[vachss]

I know H&R and Hotchkis make bars for the Z. Stock diameters on the 3.0si are 25/19mm F/R (24/18mm for non-Sport) and the aftermarket bars are something like 27/21mm. I believe the torsional stiffness of a bar varies with diameter squared so if the end link geometry is the same I'd say the stiffness of the aftermarket bars is something like 15-20% greater than stock. Aftermarket bars are usually adjustable though, so this is just a first cut.

Update: A couple of things. First the M bars are significantly thicker than those on the 3.0si - realOEM says 27/22.5mm F/R. Also while 27/21mm is correct for the H&R bars, the Hotchkis are quite a bit bigger. Here's specs I downloaded...

Specifications:

- Front 28.5mm Tubular 3 Position Adjustable
- Stiffness: +19%,+38%+61% (Compared to stock Z4 Sport)

- Rear 24mm Tubular 2 Position Adjustable
- Stiffness +59%, +86% (Compared to stock Z4 Sport with 24mm front and 19mm rear)

It looks like the aftermarket bars I'm seeing are for the non-M Z4s. I don't know if the M and non-M bars are interchangeable. Regardless, it looks like the H&R bars are roughly the stiffness of the stock M bars, while the Hotchkis set up is subtantially stiffer than either. It would be nice to find out if the M and non-M bars are swappable as lots of us 3.0si drivers would probably just as soon install stock M bars as an aftermarket kit.

Update 2: A careful reading of realOEM shows that the part numbers for the end links the sway bars fit into are the same for the M and non-M Z4 coupes. I suspect this means that they are interchangeable (but somebody should definitely verify this before ordering anything).