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Predicted vs Tested Deflection 4130 Tubing

Question asked by Mark Horne on Sep 20, 2010
Latest reply on Sep 28, 2010 by Mark Horne

I am an engineering student, Journeymen Toolmaker, Machine and Tool designer, and a Kart racer. I have convinced my instructor to let me design a Kart chassis in Solidworks as my semester project. I am using the Student edition at home and the schools computers.


I have two known chassis. Both have known handling characteristics. I modeled both chassis with 3D sketches and using Weldments with the correct tube profiles. My thought was to apply a 100lb load on the front corner of each known chassis and find the displacement. To keep things simple, the connections of the rear axle and front spindles to the frame, I used another tube of heavier wall. The idea is to focus only on how the chassis flexes under load.


So I fixed the end of the left side rear axle and the left side front spindle end. Then applied a 100lb load perpendicular to the top plane.  The displacement on the end of the spindle was recorded. I repeated a study on the rear axle right side end and found the displacement there. This was done to both known chassis.


To verify these numbers where good, I attempted to do a real world test on two samples. A straight tube held in v-blocks. The other a rectangular frame.


SIM TEST 17X19 FRAME-Study 1-Results-Displacement1.jpg  V-BLOCK TEST-Study 2-Results-Displacement1.jpg

I just cut a triangular hole, leaving the top section, rather than putting clamps in. Not even sure if I needed them.


The results: all tubes 1.125"x.095 wall 4130 n 870c.

I used a certified weight of 21.96lb with a heavy wire, 90 degree hook to slip into the end of the tube.

On the frame the predicted displacement was .0112". The dial test indicator showed .034".  (17x19" frame welded after fish mouthing tube ends)

The straight tube predicted displacement was .012". The dial test indicator showed .016".  (straight tube projected 12" from block face)


I have to show these results in my project to support my design. I used SW materials 4130 N 870c for the tubing material. My instructor is at a loss because he has not done anything with simulation except for the tutorials.


So my question, finally, is: what did I do wrong? Is simulation the correct tool for this? What would be an acceptable deviation from real world to SW?


Below is a sample of how I loaded the chassis. It will be a week or two before we can get the known chassis on the table to test those results to SW.


I was told that even if the numbers were off, the results in SW for both chassis would be relative. Another words, if one is displacing .400" and the other is displacing .600", then my new chassis would have a displacement of .500", it would be right in the middle of the two. This seems too simple to me. What if the error is not linear? It seems non linear with the two simple samples above. Then how would one begin to "guess" what the end design would be. Perhaps I am missing something.


here is the way I did the studies:

F125 CHASSIS 2 WITH SPINDLES - AXLE-Study 1-Results-Displacement1.jpg

Any help would be appreciated.