Unofficial BMW

Unofficial BMW

Google Search

What's New

Search (Google!!)









Used Cars




In Association with

Home E12 E24 E28 E30 E34 E36 Z3 E39 E46 X5/E53 ALL
Ron Stygar Carl Buckland Dale Beuning Forums Help

Unofficial BMW Nav Map

Date: Wed, 9 Sep 1998 01:22:58 +0000
From: "Aaron Bohnen" <>
Subject: <E28, All> small stabilizer bar upgrade gives noticeable improve

Hi everyone,

I just replaced the stock 14 mm rear stabilizer bar on my 533i with the 15.5 mm rear bar that came on the 535i and 535is models. It made a quite noticable difference to the way the car handles. (The front stabilizer bar in this car is already a 19 mm unit, common to the 533i and 535i and 535is models)

My neighbour said I must be kidding and asked me how just 1.5 mm of additional diameter on the stabilizer bar could change the handling of the car so noticeably. Being a university lecturer, I can't ever resist the opportunity so I told him that basically, the torsional stiffness of a solid cylindrical shaft is proportional to (among other things such as the shear modulus of the material, the length of the bar, etc.) the third power of the diameter. Doing the math at home tonight, it turns out that the 15.5 mm diameter bar is 36 or so percent stiffer than the 14 mm bar. A not unsubstantial difference for a seemingly small 1.5 mm diameter increase!

Of course this works the same way and more so as the diameters in question increase. Had I gone to the 16 mm diameter rear bar from the 14 mm unit I started with, the stiffness would have increased 49% rather than just 36%. Had I gone from the 14 mm original bar to the 19 mm M5 rear bar the stiffness would have increased a whopping 149%. This explains why those E28 owners who go with the M5 bars need to upgrade their swaybar mounts.

So, for anyone out there who may have been wondering whether to upgrade to stiffer, aftermarket (expensive) stabilizer bars - there may be versions of cars from the platform on which yours was made that came with stock larger diameter bars. If you want a stiffer anti-body-roll effect, but are unwilling to go too far with the $$$ side of things, such stock bars from other cars on your platform may be just the ticket - they often will fit in your stock fittings and use your existing swaybar links.

Now I'm speculating wildly but it seems to me at least in the E28 world that the "e" version of the car (528e) had smaller diameter bars all around than the "i" versions (533i and 535i). Perhaps the smoother ride was considered more desirable with the less performance-oriented car. At any rate, if you're an "e" owner of any description and you're interested in anti-bodyroll treatments I'd be checking out the stabilizer bars from the originally more performance-oriented cars on your platform.

Now there's always the very legitimate question of whether stiffer stabilizer bars are the way to go with a performance suspension. There is one school of thought that uses softer than expected bars and relies more on accurate wheel placement. I won't get into that here - only suffice it to say that for a daily driver such as my 533i, the upgrade to better-yet-still-factory stabilizers was noticeable and enjoyable without being at all extreme or uncomfortable.

Something to consider, anyway... :>

best regards to all,


p.s. I guess I've never listed my car and mods, etc. to the digest - so here it is:

1984 E28 533i 235k km Bronzitbeige with pretty well all the 535i trim inside - including the deluxe BMW speaker set (with upgraded tweeter crossovers - polypropylene caps, mini-Zobels, L-pads, etc. - makes a BIG difference to the high end), rear parcel shelf, 3rd eye brake light, lighted power window switches, etc. etc. No ICV (cool manually adjustable homebrew instead - works great), BL/SS (069 Euro M3 lever - works great), RL/BB (E32 750 front brakes kit - hot damn! Awesome!), Stebro SS exhaust (love it), KYB shocks (not too bad, actually - better than the stock Boge Gas, but soon to be replaced by Bilstein HD's), Red illuminated Sony in-dash CD-player (modded by me due to personal oversight at having originally purchased a model with green illumination. Duhh!), upgraded brakelight assemblies, 3.25 LSD, Redline MTL, Synquest 75W90, 15.5 rear swaybar, 2 new fuel pumps (feels like another 15 hp - I love it!), DIY SI and oil indicators reset, DIY pressure brake bled twice yearly, etc. etc... too much more little stuff. Unfortunately no Jim C. shark chip, but purposefully no K&N filter. :>

Aaron Bohnen email:
  • -Ph.D. Student, Civil Engineering Department, U.B.C.
  • -Technicraft Engineering Services

Unofficial Homepages: [Home] [E12] [E24] [E28] [E30] [E34] [E36] [Z3] [E39] [E46] [X5/E53] [ALL] [ Help ]