Originally Posted by Jon A

Gee, I guess I don't no nothing about that? Here are some questions for you. Since I knew you wouldn't be able to answer them and I'm suuuch a nice guy, I went ahead and answered them for you. You're welcome.

1) Tell me, how does the increased tensile capability of the section increase the strength of the overall part when both sections' capability exceeds that of your rod ends by a large margin?

Answer: It doesn't. The part is only as strong as its weakest link.

The commonly used 1.125 X .219 wall tubing has a cross sectional area of .622. At an ftu of 42 KSI that's over 26,000 lbs tension capability.

Your 1" solid section has a cross sectional area of .785. This results in a tension capability of close to 33,000 lbs.

[Note, that's ignoring the location at which they'd actually fail--the hollow space at the end of the shank of the rod end where the area will be .480 giving only a 20,000 lb capability (You didn't really think it would tear in half right at the strongest part in the middle of the solid section, did you?) but I'm feeling generous tonight, it doesn't matter anyway...see below.].

The ultimate static radial load capability of a 3/4" QA1 XM or equivalent rod end is about 28,000. This means a part made from the tubing using these rod ends has an ultimate tensile capability of about 26,000 lbs.

But a 5/8" rod end (the size you use) QA1 XM or equivalent is only good for 18,000 lbs. So your part will be limited to 18,000 lbs with those rod ends no matter if the center section was 2" diameter solid steel.

So the tubular part with 3/4" QA1's is about 44% stronger in tension than your parts--and that's if you used QA1 XM's or an equivalent. It's very possible (given your prices) the particular rod ends you actually use have a much smaller load rating, further weakening the parts.

2) Exactly how much tension stress are the LCA's under during a launch?

Answer: Less than zero. It's a negative number denoting compression.

As entertaining as the above was, it's really academic as neither part is likely to ever fail in tension due to non-crash loads. So now we get to the good stuff, compression.

3) You recited a definition from somewhere for tension, is it the same story for compression?

Answer: It would be if the parts were only a couple inches long. But they're not. They won't suffer a pure compression failure. Under too large a compressive load they will buckle.

4) How do you calculate that?

Answer: For these purposes, a simple Euler buckling check is more than sufficient. The equation for that is Critical Load = pi^2*E*I/L^2. The only variable that changes here is I, the Cross Sectional Moment of Inertia. The equation for that for a round section is pi*r^4/4 for the outside radius, minus the same for the inside hollow radius if the part is hollow.

This gives your section an I of .049 in^4. The "flimsy tube" noted above has an I of .068 due to its larger diameter.

Thus, the "flimsy tube" can withstand a force 39% larger before it buckles. For the LCA's this amounts to about 18,000 lbs vs 13,000 for your parts.

So as you can see, not only are your parts weaker in tension, they're also weaker in compression. They're simply weaker.

That's still rather acedemic as even 13,000 lbs is quite strong and one would be hard pressed to fail even your weak LCA's in compression.

Where it makes a very real difference is the PHB. The "whimpy tubular" is good for only about 3700 lbs. Yours is good for about 2700 lbs. That's a difference that hits home. An F-Body with sticky tires, under the right conditions can put around a 1500 lb sustained load on the PHB during cornering. Add a little for spike loads caused by bumps mid-corner, etc, and you can see the 2700 lb capability has virtually no safety margin left.

For this reason I would not recommend anybody (besides pure drag racers) use your PHB. It just isn't strong enough. The much stronger "flimsy tubular" one is the minimum I'd recommend (and I use it myself with confidence). Anything less than that is asking for failure to put you into the wall when you least expect it.

Here endeth the lesson.