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.

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

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.

Fixtures

Fixturing is central to the accuracy of the finished tube frame. Fixtures can be made in MDF, plywood or sheet metal.

Analysis

Goal:

• 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 here)
• 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 Workflow:

1. Put in a mock “engine” as just a bunch of tubes, triangulated to be stiff.
2. Put in mock a-arms (lower arms only, upper arent needed unless doing camber stiffness) and pushrods
3. Modify the appropriate joints to “hinges” (by right clicking on a member in the cutlist folder in the tree, and clicking "edit definition")
4. Apply “immovable (allowing rotation) fixtures to 3 wheel, apply 100lb upwards to the 4th.
5. 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.

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