84-85 v 86-89 torsion bar & sway rates change

[911st]

I found this neat formula for calculating the difference between torsion bar sizes.

It is the larger bar to the fourth power / the smaller bar size to the fourth.

Porsche changed the rear torsion bar from 24mm to 25mm and the sway bar sizes from 21/18 to 22/21 in 1986.

This had the following effect.

The rear torsion bar change from 24 to 25 is 17.7% stiffer.

The rear sway bar change from 18 to 21 is 85% stiffer.

The front sway bar change from 21 to 22 is 20% stiffer.

Thus, Porsche stiffened up the rear "spring rates" of the 86 and newer significantly more than the front.


I could be wrong and because of other factors this might not translate to "wheel rates" in the same relative amount.

[911st]

A popular up grade is to 22/28mm torsion bars. If starting with 18.8 /25mm torsion bars this is:

18.8 to 22mm is 87% stiffer.

25 to 29mm is 81% stiffer.

24 to 29mm is 113% stiffer. (pre 86 rear)

Caution using this by itself as the sway bar may be almost half the effective wheel spring rate.