From digest.v9.n162 Fri Sep 11 09:10:47 1998
From: Pete Read <pete.read_at_boeing.com>
Date: Fri, 11 Sep 1998 11:50:47 -0400
Subject: <E28><E24> Sway Bars and Calculated Rates

I've posted some of this before, but the recent discussion of E28 sway bars (anti-roll bars) made me look back at my notes again.

Starting in 9/84, the E28 and E24 sway bars are very simple, with only three combinations -- M5, M6, and all the rest.

E28, E24 Sway Bars 9/84-on (1/97 prices)

1. E28 M5 only, 25mm/18mm

   31 35 2 225 886 front sway bar 25mm 220.02 33 55 2 225 888 rear sway bar 18mm 167.58

2. E24 M6 only, 21mm/16mm

   31 35 2 225 420 front sway bar 21mm 219.55 33 55 1 128 392 rear sway bar 16mm 234.75

3. E28 528e, 533i, 535i, and E24 633i, 635i 19mm/15.5mm

   31 35 1 127 509 front sway bar 19mm 199.00 33 55 1 128 391 rear sway bar 15.5mm 149.10

Earlier cars aren't quite so clear-cut, as they had 18mm front bars and 14mm rear bars change at different dates. The 528e and 533i had the 18mm front bar up to 9/83, then switched to the 19mm bar above. The 633i had a 19mm front bar with a different part number that switched to the one above at 9/83. A 14mm rear bar was used up to 9/84 on the 528e and 533i and up to 11/83 on the 633i.

Yes, the M5/M6 bars (except M5 front) can be used on the cars with 19mm/15.5mm bars just by replacing the rubber sway bar bushings with standard bmw bushings of the appropriate (larger) size. The metal brackets and links are the same. The M5 front bar requires welding stronger frame mounts and a larger bracket/rubber bushing, which are all available as standard BMW parts.

Calculating Sway Bar Rate

If you're substituting a larger diameter bar that otherwise has the same dimensions, the rate increase is simply:

(new diameter/old diameter) to fourth power

So when I changed my M5's rear bar from 18mm to 19mm, the stiffness increased 24%. (19/18)^4 = 1.24

Now, if you want to calculate the actual rate or have adjustable sway bars (several adjustment holes to change length of lever arm) and want to figure out the rate increase, you'll need to measure the bar and use a formula (from Fred Puhn's "How to Make Your Car Handle").

Formula for sway bar stiffness of a solid steel bar

500,000 D^4
K (lbs/in) = -------------------------------------

(0.4244 x A^2 x B) + (0.2264 x C^3)

               B
        ----------------
  A|   /                \ C
   |  /                  \

D - Diameter bar (inches)
B - Length of center section (inches) A - Length of end perpendicular to B (torque arm - inches) C - Length of end (inches)

On the E28s, both front and rear bars have nearly a 90 degree angle between the arms and center section. So I assumed A = C for my calculations.

               B
       :----------------:
    A  :                : C (= A)
       :                :

Example calculation for M5 stock 25mm front bar D = 25mm = 0.984 inch

  A = C     = 11.25 inch
  B =         31.5  inch 

500,00 x 0.984^4
K = ------------------------------------------------ = 233 lb/in

((0.4244 x 11.25^2 x 31.5) + (0.2264 x 11.25^3))

Sway Bar Adjustment (stock bars aren't adjustable)

Changing the settings of an adjustable front or rear bar is a simple process of moving two bolts to different holes (which changes the lever arm length "A" -- see above).

Jack the car so both wheels are off the ground and there's no twist in the bar. Move the adjustment bolts to the same hole on each side of the bar. Moving the bolt inward (shorter arm) increases roll stiffness at that end of the car, while moving to the outer holes (longer arm) gives less roll stiffness.

For more understeer, either stiffen the front bar (move to inner hole) or loosen the rear bar (move to outer hole). Create more oversteer by stiffening the rear bar (move to inner hole) or loosening the front bar (move to outer hole).

Pete Read
'88 M5
Arlington, VA