Finite Element Analysis of Perforated Outrigger at ~2.5x Nominal Load
I've been doing a little design work in a tool called FreeCAD. As the name implies it's a free computer aided design package that can be used to design and analyze solid objects. Originally I started using it because I needed to fabricate some Hehr Standard window link levers (the levers used to open windows) for my Clipper. I wanted to user a laser cutting service to make some stainless steel levers and they needed the information in a digital file format. FreeCAD (and many other programs) can generate this file and there was a large amount of training available on YouTube.
In any case I had some experience in FreeCAD before I started the trailer frame design process.
I did some rough paper sketches of the trailer frame I wanted to design. I then did a more refined drawing in PowerPoint (!). I then broke down the parts needed and did some iterations to modularize the design and create common components. I know from experience that the unit cost of parts drops pretty quickly if you order more than one so common components would significantly reduce my costs.
In the case of the outriggers I need 14 total all with the same basic "outer mold line". 4 are used to bookend the wheel wells. These need to be a "solid" design because they close off the belly of the trailer from the exterior. The other 10 can be solid or they can be perforated to save weight or serve as a pass-through for things like electrical conduit, plumbing or HVAC return air.
I made measurements of the original aluminum cross-members so that I could duplicate their profile. Using these measurements I created a sketch in FreeCAD.
FreeCAD 2D Sketch of Outrigger
From the sketch it's fairly easy to create a 3D solid model in FreeCAD. And when you have a 3D solid model you can use FreeCAD's Finite Element Analysis workbench to analyze the design. It's easy (but a little tedious) to change materials or material thickness to determine how they might respond to different "loads", ie the amount of force being applied.
I focused on using mild steel (vs something like aluminum) because it's relatively cheap, easy to weld and performs well in the real world. I iterated some material thicknesses with very demanding loads (up to 2.5x max design load applied to the outside end of the outrigger) to arrive at a conservative material selection.
FEM of Solid Web Outrigger for Wheelwells
I ended up iterating the basic design 3 times. First was just an oblong hole, next was a series of 3 holes, then the final design is simply a large cutaway. The outrigger is actually designed in 3 parts: a top flange; the center "web"; and the bottom flange. I ran FEM on just the web during the iterative process as I was really primarily interested in the stress and deformation at the outer end of the outrigger under load.
Initial Iteration of Perforated Outrigger
Iteration 2 of Perforated Outrigger
Iteration 3 of Perforated Outrigger - ~2.5x Max Load at Outside
After settling on a web perforation design I then moved on to model the entire outrigger with both top and bottom flanges.
Based on the analysis I semi-finalized the design and have ordered a set of laser cut parts to fabricate 4 perforated outriggers. Why not all 10? Well I'm using a "tab and slot" design approach that I've never used before so I thought it would make sense to do a "preproduction" run before I spent an even bigger hunk of money.
I'll follow up with a post on the tab and slot design approach as applied to the outriggers.
Laser Cut Templates Uploaded in SendCutSend.com
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