An effective spring rate is the value when taking into consideration the placement, construction, and type of the sprung materials.

Torsion bar diameter is expressed as the effective diameter.  Hollow bars will always be advertised with their effective rate which may not be the actual O.D.

If using coil-over springs and T-bars, simply add the effective rates together to find the overall effective rate.

If you know the rate you are trying to achieve by adding coil-overs (such as the Koni 3012) to an existing T-bar , you can calculate the necessary spring needed.

Here's the equation: x = desired overall effective rate, y = T-bar effective rate:  "(x - y)/.56 = linear coil-over spring rate" or "<desired effective rate> less <effective T-bar rate> divided by 56% equals linear coil-over spring rate"

EXAMPLE:    OEM 23.5mm T-bar = 126#, desired overall rear effective rate = 400#

(400 - 126)/.56 = 489 this means you'll need a 500# coil-over spring to achieve the 400# effective rate.

Using the chart below, it may be easier to just subtract the T-bar rate from the desired rate, then match to up to the coil spring weight.

NOTE:  When purchasing "hollow" or "tubular" torsion bars, they are normally listed using the effective diameter so you don't have to do the math.  The actual Outside Diameter (OD) will be slightly more.  Here is a list of the most commonly available Hollow Torsion Bars and their actual OD's:

Stock Sizes
Size (mm)	Year/Model	Notes
23.5	924S, 944, 944S	includes M474
23.5	951 86-88	includes M474
24	944S2	includes M474
24.5	924S, 944, 944S (all thru '86)	with M030 or M637
25.5	924S, 944, 944S, 944S2 (all from '87)	with M030 or M031
25.5	951S 88, 951 89,