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: Fri, 15 Dec 1995 08:16:53 -0500
From: (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 throttle.

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 relabeled.

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 the Roundel.

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 caliper.

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,

Pete Read
88 M5

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