By definition, body roll only occurs when one side of the suspension is compressed (jounce), while the other extends (rebound). We can limit body roll by making it harder for the driver-side and passenger-side suspensions to move in opposite directions. One obvious method to achieve this is through the use of stiffer springs. A stiffer spring will compress less than a softer spring when subjected to an equal amount of force and less compression of the suspension on the outside edge will result in less body roll.

However, stiffer springs require the use of stronger dampers (struts or shock absorbers) and have an immediate and substantial effect on ride quality. So, even though handling is improved, they may not be the easiest or most cost-effective way to achieve the objective of reducing body roll. The use of anti-roll bars (also known as anti-sway bars, roll bars, stabilizer bars or sway bars), provides a more cost-effective reduction in body roll with minimal negative impacts upon ride quality.

An anti-roll bar (ARB) is a U-shaped metal bar that links both wheels on the same axle to the chassis. Essentially, the ends of the bar are connected to the suspension while the center of the bar is connected to the body of the car.
In order for body roll to occur, the suspension on the outside edge of the car must compress while the suspension on the inside edge simultaneously extends. However, since the anti-roll bar is attached to both wheels, such movement is only possible if the metal bar is allowed to twist. (One side of the bar must twist upward while the other twists downward.) So the bar's torsional stiffness-or resistance to twist-determines its ability to reduce body roll. Less twisting of the bar results in less movement into jounce and rebound by the opposite ends of the suspension-which results in less body roll.

There are two primary factors that determine an anti-roll bar's torsional stiffness: the diameter of the bar and the length of the bar's moment arm. Diameter is generally the easiest concept to grasp, as it is somewhat intuitive that a larger diameter bar would have greater torsional rigidity.

I wanted to experiment a little, so I source a Whiteline adjustable bar. By simply lengthening or shortening the moment arm, you can increase or decrease the stiffness of the bar. I bought my Whiteline Anti-Roll Bar from Jap Innovations.

The standard MY99/00 Impreza is fitted with a 19mm front and 20mm rear anti-roll bar. The Whiteline adjustable bar is 22mm

Installation

First you need to raise the rear of the car to get enough space to work under the car. A proper garage vehicle ramp or pit would be ideal as it would give you lots of room to swing the original bar out from under the car, but some simple wheel ramps are fine.

You'll need a 12mm socket wrench to remove the hanger bolt and a 14mm spanner and socket wrench to remove the drop link bolt. I have replaced my OEM drop links with some from Whiteline, so the bolt has a larger head but other than that the process is identical.

Once the bolts from the drop-links and hangers have been removed from both sides, the bar can simply be removed although it is slightly awkward since the bar bends around the backbox exhaust pipe.

Then it's just the matter of reversing the procedure and fitting the new bar. The Whiteline bar comes with some replacement rubber bushes, so leave the original mounts on the OEM bar. You just need to remove the metal bracket to re-use with the new Whiteline bushes, but this should easily come away. Use plenty of the white grease on both the inside and outside of the bushes and clip the new bushes onto the Whiteline bar. Then push the metal bracket onto the bushes and slide the bar behind the exhaust pipe and this should allow the bar to swing into place. Then just screw the brackets back into their original position and attach the bar to the drop-links - this is where you can decide which adjustment you want on the bar.

There are basically 3 positions that determine the 'stiffness' of the bar :-

1. The outer holes (closest to the ends of the bar) - minimum settings appearing as a 22mm diameter bar,

2. The middle holes - a good balance in stiffness (similar to a 23mm bar) and should take away much of the Impreza's in-built understeer,

3. The inner holes - stiffest setting (similar to a 24mm bar) and without upgrading the front anti-roll bar would probably induce too much oversteer.

After hearing from other users, I decided to try the middle setting as this gave the best neutral balance to the cars handling.