<E28, All> small stabilizer bar upgrade gives noticeable improve

["Aaron Bohnen" <bohnen@xxxxxxxxxxxxxxxx>]

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,

Aaron

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: bohnen@domain.elided
- -Ph.D. Student, Civil Engineering Department, U.B.C.
- -Technicraft Engineering Services

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