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I should say that the "shape" is very light and you have to hold it parallel to something and have a very good eye to notice. You should perceive the unit as "arrow straight" as dan says if this is your first suspension adventure and have never researched extensively or look moer than 30 consecutive minutes into that

All torsion bars are "indexed". Smaller diameter bars have the hex offset to rotate the control arm down otherwise they wouldn't support the car. Just as a soft coil spring has to be longer to support the car. You have to put more twist in the spring whether it's straight or coiled. Softer springs allow the front end to lift more because lifting the front end 1" may reduce the loading on the tires only 82 lbs for .820" bars, but 195 lbs for 1.00" bars. Has nothing to do with the shape of the spring.
The shape does not affect the spring rate. Even if the spring is bent into a coil the rate doesn't change. You are still twisting the wire along it's length. Having some arc to the torsion spring is not essential to it's function. It's a byproduct.
When you say progressive you are describing a variable rate spring. Variable rate coil springs are possible only because coils are allowed to go solid which changes the effective number of coils thereby shortening the spring. How are you shortening a torsion spring to increase the rate?
if smaller diameter bars wouldn't have shape, they would break because of the massive amount of axial stress that offsetting the hex head has. They're indexed because in the process of offsetting the hex head, they buil in a "bow" shape in that bar wich relieves part of the axial stress.
and incresing the rate is achieved by diameter on a torsion bar. Since the lenght is not a variable, that's the reason for different diameters. You "shorten" the torsion spring by aughmenting the diameter.
Shape indeed afects spring reaction, not rate. you put shape into a piece of wire or flat spring and it would react differently than a straight piece of the same material.
Think of one reason to prove me wrong. If the only reason for indexing a bar is the offset that would push the arm downwards, and being exactly mirrored symmetric (each side) why there's left and right T-bars?
And more importantly, have you ever checked about a dozen of OEM fresh .900 or .860 or .99 t bars? I have, and they have a shape. I'm just pointing out what I have observed.
I didn't mean "progressive", I mean "sort of", as in "behaves somewhat alike progressive"