in a three-dimensional structure with the body panels having little or no structural function. Stiffness is maximised by triangulating the tubing to ensure that the tubes are not loaded in bending.
Contents
Design
Designing the tube frame is a compromise between structural rigidity, weight and manufacturing complexity.
Triangulation
Proper triangulation makes sure that the tubes are not loaded in bending. Full triangulation is probably impossible given that the driver like, can't have a frame member through their torso, but please try?
Manufacturing
Many teams use "VR3 Engineering" to produce their tubeset. This is an all inclusive (bending and cutting) service, and will be in the 3000-5000$ range for frame and suspension, depending on complexity and shipping. The upside is substantial time saved - their process is nearly fully automated, and the manufacturing process limits are fairly minimal.
Cutting tubes
Tubes can be cut by hand, using holesaws and a drill press, or by using water jet or laser jet cutting.
Tube bending
Bending the tubes by hand
Using CNC tube benders
Any motorcycle builders around? They can have tools for 1" tubing.
VR3 Engineering - Specifics
This is a schematic of the tube cutting setup:
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Their machine has a vertical mill head (1/8in diameter) and a rotating feeder cuff. This means that in the default 3-axis configuration (and for round tubes), the end-mill is always perpendicular to the tube surface being cut. This matters for tolerances/fit:
You can request them to use the 4th axis on round tubes where a very tight tolerance is required. For the Ryerson 2020 car, we asked for it on the a-arm tubes, which are .5in diameter.But for all other frame tubes, it is of no consequence, the tube will still fit very well, as intended.
Fixtures
Fixturing is central to the accuracy of the finished tube frame. Fixtures can be made in MDF, plywood or sheet metal.
Welding
Analysis
Goal:
<a class="image"><img alt="" src="/images/thumb/0/0b/Image152.png/300px-Image152.png" decoding="async" width="300" height="176" class="thumbimage" srcset="/images/thumb/0/0b/Image152.png/450px-Image152.png 1.5x, /images/thumb/0/0b/Image152.png/600px-Image152.png 2x"></a> <a class="internal" title="Enlarge"></a>Torsional stiffness FEA in solidworks
- To test totsional rigidity, camber (lateral) rigidity, and perhaps also toe rigidity
- To stiffen frame where needed, and change up layouts
- To make sure it doesnt break, or is too close to low FOS where welds may crack
- Frequency analysis perhaps?
Problem:
- Solid body FEA with a triangle mesh is too time consuming, for a 50+ tube frame. IF you have to, export small sections and do a small contained study (outlined below)
- Theres a quicker method - beam FEA. It makes a mesh of “pie cuts” of tubes. Way less mesh data, but very good accuracy still - a whole frame sim can run in ~30sec, on a laptop.
- It does not capture tab interfaces etc, but it does caputre member stresses.
- Cannot do an assembly simulation in it, but can simulate pivotable “joints”
Torsional FEA (Beam sim) Workflow:
- Put in a mock “engine” as just a bunch of tubes, triangulated to be stiff.
- Put in mock a-arms (lower arms only, upper arent needed unless doing camber stiffness) and pushrods
- Modify the appropriate joints to “hinges” (by right clicking on a member in the cutlist folder in the tree, and clicking "edit definition")
- Apply “immovable (allowing rotation) fixtures to 3 wheel, apply 100lb upwards to the 4th.
- Bellcrank can be done but its complex computationally and my best way was to treat as direct actuation (getting decently accurate), and then performing a separate exported section get simulated with an assembly simulation - with the actual bellcrank, tabs etc. Then just add up the two deflections.
Beam FEA Notes:
- Beam FEA doesnt like short members. If you have a short member, ask yourself if it can be “combined” (i.e combine operation) with another. An example is suspension tabs. In our case, they will be combined with the arm tubes.
- Beam FEA also doesnt like arcs - sharper bent tubes (making arcs that are fairly long, like 3inch plus) as nodes should be avoided, as ive had a bunch of weird bugs in FEA where the SIM node of the RRH being an arc just randomly detached
- Beam FEA works with “nodes” that it automatically computes. You can get it to manually compute, based on custom set distances - this helps eliminate (filter out) 2 super close together nodes. But keep in mind, every time you recompute the nodes, your fixtures may change - since their referenced node will change number.
<a class="image"><img alt="" src="/images/thumb/4/4d/Image41.png/300px-Image41.png" decoding="async" width="300" height="134" class="thumbimage" srcset="/images/thumb/4/4d/Image41.png/450px-Image41.png 1.5x, /images/thumb/4/4d/Image41.png/600px-Image41.png 2x"></a> <a class="internal" title="Enlarge"></a>"Hinge" Joint Configuration
Solid-Body Frame FEA
<a class="image"><img alt="" src="/images/thumb/2/25/Image75.png/300px-Image75.png" decoding="async" width="300" height="131" class="thumbimage" srcset="/images/thumb/2/25/Image75.png/450px-Image75.png 1.5x, /images/thumb/2/25/Image75.png/600px-Image75.png 2x"></a> <a class="internal" title="Enlarge"></a>FEA setup
Frame/Susp - Sending out
If using VR3 engineering to produce your tubeset for you, you have to send out a:
- BOM Drawing
- VR3 template excel sheet, with a qtys summary
- Once quote approved, indiv tube files.
Fortunately, a superb guide already exists - on the VR3 website. Its not a super light/easy thing to figure out, and yes you will have some late nights (or one all nighter) trying to send the frame out - usually, due to struggling with the BOM etc - but what do you expect? Not everything in life is a light read.
The doc is “SAE Student Guideline”, and is in their documents section of their website
