Why are M and non-M sway bars spec'd so radically different?

[tetsuo111]

I posted this over in 3-series, but read here often. This group seems a little more performance-focused

My 328i xDrive is a daily driver and also tracked as often as I'm able. I'm preparing to drop my rear subframe to install Al RSBs and delrin differential bushings. I planned to further upgrade the rear suspension in the future with M3 control links, but since this is such a large job, I decided to do all rear suspension upgrades at once. The final piece of the puzzle is the rear sway bar. To be very clear, I don't feel excessive body roll since replacing the OEM suspension with TC Kline. In fact, I'm inclined to leave the OEM rear sway bar alone.

It was interesting to learn the M and non-Ms share the identical front sway bar size of 26.5 mm. All the 3-series, from 325i to M3 have this size front bar, even though part numbers are different between the M and non-Ms.

The rear is different - the M3 rear sway bar is 22.5 mm while the non-Ms share an identical 13 mm sway bar. Can someone share any insight on the different suspensions? This will help me to decide whether or not to include a new rear sway bar in my project.

I can understand non-Ms having a softer rear bar to make the car safer to drive (more understeer) for the average driver. But considering the front bars are identical, the difference in size between the rear bars is a mystery to me. Especially since bar stiffness is an exponential function of diameter. The massive difference in size strikes me as impossible for both suspensions to be balanced.

What am I missing here?

[aloksatoor]

The LSDs on the rears of the M cars.

[tetsuo111]

I knew the LSD matters for suspension balance. I didn't realize just how much. Thanks for the insight.

[aloksatoor]

yep this is discussed in the hotchkis rear sway bar thread as well extrnsively.

[John_01]

The Front bar may have the same diameter spec, but the M3 version is firmer. The stock bar tapers to around 23mm in the center part but the M3 version is constant diameter. Also the M3 front sway bushings are a lot firmer than the tyre found on Non-M cars.

[tetsuo111]

I spent a good chunk of time reading that thread. I think that I just missed the magnitude of the LSD on the sways. Anyway, thanks again for bringing it to my attention. It's important for me to understand why a mod works or not. I'm going to sit tight and leave the OEM rear sway in place, but am upgrading the upper links to M3 parts, replacing the OEM toe arm with an adjustable arm, installing solid RSBs, and also delrin diff mounts. I'm looking to tame the sloppy rear end.