Many load-carrying tesion-type lower ball joints on General Motor and Ford rear-wheel drive applications have a built-in "wear indicator" to show how much wear has taken place inside the joint.
If it weren't for ball joints, suspensions would be much heavier and stiffer than they are today. Up until the 1950s, king pins served as the hinge pins and pivot points for the front steering knuckles. King pins are still used today in heavy-duty trucks. But tough as they are, king pins lack the flexibility and range of motion of a ball joint that can swivel and flex at the same time.
The first ball joints were used by the French automaker Citroen back in the 1930s. But it wasn't until 1952 that ball joints were used on a domestically-built car (Lincoln). By the end of the decade, almost all passenger cars were running on ball joint suspensions.
The steel-on-steel ball joints that were used in these applications began to change in the 1970s. To reduce friction for improved steering return and better overall ride quality, ball studs were made smoother and new types of bearing materials were introduced including powder metal and polymer bearings. In the 1980s, "low friction" ball joints with polished ball studs and polymer bearings came into widespread use on a variety of vehicle applications, including European and Japanese imports, domestic front-wheel drive passenger cars and even trucks. At the same time, joint seals were improved to eliminate the need for grease fittings in many instances. So today you're apt to find a wide variety of different types of ball joints in use.
ON THE BALL
Ball joints can be classified according to their location or function. There are upper and lower ball joints. Short and long arm (SLA) suspensions have both uppers and lowers as do "wishbone" strut suspensions. But ordinary MacPherson strut suspensions only have lower joints.
Some ball joints are load carrying while others are nonload carrying depending on their position in the suspension. And the loaded variety come in two basic types as well: compression joints and tension joints.
Compression ball joints are those designed to carry loads that bear down on the ball stud. Consequently, most joint wear occurs where the ball stud presses into its bushing. This type of joint is found in SLA suspensions where the spring is mounted over the upper control arm.
Tension joints, by comparison, are designed to carry loads that try to pull the joint apart. Wear occurs in this joint at the point where the shoulder of the ball stud pulls against its seat. The lower ball joints on SLA suspensions where the spring sits on the lower control arm are tension joints. Tension joints are also found on "modified" MacPherson strut suspensions (Ford Mustang, Fairmont and T-Bird) where the spring sits on the lower control arm rather than on the strut.
Many load-carrying tension-type lower ball joints on General Motors and Ford rear-wheel drive applications have a built-in "wear indicator" to show how much wear has taken place inside the joint. This same type of joint is also used in the rear suspension on some of GM's big front-wheel drive cars (Cadillac, Buick and Oldsmobile). The wear indicator is the shoulder on the grease fitting. As the joint wears and the stud sinks deeper into the housing, the grease fitting also recedes into the housing. Joint wear is considered acceptable as long as some shoulder protrudes above the face of the housing. But once the shoulder becomes flush with the housing, it's time to replace the joint.
The third type of ball joint is the nonload-carrying joint (also referred to as a "dampening" or "follower" ball joint). This type of joint is used for the upper ball joint on SLA suspensions where the spring seats on the lower arm, the lower ball joint on suspensions where the spring is over the upper control arm, and the lower ball joint on MacPherson strut suspensions where the spring is mounted around the strut. Because this type of joint does not carry weight, it is preloaded to keep it tight and to provide a resistance for improved steering stability. Any play in this type of joint means it should be replaced.
Wear-indicator load-bearing ball joints.
WHY JOINTS WEAR OUT
Like any other part that is subject to friction and loading, ball joints eventually wear out. Nobody can say for certain how long a ball joint will last, because wear depends a lot on the type of driving that's done (rough roads as compared to smooth ones), how often the joint is greased (if greaseable), the design of the boot (how tightly it seals out contaminants) and exposure to water, dirt and road salt.
Two types of friction ball joints.
Load-carrying ball joints do tend to wear at a faster rate than their unloaded counterparts because of the weight they carry. That's why the lower ball joints on an SLA suspension typically wear out before the upper joints.
The higher the mileage on the odometer, the greater the likelihood that the ball joints may be worn. Any vehicle that has more than 50,000 miles on a set of ball joints should be closely inspected for possible ball joint wear.
Some of the symptoms of worn ball joints include:
Front wheel shimmy at low speed;
Clunking noises from the front suspension;
Camber wear on the tires.
The different types of ball joints require different inspection procedures - which means first identifying the type of joints on the vehicle itself.
On vehicles that have ball joints with built-in wear indicators, checking the joints is relatively easy. As long as the shoulder on the grease fitting is still above the face of the housing, the joint is considered to be within normal wear limits. But for an accurate wear indication, the joint must be checked with the full weight of the vehicle resting on the suspension, and the tires in full contact with the ground.
Wear indicating ball joints should not be checked with the wheels raised off the ground because the weight of the tire, wheel, steering knuckle, brake rotor and caliper are not supported and push down on the joint. This can push the indicator out giving the false impression that the joint is not worn.
Joints (all types) should also not be greased prior to inspection because filling the joint with grease can make the joint appear to be less worn than it actually is.
Something else to keep in mind about ball joints with built-in wear indicators is that the wear indicator only shows axial (up and down) wear, not radial (sideways) play. Radial play is usually not a factor because of the way in which the joint carries the load.
On certain Chrysler applications, a slightly different type of wear indicator is used. As the joint wears, the grease fitting becomes progressively more loose rather than receding into the housing. If the fitting can be wiggled by hand, the joint is worn and needs to be replaced.
On vehicles that have loaded lower ball joints without built-in wear indicators, the joints have to be unloaded to check for looseness. This is done by raising the wheels off the ground and supporting the lower control arms. The support stand should be placed as near to the joint as possible. The lower ball joint can then be checked for axial (vertical) play by wiggling the wheel up and down. Radial (lateral or sideways) play can be checked by rocking the wheel in and out. Don't use a pry bar between the lower control arm and knuckle because doing so may make a good joint appear to be loose.
The amount of play that's considered acceptable will vary according to the application, so always use a dial indicator to get an accurate reading. Then you can compare joint movement to the specifications listed by the manufacturer to determine if the joint is worn or not. Axial play should be measured between the knuckle and lower control arm, and radial play between the lower control arm and wheel rim.
The old rule of thumb that says any more than .050" of vertical play calls for joint replacement doesn't necessarily hold true for all applications because some specifications allow no movement while others allow up to several times as much vertical play! The maximum limit for radial (sideways) play in most applications is 0.25", but again always check the specifications to make sure.
The upper nonload-carrying ball joint can be checked by grasping the top of the wheel and attempting to rock it in and out. Any looseness in the upper ball joint means the joint should be replaced.
In suspensions where the upper ball joints are the loaded ones, the upper control arms must be supported to take the weight off the joints. This can be done by wedging a block of wood between the upper control arms and frame to support the vehicle's weight when the wheels are raised off the ground. The lower control arms must be allowed to hang free, with the supports positioned under the frame, not the lower control arms. Both axial and radial movement in the upper joint can be checked by rocking the tire up and down, and in and out. As with loaded lower ball joints, a dial indicator should be used for accurate measurements. The limit for lateral play is usually 0.25". Refer to the manufacturer's specs for vertical play. The lower ball joint in this type of suspension is unloaded, so any looseness would indicate a new joint is needed.
On vehicles with MacPherson struts, the strut carries the load and the lower ball joints are unloaded. Play can be checked by raising the wheels off the ground so the lower control arms hang free with the strut fully extended. Rocking the wheels in and out should produce no horizontal play between the control arms and knuckles if the joints are good. On Chrysler FWD cars or others with built-in wear indicators, the wheels must be left on the ground as described earlier.
On some vehicles, the vehicle manufacturer may recommend a more involved joint checking procedure. Toyota, for example, recommends the following procedure for preloaded ball joints: disconnect and separate the ball joint stud from the steering knuckle. Then use a torque wrench to measure how much effort it takes to rotate the stud. If the amount of force required is less than the specs (typically 9 to 35 in.-lbs.), the joint is worn. Excessive friction would indicate binding inside the joint, probably due to dirt contamination.
When a ball joint is found to be worn (wear exceeds manufacturer's specifications), it obviously needs to be replaced. But should you also recommend replacing the companion joint on the opposite side, or even all four joints if the vehicle has an SLA suspension? It depends. Some states have strict rules about ball joint replacement. No replacement is allowed unless wear exceeds manufacturer specifications.
When one joint is badly worn, chances are its companion joint on the opposite side will also be badly worn or nearing the end of its service life. The other joint may still be marginally within specifications, but for how much longer is anybody's guess. Probably not much longer. That's why many technicians recommend replacing both joints even if one is still within specifications.
As for replacing all four joints, it depends on mileage and play. Again, if the other joints in an SLA suspension are still within specifications, but have a lot of miles on them, it may be wise to go ahead and replace them all in spite of the extra cost. Inconvenience is a big factor in auto repair these days, so replacing all the joints at one time means the customer won't have to come back when the next joint fails, and when the next one after that fails, and so on. Do it once and be done with it. Just make sure your customer understands that it's a voluntary decision and is being done to restore the suspension to "like-new" condition rather than a necessary repair.
Actually, a ball joint can have a fair amount of play in it before it will have a noticeable effect on tire wear or steering stability. A "tight" joint, on the other hand, can cause a very noticeable and undesirable change in steering effort and steering return. So the condition of the joint is another factor that must be considered when making a replacement recommendation.
There is a valid danger to postponing the replacement of worn joints, and that is that waiting too long increases the risk of a catastrophic joint failure. If the joint wears to the point where it pulls apart, the suspension will collapse causing the driver to lose steering control. That's something nobody wants to happen.
Fortunately, it takes a lot of wear for that kind of failure to happen, but it can and does happen. So if one or more ball joints are found to be worn, repairs should not be put off for too long. And if a joint is found to be really bad, repairs should be made on the spot.
Another item that should be checked when ball joints are replaced is the stud hole in the steering knuckle ó especially if the ball joint stud has broken or is loose. An out-of-round hole can allow flexing that leads to metal fatigue and stud breakage. The new ball joint stud should fit snugly in the hole without rocking, and only the threads of the stud should extend above the hole.
On front-wheel drive suspensions that use a pinch bolt arrangement to lock the ball stud in the knuckle, do not use a chisel to spread the flange as doing so may damage the flange. Replacing the pinch bolts and nuts is also recommended.