Indy Suspension Build

[Abeaw]

I am enjoying reading your posts. I have been interested in building a trailer for over landing for a while.

I am a Mechanical Engineering student at WSU and I thought I would toss my two cents into the square tube orientation conversation. I would like to emphasize that I am just a student, but I might be able to provide some technical considerations.

First I would like to say that the stresses that square tube will experience will be largely affected by the area moment of inertia with respect to the neutral axis of bending. In simpler terms the amount of material that is further from a line running horizontally through the cross section. Off the top of my head I can tell you that for the flat orientation of the tube the moment of inertia will be (1/12 * Base length * Height of tube)-(1/12 * Inside base length * Inside Height). The rotated orientation is a little less common and it I would have to do some calculation to tell you what it is exactly. That said, I think that the flat orientation will most likely have the higher moment of inertia because it looks like there is more material further from the neutral axis, but I can't say for sure until I do the calcs. Just for clarity the configuration with the higher moment of inertia will resist more bending stress.

Now that I have said all that I think that you could make it either way and probably be fine. I haven't addressed the subject of the stress concentrations that the corners produce. I would say (educated guess) that the rotated design would most likely produce larger stress concentrations then the flat configuration.

I hope this has been helpful. I will, if I have time, calculate the moment of inertia for the other configuration.

I look forward to seeing how your design works.

Abe

 

[ntsqd]

Using 2" x .12" wall I just did a qwik model eval in SolidWorks. It comes up with the "I" being the same whether turned on a corner or laid flat. Which was not what I was expecting.
Laid flat would be easier to work with.

 

[Abeaw]

The same either way huh? That is interesting, I wouldn't have guessed that. Well I guess that it doesn't matter which way you do it then. If you have ANSYS or the finite element package that comes with Solidworks you could find the max stress, but as I said before you could probably do it either way with a reasonable expectation of success.

What are the dimensions of the steel you are using? also do you know what kind of steel it is? just curious.

Abe

 

[ntsqd]

I'm not convinced that the results are correct. It was just a quickie trial.
With no idea of the loads finding the stresses would be a guess.
I used 1020, but it wouldn't matter to the I calc.

Re-visiting it: With a 1' long extrusion of 2" x .120 wall square tube I get these values when laid 'flat':
Ixx = 38.0933
Iyy = 38.0933
Izz = 3.5363
And these when laid on a 45*:
Ixx = 38.0933
Iyy = 38.0933
Izz = 3.5363

 

[GeoTracker90]

OK, I have an answer to the flat vs. 45* question that I posed and the result was suprising , at least to me. This started Tuersday night after work when I was discussing this project with one of the mechanical engineers at work. I posed to him the idea of turning the square tube 45* and we talke some about the forces that it would see. Friday morning he presented me with print outs from Solid Works. I think that he did the beam analysis. Here are the results:

45*

Flat

My understanding of this evaluation is that the end of the tube to the left is fixed and a verticle force (either positive or negative) is applied to the free end. The higher the number at the fixed end the higher the strain that it is under. The 45* tube has a value of 3.329e+004 while the flat tube has a value of 2.919e+004. Im not sure if the pictures show it in enough detail but, along the top of the 45* piece is where all of the stress is being transmitted to. With the flat piece the stress is distributed along the full width of the tube, and there is less stress to the part and joint overall.

So I guess when I have the tubes coped I'll do it in the flat configuration.

Hope that I did a good job of describing the engineering behind this in a way that is accurate and easy to understand. also, I hope that others find this information helpful to them as well.

Mike