Rescued
from PARAGON
PRODUCT'S now-defunct
tech-session website.
AUTHOR:
Skip
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.
924/944/968
Rear Coil Over
|
|
924/944/968
Torsion Bar
|
Coil
Spring
Rate (mm)
|
Effective
Rate
(lbs/in)
|
|
Torsion
Bar
Diameter (mm)
|
Effective
Rate
(lbs/in)
|
200
|
112
|
|
18.5
|
48
|
225
|
127
|
|
19
|
54
|
250
|
141
|
|
23.5
|
126
|
275
|
155
|
|
24.5
|
149
|
300
|
169
|
|
25.5
|
175
|
325
|
183
|
|
26
|
189
|
350
|
197
|
|
27
|
220
|
375
|
211
|
|
28
|
254
|
400
|
225
|
|
28.5
|
273
|
425
|
239
|
|
29
|
292
|
450
|
253
|
|
30
|
335
|
475
|
267
|
|
31
|
382
|
500
|
281
|
|
32
|
434
|
525
|
295
|
|
33
|
490
|
550
|
309
|
|
34
|
553
|
575
|
323
|
|
|
|
600
|
337
|
|
|
|
625
|
352
|
|
|
|
650
|
366
|
|
|
|
675
|
380
|
|
|
|
700
|
394
|
|
|
|
725
|
408
|
|
|
|
750
|
422
|
|
|
|
775
|
436
|
|
|
|
800
|
450
|
|
|
|
825
|
464
|
|
|
|
850
|
478
|
|
|
|
875
|
492
|
|
|
|
900
|
506
|
|
|
|
925
|
520
|
|
|
|
950
|
534
|
|
|
|
975
|
548
|
|
|
|
1000
|
562
|
|
|
|
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:
Hollow
vs Solid Torsion Bars
|
Hollow
-
actual
OD (mm)
|
Solid
-
effective OD (mm)
|
27.6
|
26
|
28.5
|
27
|
29.4
|
28
|
30.2
|
29
|
31
|
30
|
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,
|
|