Your link clearly shows that a larger hollow bar has a higher spring rate than a smaller solid bar so I am not sure what you trying to make a point with. All of my calculations are from equations found in my machine design text book. Have you looked at the cross-sectional area of the two? The areas are with 2% of each other. Can you tell me a why a smaller bar would have a larger spring rate when the areas are nearly the same? No, you can't because you are wrong. Even though I was incorrectly using I initially, it is closely related to J which is why the numbers are nearly identical. I ran the calculations for the SC bar before selling it to Justin and if I felt it was crap I would not have sold it to him. I worked in suspension systems for four years, I have a good idea of how they work.
Ok Mr. Wizard, since you apparently can't grasp the concept from what I have given you, I will walk you step by step through it. I've given you the two relative portions of the equations for this discussion, D^3 for the solid bar and (D^4-d^4)/D for the hollow bar. Let's apply these to a couple of bars in the link from above to show that the theory is correct. We will look at the 10" long bars in 1.25" diameter. We will compare the solid bar and the .120 wall bar. First, D^3 for the solid bar is 1.953. For the hollow bar, (D^4 - d^4)/D is 1.120. Ok, if we divide 1.120 by 1.953, we get .573, which means that the hollow bar is 57.3% as strong as the solid bar. So lets check now, the listed rate for the solid bar is 664. If we multiply that by .573, we get 381. If we check the chart for the hollow bar, low and behold it lists the rate of 381.
Now, we will apply this principal to the t-bird bars. From the tech articles, it lists the diameter as being 1.10, so we will use that, along with the .120 wall thickness dimension for a hollow bar. In this instance, D^3 equals 1.331, and (D^4 - d^4)/D equals .834. If we divide .834 by 1.331, we get .627, thus the hollow bar has a rate of 62.7% of the solid bar. Now, if we want to find the diameter of solid bar with the equivelant rate of the hollow bar from above, all we have to do is take the cube root of .834, which just so happens to equal .941. Do you think it was a coincidence that a lot of the later model cars got .94" diameter solid rear bars? I don't think it was.
By the way, what suspensions have you worked with? I would like to make sure and steer clear of them.