First, torsion bars are designed for rotation. Torque in ft-lbs (NOT POUNDS) is related to rotation (in degrees or radians NOT INCHES). To think of a stiffness at the wheel, you MUST INCLUDE ALL THE OTHER COMPONENTS - LINKAGE / DIMENSIONS necessary to convert the twist to a linear (up and down) motion. These will also affect the net stiffness at the wheel. So if you are talking in terms of 125 or 130 POUNDS stiffness (force) you need to know the rest of the geometry including where the force is applied and where the linear motion is calculated.
Second, the torsion equation is Rotation = (Torque x Length) / (G x J) where G is the Shear Modulus - a material constant and J is a function of the cross section. In the case of a round bar J = 1/2 x Pi x Radius to the 4th power. G is essentially a constant for steel (as opposed to Aluminum or other materials). So you are correct, a small change in radius can be significant to the rotational component of the motion.
In the case of a 1.000 bar vs an 0.980 bar, the difference is the
1.000 inch bar is torsionally 8.5% stiffer. A 1.06 bar vs an 0.982
bar is torsionally about 35% stiffer - and to me that would be a
concern.
But if you are debating an 0.983 vs 0.982, the difference in the torsion bar is only 0.4% stiffer (probably within tolerances).
Third, the difference in sanding paint off is negligible. You want the steel dimension as even a few thousandths of paint does not contribute to the stiffness. Further, spring steel is tough - you are not going to affect the diameter of a hardened steel bar with sandpaper (don't ask how I know). If you do, believe me, you have other problems.
Hopefully someone can get you the measurements you need - but be aware changing the wheel rate (stiffness) on one end of the car without changing both ends proportionally will also affect the handling. Stiffening just the front may add understeer while stiffening just the rear (or softening the front) can add oversteer. Small changes may be negligible but a 35% increase in front stiffness could add to the understeer that most cars of that vintage had.
Best, Ed
