The stabilizer bar is known by many names on both the left and right sides of The Pond; sway and anti-sway bar, roll and anti-roll bar, among others. But no matter what you call it, this vital suspension component acts as something of a counter-balance for your car's main springs. Over time, the bushings that connects it to the chassis can wear out and fail, reducing its effectiveness.
Your car's main springs -- be they coil, leaf or air -- work by pushing each of the wheels toward the ground. When you turn, the springs on the outside of the turn will compress to control body movement, while the springs on the other side will expand. The problem with relying solely upon the main springs is that the springs on the inside of the turn will attempt to push the car over, thus negating the effects of the outer springs and encouraging body lean.
The sway bar is a roughly U-shaped, spring-steel bar that connects the wheel on the right side of the car to that on the left. The "top tips" of the U connect to the suspension just behind the wheels, and the long "base" of the U goes through circular brackets mounted to the chassis. It pivots freely inside these circular brackets, riding inside of cylindrical bushings or "isolators" made of rubber or plastic. When the car body attempts to lean, the rising inside portion of the body tries to bend the bar, and the bar resists body roll by pushing back.
The isolator's primary purpose is to keep vibrations from the suspension from snaking up into the car's body and rattling your fillings lose. But it does something else, too; it momentarily slows the action of the sway bar, allowing a degree or so of body roll before the rubber compresses and the sway bar bends.
Rubber components work kind of like brake pads, converting motion on a large scale into heat spread throughout the rubber's molecular structure. Over time, constant heating and cooling will break the molecular strands that hold the rubber together, causing it to permanently soften. When that happens, the sway bar engagement will slow, allowing for more body lean before the sway bar resists movement. On practical terms, the extra body lean caused by bad sway bar bushings is very subtle; you might not even notice it apart from the fact that the car feels a bit sloppier and more vague during cornering and quick lane changes.
Not all bushings get softer over time; some actually dry out, get hard and shrink slightly. Hard, shrunken bushings will allow more immediate slop in terms of body roll, followed by a sudden cessation of body roll and changes in handling balance during cornering. The same thing will happen if the bushing actually tears or falls out of the bracket, but to a much greater degree. If left unchecked, the sudden and violent engagement of the sway bar can damage the sway bar end links, or break the D-shaped bracket that hold it to the frame.
