Ball joints are chassis parts that connect the steering knuckles to the control arms. The ball and socket design of the joint allows it to swivel so the knuckles can pivot as the wheels are steered, and to arc so the knuckles can follow the vertical motions of the suspension as it reacts to changes in the road surface.
Vehicles with short-long arm (SLA) front suspensions have four ball joints (two uppers and two lowers), while most strut equipped vehicles have only two lower joints in the front suspension (except those with "wishbone" strut suspensions such as Honda that have four). Rear ball joints are also used in some front-wheel drive cars with independent rear suspensions.
Ball joints that are "loaded" carry or support weight (for example, the lower joints on rear-wheel drive cars and trucks with SLA front suspensions). Ball joints that are "unloaded" carry no weight. These include the lower joints in front-wheel drive cars with MacPherson struts, and the upper joints on rear-wheel drive cars and trucks with SLA suspensions where the coil spring is on the lower control arm. On SLA applications where the coil spring is over the upper arm, the upper ball joints are loaded and the lower ones are unloaded.
Though most original equipment ball joints today are designed to provide lasting durability, they may fail prematurely for a variety of reasons. One is wear. The constant friction created by turning and driving on rough roads creates friction between the ball stud and bearing. And the rougher the roads and the more frequent the turns, the faster the rate of wear.
Wear can be further accelerated by contamination and/or lack of lubrication. The ball joints in most passenger car as well as many light trucks today are sealed for life and do not require periodic maintenance. But the ball joints on many older cars and trucks do have grease fittings are require periodic greasing. So do some aftermarket replacement ball joints.
If a ball joint is greaseable, use a grease gun to add chassis grease periodically. This will minimize friction between the ball stud and bearing within the joint, and flush out old grease and contaminants that can shorten the life of the joint.
Ball joints that do not have grease fittings also rely on grease for lubrication, but the grease is permanently sealed inside. Such joints typically have much tighter seals to keep contaminants out. Sealed joints are often referred to as "low friction" joints because they typically have polished ball studs and synthetic rather than steel bearings. This reduces internal friction and makes steering easier, which is why this type of joint is used as original equipment on most front-wheel drive cars and many late model trucks.
Ball joints should be replaced when the amount of play or movement in the joint exceeds the maximum allowed by the vehicle manufacturer. A loose ball joint can cause misalignment, uneven tire wear, sometimes a steering pull to one side, and/or suspension noise.
The amount of wear that can be tolerated in a joint varies from one vehicle application to another. The old rule of thumb that ball joints with more than .050 inches of play are worn does NOT hold true for all vehicles. Some ball joints should have NO visible play while others can handle up to .250 inch or more of play and still be considered okay. The only way to know if a joint is worn, therefore, is to refer to the OEM service specifications and to measure play (both horizontal and vertical) with a dial indicator, or check the joint's built-in wear indicator if it has one.
Load carrying ball joints deserve the closest scrutiny because they usually wear out first. On an upper/lower control arm suspension (SLA suspension) where the spring seats on the lower control arm, the LOWER ball joint carries the load and is the one most likely to be worn. If the ball joint does not have a wear indicator, tension must be relieved on the lower ball joint by raising the wheel off the ground and supporting the lower control arm near the joint. The lower ball joint can now be checked for axial (vertical) looseness by wiggling the wheel up and down. Radial (lateral or sideways) looseness can be checked by rocking the wheel in and out.
Load carrying lower ball joints with wear indicators must be checked with the weight of the vehicle on the wheels. Joints with wear indicators are considered good if the indicator is still protruding from the surface of the joint housing.
Most wear indicator joints have a raised collar or shoulder at the base of the grease fitting that protrudes about .050 in. out from the base of the joint. As the joint wears, the raised shoulder sinks into the joint. When the shoulder is flush with the joint or has receded into it, it's time for a new ball joint.
Some ball joints have a separate wear indicator pin. The tip of the pin extends through a hole in the bottom of the joint to show how much the joint has worn. Measuring the height of the pin shows if the joint is worn. If the pin is flush with the bottom of the joint or no longer protrudes through the hole, the joint needs to be replaced.
A third method that's used to indicate wear in some lower ball joints (Chrysler front-wheel drive cars, for example) is a grease fitting that becomes progressively loose as the joint wears. If the fitting can be wiggled by hand, the joint has too much play and needs to be replaced.
In suspensions where the spring seats on top of the upper control arm, the upper ball joint carries the load as is the one most likely to be worn. The upper joint must be unloaded by wedging a block of wood between the upper control arm and frame to support the vehicle's weight when the wheel is raised off the ground. The lower control arms must hang free, so the jack is 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. Lateral play should not exceed 0.25 inches and vertical play should not exceed the manufacturer's specifications.
The lower ball joint in this type of suspension is unloaded, so any looseness would indicate a new joint is needed.
With a MacPherson strut suspension, the strut carries the load and the lower ball joint is an unloaded follower. Looseness in the lower ball joint can be checked by raising the wheel off the ground with the lower control arm hangs free with the strut fully extended. Rocking the wheel in and out should produce no horizontal play between the control arm and knuckle if the joint is good. On Chrysler front-wheel drive cars and minivans, or other vehicles with wear indicators on the lower ball joints, check the indicator with the wheels on the ground and the weight on the suspension.
Ball joints may last 70,000 to 150,000 miles or more. Their lifespan depends on usage, road conditions and exposure to road splash and salt. If a joint is worn, it has reached the end of its service life and should be replaced. Worn ball joints can be dangerous and should be replaced. But should only the worn joint be replaced or should the other ball joints be replaced, too? The Motorist Assurance Program (MAP) guidelines say replacement is only required for a joint that is worn beyond factory specifications. Even so, many experts recommend replacing both joints (both uppers, both lowers, or all four). Doing so may actually be more convenient and less expensive in the long run than replacing individual joints as each wears out.
As a rule, replacement ball joints will often be the same type (greaseable or low friction) as the original. For some applications (such as trucks), you may want to upgrade to a stronger or greasable aftermarket ball joint if the original equipment ball joint failed at low mileage.
If a broken ball joint is being replaced, the tapered hole in the steering knuckle should also be inspected for damage (a worn taper can cause a repeat failure).
On suspensions that use a pinch bolt to lock the ball joint stud in the knuckle, a new bolt should be used if the ball joint is replaced.
Some ball joints are difficult to replace because they are pressed into the control arm. This will require taking the control arm to a parts store or shop that has a hydraulic press for disassembly and reassembly.
Some ball joints are attached to the control arm with rivets. These must be chiseled or drilled out to replace the ball joint.
IF the ball joint is permanently attached to the arm, the whole control arm assembly must be replaced if the joint has worn out.