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or this;

Torsion bars
By Wil Ferch – 1/22/2000
Listers:
Every once in a while, questions come up on torsion bar sizes , and of
the suitability of certain size bars. I've compiled the following information
that can be used for all pre-G50 911's.
Torsion bar spring rates can be determined by the formula:
K= 1,178,000 ( d)*4 / (L) (A)*2
-
d is to the fourth power, and A is to the second power.
-d is diameter of torsion bar
-L is torsion bar length
-A is the lever arm length
A fellow Rennlister, Joe Winn, measured torsion bars he had for sale,
and the following is for the FRONT bar:
-length of 611 mm ( or ~24" )
-spline length at each end of about 1".....this makes for an
"effective" length of 22" , assuming full length spline engagement as
installed. Also, I'm using a lever arm measurement of 12"...the distance from
the torsion bar centerline, to the brake caliper centerline. With this , I
come up with:
SIZE ( mm) SPRING RATE ( Lb/in)
18.8 110
21 173
22 210
23 250
24 296
25 350
Using the same principle for the REAR bars, we have:
- bar length of 626 mm ( 24.65 ")
- spline length at one end of 19 mm, or 0.75"
- spline length at other end of 26 mm, or 1"
- "effective" bar length of 22.9", assuming full spline engagement
- lever arm of 18.5"...centerline of torsion bar to centerline of
rear wheel.
With these, I get:
SIZE (mm) SPRING RATE ( lb/in)
23 100
24 120
25 140
26 165
27 191
28 221
29 254
30 294
31 332
32 377
33 427