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 maximized by triangulating the tubing to ensure that the tubes are not loaded in bending.

Required Reading

These 5 papers teach you more than the wiki ever could.

• Riley, W. and George, A., "Design, Analysis and Testing of a Formula SAE Car Chassis," SAE Technical Paper 2002-01-3300, 2002, https://doi.org/10.4271/2002-01-3300.
• Auer, B., McCombs, J., and Odom, E., "Design and Optimization of a Formula SAE® Frame," SAE Technical Paper 2006-01-1009, 2006, https://doi.org/10.4271/2006-01-1009.
• P Baskara Sethupathi et al 2018 IOP Conf. Ser.: Mater. Sci. Eng. 402 012184, https://iopscience.iop.org/article/10.1088/1757-899X/402/1/012184/pdf.
• Chassis Design: Principles and Analysis Milliken 0768008263.
• Dodd, C. et al, "MEEG 402-010 Chassis Design Report". 2017 https://udel.edu/~pgeneva/downloads/fsae/2017_FSAE_chassis.pdf. ("The Delaware Paper")

Design Considerations for Systems Integration

Designing the tube frame is a compromise between structural rigidity, weight, and manufacturing complexity while complying to the rules. One thing to remember is that the frame is basically a glorified bracket. From a systems perspective, the frame designer is primarily concerned with packaging the other vehicle components. In approximate order of importance:

1. Ergonomics
2. Suspension
3. Powertrain/drivetrain
4. Aerodynamic devices and bodywork
5. Electrical

Driver Accommodation and CAD basics in SolidWorks

Start the frame CAD with a "driver sketch", so that the driver dimensions and margins of safety past the rules will set the dimensions of the entire frame. A driver sketch can look like this:

Driver Sketch

with a separate sketch building up the Side view frame around it:

Driver Sketch and Side View Frame

The frame CAD is best set up with the help of both 2D and 3D sketches, and then with the use of "Weldments_Overview.htm Weldments" - basically pre-configured sweep features, that are generated on top of the sketches (the sketch being the neutral axis of the tube). It makes things quite easy to make, and to export later - SolidWorks generates a "cut list": a BOM of tubes.

Teams may be required to create or import Weldments_Creating_a_Custom_Profile.htm custom weldment profiles.

As part of smart CAD practice, it is best to have a "master sketch" (or a few in this case due to the complexity) such that the entire frame design is controlled at the top of the part tree, and so that you don't have to go to 10 different features just to make one change. Learn to work with inserting_reference_dimensions.htm driven dimensions, and construction geometry. 3D sketches are computationally difficult, so you may occasionally see weird bugs with things going over defined even though they shouldn't. To mitigate this, reduce the number of dimensions and sketch lines per 3D sketch.

Example Layout

• Driver Sketch (2D)
• Side View frame (2D)
• Bulkhead (2D, generally)
• Front Hoop (2D or 3D sketch on a plane)
• Main Hoop (3D sketch on a plane)
• Front tubes (3D)
• Middle tubes (3D)
• Rear tubes (3D)
• Suspension (2D and 3D)

Front Tubes, with bulkhead and FRH visible

<a class="internal" title="Enlarge"></a>Front Tubes, with bulkhead and FRH visibleIn-context relations deliver the suspension geometry to the a-arms, bellcranks, uprights, etc. That way, all adjustments to the geometry can be done in the frame file, and the changes would be up dated on assembly rebuild.

Suspension Integration into Frame file

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?

Note that because some tubes will inevitably take bending (primarily the side impact structure, due to impossible triangulation from the big empty space the driver occupies), there should be emphasis on reducing tube aspect ratio (by either upping diameter or reducing length), and adding gussets.

Tube Choice

Alloys

10XX steel

is cheap, easy to weld, readily available, and easy to machine.

41XX steel is more expensive but stronger, more difficult to weld and machine (prone to cracking, heat treat might be required).

Method

ERW, DOM

Manufacturing

Outsourcing

Many teams in North America use "VR3 Engineering

" to produce their tube set.They will bend and cope tubes for the team in the $3000-5000 range for frame and suspension, depending on complexity and shipping. VR3 has strict requirements on documentation sent to their sales engineers and is detailed on their website at<a href="https://vr3.ca/request-a-quote/">https://vr3.ca/request-a-quote/.</a>

In-house Fabrication

Cutting tubes

Cutting tubes to intersect each other is called coping or notching. Tubes can be notched by hand typically using a hole saw notcher, bench grinder, or die grinder. This process is very time consuming and meticulous. Tubes can also be profiled using a water jet or laser cutter.

Tube bending

Manual Bending

CNC Bending

Fixtures

Fixtures (also known as jigs) indicate the location of tubes within the frame. Fixtures can be made of many materials. Fixtures should start measurement from a datum . From the datum, important tubes should next be fixtured (suspension points, roll hoops, etc...).

(Fixture gallery?)

Welding

Main page: Welding

Tubes should be cleaned before welding. Joints should have minimal gaps, otherwise welding will be difficult. Tubes must be welded around the entire circumference.

Analysis

Goal:

Torsional stiffness FEA in SolidWorks

• To test torsional rigidity, camber (lateral) rigidity, and perhaps also toe rigidity
• To stiffen frame where needed, and change up layouts
• To make sure it doesn't 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)
• There's 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 capture member stresses.
• Cannot do an assembly simulation in it, but can simulate pivot “joints”

Torsional FEA (Beam sim) Workflow (SolidWorks):

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 aren't needed unless doing camber stiffness) and pushrods
3. Modify the appropriate joints to “hinges” (by right clicking on a member in the cut list 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.
6. 

   FEA setup
   Run, then view results! to understand if it makes sense, animate it - with the stress view.

Beam FEA Notes:

• Beam FEA doesn't 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 doesn't like arcs - sharper bent tubes (making arcs that are fairly long, like 3inch plus) as nodes should be avoided, as I've 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 re-compute the nodes, your fixtures may change - since their referenced node will change number.

Configuring Hinge Joints

Solid-Body Frame FEA (SolidWorks)

Its not computationally realistic to do a solid body simulation for the entire frame. As such, I recommend exporting a section of interest, and analyzing it separately. For example, the bellcrank mounting, or a-arm tabs. In the case of the bellcrank, you'd be best to include the actual bellcrank as well and perform an assembly simulation. You never know how much it will contribute!

How to export a section? Start from having a fully merged body that includes the tubes/tabs of interest. The extruded tabs have to properly intersect the tube for merging to work. You can use the "combine" feature (just search for it in the command search box) to combine all the bodies together. Once done, right click the body, and click "insert into new part".

Proceed with solid body FEA - NOT beam FEA. Meshing is sometimes tricky, so the addition of fillets as well as mesh control (mesh per part size) helps auto refine the mesh in the sharp edges and prevent the meshing from failing.