The main function of a sway bar is to tie the two sides of the vehicle's suspension together, thereby preventing the vehicle from swaying back and forth while turning. The sway bar connects to the lowermost suspension component on the vehicle--the lower control arm--on both sides. There are a total of nine bushings on a control arm. There is one directly in the centre and four on each side, where it connects to the control arm.
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The most common symptom of a failed sway bar bushing is a rattling sound. This typically happens when one or more of the bushings on the side goes bad. These bushings sit atop and below a sway bar link; when the bushings go bad, the sway bar link will begin to make a metallic rattling sound because it is moving freely. This noise is typically constant and never changes. The repair is fairly simple and just requires replacement of the sway bar link bushings.
If the main bushing of the sway bar--the centre bushing--goes bad, the entire sway bar moves up and down. If you hit a hard enough bump or take a turn too hard, you can hear a loud thumping noise from the front end. This noise is the sway bar hitting the mounting bracket, where the bushing used to insulate. The volume of this noise depends on the intensity of the bump or turn. This involves a more difficult repair, compared to the links; the entire sway bar must be removed in some cases.
The bushings on the sway bar link are what give it the rigidity and flexibility needed to maintain a proper vehicle stance. When these bushings are compromised, the sway bar has now lost both its rigid positioning and the impacting absorbing ability of the rubber bushing. The vehicle's body will tend to flex a lot more and stop flexing abruptly when the sway bar impacts the metal mount or link. This flexing, also known as body roll, creates an inertial pull in the direction it rolls and the abrupt stop jars the vehicle in that direction. At high enough speeds, this loss of handling is an easy symptom to notice.