Sway-bar size...?

[Allen K. Littlefield]

Originally Posted by blue2000s

Stiffness goes with the 4th power of radius. In other words, a little larger diameter makes a very large difference in roll stiffness.

Lord have mercy, what a difference. I took a dogleg left and right turn at 60 on the way home from having the M030 bars installed and that little sucker din NOT lean at all. For the cost from Sunset out in Oregon, $235.00 including shipping for both bars and $170.00 installation, I would say that this is the most bang for your buck handling wise. I kept my 17" wheels and stock strut/spring set up and you can really feel the difference. Highly recommended if you like fast corners. What a difference a mm makes.
Down side is I found I really need a set of rear tires and I had not provided for that in the budget so some adjustments have to be made financial wise.

AKL :dance:

[John V]

Remember that while the M030 "S" bar is 24mm, it's also hollow. The 24mm Eibach bar is SOLID! Big difference in stiffness.

Front bars:
986 non-s standard suspension 23.1mm diameter 3.4mm wall thickness
986 non-s M030 suspension 23.6mm diameter 3.5mm wall thickness
986 S standard suspension 23.6mm diameter 3.5mm wall thickness
986 S M030 suspension 24.0mm diameter 3.8mm wall thickness
996 GT3 Bar 26.7mm diameter hollow
Eibach bar 24mm solid

Rear bars:

986 non-s standard suspension 18.5mm diameter 2.5mm wall thickness
986 non-s M030 suspension 19.6mm diameter 2.5mm wall thickness
986 S standard suspension 18.5mm diameter 2.6mm wall thickness
986 S M030 suspension 19.0mm diameter 2.7mm wall thickness

Yes the non-S M030 rear bar is the stiffest.

[blue2000s]

Originally Posted by John V

Remember that while the M030 "S" bar is 24mm, it's also hollow. The 24mm Eibach bar is SOLID! Big difference in stiffness.

Front bars:
986 non-s standard suspension 23.1mm diameter 3.4mm wall thickness
986 non-s M030 suspension 23.6mm diameter 3.5mm wall thickness
986 S standard suspension 23.6mm diameter 3.5mm wall thickness
986 S M030 suspension 24.0mm diameter 3.8mm wall thickness
996 GT3 Bar 26.7mm diameter hollow
Eibach bar 24mm solid

Rear bars:

986 non-s standard suspension 18.5mm diameter 2.5mm wall thickness
986 non-s M030 suspension 19.6mm diameter 2.5mm wall thickness
986 S standard suspension 18.5mm diameter 2.6mm wall thickness
986 S M030 suspension 19.0mm diameter 2.7mm wall thickness

Yes the non-S M030 rear bar is the stiffest.

The solid bar will deflect about 28% less for a given torque than the hollow one. But I wonder if you'd notice that difference if the car is barely leaning with the hollow bar.

So the solid bar would be a 1.549 on this chart:

http://www.cb-racing.com/boxster_030.html

[John V]

So slightly less stiff than the GT3 bar on full soft. Interesting.

[blue2000s]

Originally Posted by John V

So slightly less stiff than the GT3 bar on full soft. Interesting.

I don't know the thickness of the GT3 bar, but about 75% of the stress on a torsion bar is taken by the outer 25% of the radius, so the OD has a much larger influence on stiffness than if it's solid or hollow. So it makes sense that the GT3 bar, being so large in diameter, would still have a higher stiffness.

[John V]

The effective diameter of a tubular bar is calculated by raising the inner diameter and the outer diameter to the fourth power, then taking the fourth root of the difference between those two numbers.

The GT3 bar (26.7mm and 3.5mm wall thickness) on full soft is 67% stiffer than the standard 23.1mm bar. The 24mm solid bar should be 55% stiffer than the same bar.

Granted this assumes the 24mm solid Eibach bar has the same lever arms as the stock bars and this calculation neglects the bending effect of the lever arms, which could be significant with a tubular bar. The GT3 bar is bent differently than the stock bar (which is what allows it to be adjustable) but it does have the same length lever arms.