Date: Fri, 15 Dec 1995 08:16:53 -0500
From: read_at_engr05.comsys.rockwell.com (Pete Read)
Subject: Sway Bars
Don Stevenson asks about E30 Sway Bars:
>I need some Sway bar advice...
>Does anyone have experience with any of these?
I had similar sway bar questions for my 88 E28 M5 several
months ago. To figure out sway bars, and other suspension
basics, I purchased "How to Make Your Car Handle" (Fred
Puhn, HP Books).
Sway Bar Basics
When cornering, sway bars (and springs) control body roll
and the amount of weight transfer at each end of the car.
More weight transfer causes greater tire loading and slip at
that end of the car. So, for example, larger/stiffer rear
bars cause more tire loading and slip at the rear (oversteer)
and larger front bars generally understeer more. Front and
rear stiffness has to be balanced. Ideally you'd have slight
understeer and the ability to induce oversteer with the
Sway bars are just torsion springs. Stiffness increases to
the fourth power of bar diameter. So the easy way to
determine sway bar stiffness increase is:
(new diameter/old diameter) to fourth power
So for my car, changing the rear bar from 18mm to 19mm
increased sway bar stiffness 24%. (19/18)^4 = 1.24
This isn't a 24% increase in overall rear roll stiffness.
The rear springs haven't been considered and the rear sway
bar end may not move in a one-to-one ratio with wheel travel.
>How "adjustable" are the Suspension Tech bars?
The E28 M5 Suspension Technique bars have three adjustment
holes in front and two in the rear, at one inch increments.
To figure out the actual bar stiffness or the impact of
using different holes, you need to measure the bar and use
the formula below. My rear bar stiffness increases 28% by
adjusting to the inboard hole (calculation below).
Calculating Sway Bar Stiffness
>From the Puhn book, page 150.
The formula for sway bar stiffness of solid steel bar
(Factors both twisting of section B and bending of arms C)
K (lbs/in) = 500,000 D^4/((0.4244xA^2xB) + (0.2264xC^3))
A| / \ C
| / \
D = Dia bar
B = Length of center section
A = Length of end perpendicular to B (torque arm)
C = Length of each end
On my E28 M5, the rear sway bar measured:
B = 34.5 inch
A = C = 9.25 inch to outer hole (bar is shaped like |__|)
D = 0.748 inch (19mm Suspension Techniques)
Using the formula above for the outer hole:
K = 500,00x0.748^4 /((0.4244x9.25^2x34.5)+(0.2264x9.25^3))= 109
Change A and C to 8.25 inch for bar adjusted to inner hole
K = 500,00x0.748^4 /((0.4244x8.25^2x34.5)+(0.2264x8.25^3))= 139
So a 28% bar stiffness increase by adjusting to the inner hole:
139/109 = 1.275 or about 28% increase
Suspension Techniques (ST)
There aren't that many choices in sway bars for the older
E28 and E30 cars as far as I can tell. ST seems to
manufacture bars for several other companies, which are then
I'm happy with the performance of my ST bars, but the
quality control was less than perfect. The front bar, third
hole in, was 0.080 inch thicker at one side of the hole than
the other (end wasn't completely flat before the holes were
drilled). Without a flat surface, the suspension link bolt
can't be tightened correctly to that hole in the bar.
Eurasian Auto Parts, where I bought the bars, checked two
other ST M5 sway bar sets and found the same problem.
Also the front urethane bushing was sized for a E28 535i
sway bar mount - the E28 M5 mount is much larger (rear is
the same as a 535i). ST was contacted, I explained the size
difference, and they sent me the wrong bushing again. After
several go-arounds, Eurasian finally compensated me with a
parts credit because it was easier for both of us. I still
have the stock rubber front bushings and can't use the front
inboard hole until it's machined flat.
Eurasian is advertising E30 22mm/19mm ST bars for $275 in
Stock Bar Size
>Are the Stock bars 22MM front and 16MM rear?
Usually the parts fiche has the sway bar sizes listed. Or
just reach under your car and measure the diameter with a
Based on the advertised 22mm/19mm ST bars, I'd be surprised
if the stock bars were 22mm/16mm unless your car understeers
like crazy. If the 22mm/16mm numbers are right, ST bars
leave front bar stiffness the same (same size), but almost
double rear bar stiffness (19/16)^4 = 1.99.
Hope this helps,
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