torsion bar stiffness % ??

[garrege]

When upgrading coil spring suspensions, it's common to refer to a new spring as xx% stiffer than the factory spring. Does anyone know the percentages for the torsion bars offered?

In the front, 22mm is what % stiffer than 19mm?
In the rear, 27mm is what % stiffer than 23mm?

thanks,
geoff

[BeauBlues]

I'm not 100% sure, but I believe that for every mm, you increase stiffness by 4x.

[garrege]

do you mean 4 times or 4%?

4 times is a little extreme, right?
so going from 19mm to 22m would mean 1200% stiffer.

[David]

Like this:

[Zeke]

Here's an old thread from Will Ferch:

"I'm providing you with this to be an "informed consumer"...
Your 86 was likely delivered with a 18.8 front bar and a 25 rear bar. The equivalent spring rates are 110 lb/in front... 140 lb/in rear. Stiffer rear ( only) will promote oversteer....stiffer front ( only) will promote understeer. Here's a breakdown of front and rear TB sizes and their spring rates, after my exhaustive investigation on sizes, bar lengths, pivot arm lengths, etc:

Front:
18.8= 110 lb/in
21= 173
22=210
23=250

Rear:
24=120 lb/in
25=140
26=165
27=191
28=221
29=254
30=294
31=332
33=427

Notice some interesting trends.... many people say to upgrade to 21/27...or 22/28. The 21/27 combo equates to 173/191...and the 22/28 combo equates to 210/221. In both cases...fairly equal. This maintains the front/rear balance Porsche designed into these cars, but some say (me included) that we can go proportionately stiffer in the rear to dial-out some unwanted low speed understeer. So...maybe 21/28 ( 173/221)....or 22/29 ( 210/254). But look what you're contemplating:
21/31 ( or 173/332). IMHO...this is too much rear bar...and the overall stiffness for the rear is over 2.3 x as stiff as stock.... !"

What this doesn't cover is the wheel size and offset. That factor will change the rates if not stock.

[ToddM]

Originally Posted by garrege

When upgrading coil spring suspensions, it's common to refer to a new spring as xx% stiffer than the factory spring. Does anyone know the percentages for the torsion bars offered?

In the front, 22mm is what % stiffer than 19mm?
In the rear, 27mm is what % stiffer than 23mm?

thanks,
geoff

The formula for calculating spring rate is: k=d^4G where k=spring rate, d=diameter, G=modulus of elasticity (constant for steel)so ignoring the constant:

% increase in stiffness = ( (d2^4)- (d1^4) ) / (d1^4) * 100

where d2 is new diameter and d1 is original diameter

To answer your question:

19 to 22m front active rate stiffness increase as a %: 49.23228029
23 to 27mm rear active rate stiffness increase as a %: 89.90819787

[garrege]

thanks guy, good information.

[dshepp806]

you better believe it,....good stuff!!

Thanks!

Best,

Doyle