Automobiles are oddly organic things, at least insofar as design. Fluids pump through lines like veins and arteries, engines convert hydrocarbon fuel to energy just like cellular mitochondria; even the ball-and-socket design of your shoulder and hip joints has found its way into the car through your suspension. The ball joints that allow your steering suspension to both move vertically and horizontally and pivot about an axis work on exactly the same design principles that allow your arms and legs to do the same things. And when they fail, you could find yourself standing on some very shaky ground.
The first indication of wearing ball joints can be very subtle, and develop slowly enough that you may not notice. Ball joints, because they wear in all directions and are tied into so many other parts, will tend to make a vehicle feel vague and wandering where it was once sharp and precise. This is particularly noticeable in the steering, where even a small amount of wear can allow your steering wheel to wander a bit just off-center before it turns the wheels, and wander a bit more as you turn them back. But this initial vagueness is easy to miss or ignore. Less so are later driving symptoms. When ball joints start wearing out more, the vehicle may start pulling to one side, and may feel darty, loose, and spookily slow to respond to corrections when it does decide to veer off course. Alternately, depending on how the joint is worn, and which joints are worn, the steering itself could become very tight and "notchy" when it moves, while the vehicle responds in a nervous and spastic manner to inputs.
Loud banging or popping noises are a primary indicator of bad ball joints, but the type and degree of the noise will vary from joint to joint. Steering ball joints don't often make much noise, aside from perhaps a slight tap when you turn the wheel. It's quiet enough, though, that you're as likely to feel it through the steering wheel as hear it. Upper and lower suspension ball joints aren't nearly as restrained; a really bad one can be about as subtle as a sledgehammer to your fender. They'll start out making quieter thuds and clunks when you hit a pothole or speedbump, eventually progressing to a single, loud bang over smaller imperfections, and as you enter into a left or right turn. Once you start hearing that, address the ball joint immediately before your suspension hammers itself to pieces.
Ball joints will wear out fairly slowly, at least in the initial phases. Even before you start hearing noises or notice changes in the way your car drives, you may see evidence of failure in your tires. As ball joints fail, they'll typically allow the front of the wheels to point outward, away from each other in a condition called "toe-out." This toe-out is actually what makes a vehicle with worn ball joints feel twitchy and unstable, because the tires are always trying to turn away from the car. It also causes a particular kind of tire wear. Toe wear starts as uneven wear on one side of the tire tread, and feathers gently across the tread toward the middle. This is a subtler variation of "camber wear," which is also common on tire with ball joint issues. Camber wear happens when the tire leans either in or out at the top, causing a hard line of excess wear along one edge of the tire. Either or both of these can indicate ball joint problems.
Diagnosis -- Rocking the Tire
Lift the vehicle off the ground so the wheels are hanging. Grasp one tire at the top and bottom, and try to pull and push the top and bottom in and out. If the tire rocks by any noticeable degree, and especially if the movement is accompanied by a clinking or clunking sound, you likely have a bad ball joint or two. This test can also indicate a bad wheel bearing, but that will also grumble and vibrate as you drive in a straight line. Next, try rocking the tire side to side, as though you were attempting to turn it. A certain amount of play is fairly normal, but listen for clicking noises, and watch the other tire and the ball joint on the steering end-link. If the tire is just moving the end-link back and forth without moving the linkage or the other tire, it's likely worn out.
Some lower ball joints also often have wear indicators on the bottom. The ball of the joint presses on the cup-shaped socket in the bottom. As the ball and socket wear, the socket sinks up into the joint, and the case slides down over it. The exposed, protruding part of a new ball joint is called -- appropriately -- the "shoulder." If your vehicle has those type of ball joints, lay a straight edge on the bottom of the ball joint. It should come to rest on the shoulder protruding from the bottom of the joint, not the edges of the joint case. If the bottom of the joint is completely flat, or the shoulder is recessed into the case, it's worn out. A set of feeler gauges can be handy here, since the clearances involved are so small that they may not be visible to the naked eye.
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