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tperazzo tperazzo is offline
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Join Date: Nov 2008
Location: Huntington Beach, CA
Posts: 337
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Quote:
Originally Posted by aoncurly View Post
Not trying to be critical, but I see a couple of questionable elements to your setup and test. First, I question why you cut and instrumented the top brace in the middle of the brace? By so doing, you create a "pin" in the middle. In tension, this is not an issue, but in compression it would significantly weaken the strut/brace. For the loads you are seeing, I suspect you are nowhere near the buckling force (again, in compression) of the brace, so you would not measure much difference. At more extreme loads, the strut or brace would deflect under compression, creating a weak system, and in theory, a collapse mode. A better (but not ideal) solution would have been to move the load cell to the end of the brace, where there is already a "pin".
Secondly, the addition of a diagonal for the loads you showed earlier in your video will not see much if any load. If the top strut/brace is in tension or compression, the diagonal sees no load. Only if there is a vertical load at the strut tower applied to the system (acting perpendicular to the top strut/brace) will the diagonal see any load. Also, if the diagonal sees any vertical load, it all ends up at the lower diagonal attachment point. If you are designing for some vertical applied load at the strut tower, you should have a diagonal in each direction or cross.
So, in summary, for the loads you have described in your video, the addition of the diagonal has no or little effect on the torsional rigidity. If the top strut/brace is significantly loaded, the strut/brace would collapse under compression.
I offer this commentary for your consideration.
Hi and thanks for you comments. I appreciate any constructive criticism and will gladly debate on technical grounds.

Regarding the buckling: I agree with your placement of the sensor recommendation that relocating on the side is a better choice for high loads. But as you say 70 lbs is no where near the buckling strength of the bar/sensor setup. So, if I did this test again with your consideration, I would get the exact same results.

Regarding the diagonal bar forces, I'm not clear why you think there is no load in the diagonal when the chassis is twisted. When the diagonal bar was connected the deflection dropped by 9%. The top bar did have some load as I showed on my computer screen, but I think the results speak for themselves: The diagonal bar improves torsional rigidity and a second diagonal bar or X may further improve it.

Your vertical load scenario in the strut only comes into play when I dropped the car on the right front tire. The strut doesn't have any springs in it and only transfers vertical load through the damper piston/fluid design.

I wish I had second load cell because several questions about that have come up. However, I feel like I've learned enough about this that I can make an informed decision on how to proceed.

I will size the top bar to be fairly lightweight but will avoid buckling and I will add a second diagonal bar in an X configuration.

Cheers,
Tom
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Old 01-21-2019, 12:29 PM
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