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just a note on the FEA... edge loads aren't the most accurate method of modeling a force because they essentially produce a moment instead of an actual face load. You'd be better off applying a face load, model in a boss in the area that you want to apply the load, to simulate the tube or tubes that the load will be transmitted from, and apply the load on that face. You'll get more accurate results.

Also, fixed constraints assume that the face constrained is rigid, IE, can't bend. A displacement constraint allows bending in the constraint face, just not displacement in either 3 axes. This also more accurately simulates the modes present in a race car chassis, because no part of it is truly fixed.

Here are some screens of an FEA comparison I did of triangulation methods...

I applied a vertical face load at the closest corner, and displacement constraints at the other 3 (allowing bending) to test torsional rigidity of the structure.

nice progress! FEA is wonderful if you have access and the ability to truly use it.

couple more thoughts:

-in re: to balsa wood. i didn't mean to model the entire car, just do exactly what you just did with the FEA, creating small pieces to see how different designs effect load paths/deflection/etc... might be easier to get a feel for how it reacts in real world situations. at this point, depending on how good you are with balsa, i don't know if it'd be faster, but i really think it would tell you a lot just getting to "feel" how the design changes effect the small models.

-in re: adding x bracing increasing weight. did you try it with smaller sized tubing "x" tubes to see where the "knee" is in the relationship of weight from tubes added to increased rigidity?


doggone it. i think today i'm going to have to finally put my copy of solidworks on my work computer

Blue j: The relationship depends on whether you want compression stiffness or bending stiffness... IE, you can get the same compression stiffness with a 1.5" x .035 wall as you can with a 1.0" x .095 wall tube, it's all about face area. In terms of bending stiffness though you do need some wall thickness, and there isn't really a "knee" in the curve.

The bracing styles have a linear relationship with weight vs. thickness of tubing.

I have a spreadsheet I made up some time ago that calculates the bending and compression stiffness of several shapes and sizes of square and round tubing. The interesting thing is what 2 different kind of tubes you can use to do the same job. (large dia, thin wall vs. small dia thick wall). Then just choose the lightest combination and you have optimized your design.

Lets face it, you need bracing of some sort, so you'll have to add weight. The trick is figuring out how MUCH bracing you need, and where to put it.

The X bracing adding weight was only as a function of stiffness in my FEA comparison... IE the parallelogram(sp?) style bracing was lighter per unit of torsional rigidity than the x bracing.

Thanks for the advice on the FEA. I am trying to rerun the parts using the loading and constraints you mentioned. It's not going so well. I keep getting an error concerning the coordinate system of the constraint compared to one of the nodes. I am going to have to read up and practice some more. I'll keep trying and post some more results when I can.

Ken