Driving on a flat smooth road, your vehicle rides quietly and smoothly but hit the slightest of a rough road and your vehicle knocks and rattles along. Otherwise, as far as you’re concerned, your vehicle drives perfectly. “What is with the rattle? It sounds serious.”
There are a lot of sources for rattles on vehicles but one that is very common to late model vehicles is sway bar links and bushings.
Wikipedia says “a sway bar, anti-roll bar, or stabilizer bar is a part of an automobile’s suspension that helps reduce the body roll of a vehicle during fast cornering or over road irregularities.”
Most vehicles have one of these bars on both the front suspension and the rear suspension. They connect opposite (left/right) wheels together with a torsion spring and short lever arms.
Body roll is that feeling that your vehicle is going to tip over in a corner. The sway bar moves the force, that is pushing the body down, on the outside wheel of a turn to the opposite wheel. It is also pushing it down thus reducing the roll.
The design of these sway bar mechanisms is such that the automobile engineer achieves the level of ride and handling that is desired. Sports cars have high roll stiffness and luxury cars have less roll stiffness. There are genuine off road four by four vehicles where roll stiffness is not desired at all.
Front and rear sway bars are used to tune vehicle handling traits. Together or singularly they can determine the degree to which your vehicle understeers or oversteers. An oversteering vehicle will slide the rear tires before the front tires in a corner. An understeering vehicle pushes through corners because the front tires slip before the rear tires. Generally vehicle are tuned to understeer because that behaviour is easier to control in an emergency.
Getting back to the rattle or knocking coming from your suspension, the sway bar is usually the shape of a wide “u”. It twists about two rubber mounts or sway bar bushings to the vehicle body. The end of the bar is then connected through a short link connected to the suspension at each wheel. These links come in many different forms. What used to be a threaded rod with rubber donuts connecting the bar and the suspension is now two ball and sockets.
These ball/socket type joints are the most likely source of rattle as the ball wears in the socket. Left too long and the ball will pop out of the socket. Yes, now the noise may be gone. At this point your sway bar is no longer functional. The emergency handling of your vehicle is compromised severely.
The bushings mounted to the body also wear out. They are many times the source of rattles as well as squeaks.
These components are not only annoyances as they are wearing out. Worn sway bar components upset the handling balance of your vehicle. As is typical with suspension components the wear is gradual and because of that the driver does not recognize the change unless he/she is lapping the Nurburgring for pink slips.
Seriously though, worn sway bar components will come to play when you try to avoid that deer that steps gingerly in front of your vehicle. The difference being a successful evasive maneuver or a trip into the fajoles.