An anti-sway bar is designed to reduce body roll.  It keeps your vehicle flatter in the turns by transferring the pressure exerted on the outboard wheels and applying opposite pressure on the inboard wheels.  In this way weight is more evenly applied to all four wheels, helping the vehicle stay more level with the road.

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The E36 and E46 3-series are indeed very similar in suspension design and specifications.  For our discussion about sway bar sizes, however, we shall focus specifically on the E36 M3 to demonstrate what altering the sway bar sizes does to handling characteristics. 

When the 95 M3 was introduced, it was said to be a agile handling car, with some body roll, with a tendency to understeer.  When the 96 M3 was introduced, the car is said to have increased tendencies to understeer more with no improvement in reducing body roll.

Obvious changes from 95 to the 96-99 model year M3s included tire size.  This was a contributing factor to the increased understeer of the car:

By specifying more narrow front tires with wider rear tires, the car will want to continue to stay "planted".  Since US M3s do not have enough horsepower to effectively throttle steer, the this new balance causes the newer M3s to understeer more (or plow) when pressed to the limits.

While most current aftermarket sway bars reduce the amount of roll with larger diameter bars (being stiffer in nature), they do not address the issue of restoring "balance" at the same time.  Using the formula for the polar moment of inertia (in⁴), sway bar stiffness can be calculated assuming the following parameters:

• Bushing material remains the same
• Mounting location remains the same
• Metallurgy remains the same
• Adjustments can be made to duplicate the stock position
• No variations in end link length, tension or compression

When comparing the stock sway bar sizes to aftermarket bars, to the 95 M3, you find some interesting results.

Racing Dynamics' dimension further increase the car's tendency to understeer since the new sway bars increase the front stiffness more than the rear.  It's increases in stiffness do much to reduce body roll.

Dinan's sway bars are designed to dial in more oversteer, to counteract the factory designed understeer.  However, this is at the expense of increase front body roll.

Yet, when applied to the newer M3s, the picture changes.

Racing Dynamics' near equal increases in sway bar stiffness as shown on the 95 M3 are now very skewed.  The 96-99 M3's tendency to understeer more than the 95 M3 is increased with the imbalanced nature of the rates of stiffness.  Dinan's increases are now less dramatic, but still does not do address the issue of front body roll.

What do the numbers mean to me?

With all of the "mathematical" increases in stiffness, what does it mean to you, the driver?  Extensive testing has shown that as driver skill increases (at the track/auto-x) that sway bars that increase stiffness too much cause inside front wheel lift in corners.  By making a front end too stiff (to decrease body roll), the chassis is not able to plant both wheels under turns with increased speed.  Anything stiffer than a 1.0" front bar has shown to be stiff enough (and can be reduced in stiffness) to be the ideal size.

What about the rear bar?

A common misconception is that the increase in rear bar stiffness alone is the sole cause for bent (or broken) rear sway bar tabs (the tabs which mount the sway bar bushings).  In fact, even under race conditions, most racecars do not break the tabs.  Rather, rear sway bar tabs bend (or break) because of daily obstacles such as potholes, construction, and other road hazards.  It is the sharp transition and weight shift in such conditions which will weaken and stress the factory tabs.  In the events that the tabs do break, they are built into the rear subframe of the vehicle, which becomes a costly replacement.

Click here for important information about 1999 M3s.