Ball Joints

The ball joint is the suspension pivot closest to the wheels. It is mounted to the control arm and steering knuckle, or spindle. The ball joint stud swings in its socket to compensate for up-and-down suspension movement and rotates in its socket for steering. The ball joint works in the steering and suspension systems simultaneously. Ball joints can be load-carrying or non-load carrying. The load-carrying ball joint supports the vehicle’s weight and generally wears quicker. The load carrier is generally the joint closest to the spring. The follower, or non-load ball joint does not support vehicle weight and doesn’t get the same amount of stress. Most MacPherson strut suspensions have only one ball joint per side, and that is a non-load ball joint.

 
Load-Carrying Type
Load carrying ball joints can be divided into two types. In the tension-loaded type, forces applied to the joint housing are pulling the stud against the housing. In the compression-loaded type, the same forces are pushing the stud into the housing. The tension-loaded is the more popular design.

 
Inspection Procedures – NON-Wear Indicator Ball Joints

In order not to confuse wheel bearing end play with ball joint looseness it is recommended that the amount of bearing play be established first. With the vehicle's wheels off the ground, grasp the wheel at 6 and 12 o’clock and gently rock the wheel. Sometimes, the brake pedal can be applied to lock out wheel bearing play. To check ball joints (other than wear indicator) they must be unloaded. Unloading ball joints is removing vehicle weight and spring force from acting on the ball joint.
To unload ball joints on an unequal length control arm suspension (spring mounted on lower arm), jack under the lower arm, as close to the ball joint as possible, until the upper strike out bumper is no longer in contact with the control arm or frame. A quick check for axial looseness can be made with a pry bar between the tire and ground. Radial looseness is tested by placing your hands at 12 & 6 o'clock and moving the tire. If looseness exists, it should be measured and compared to the manufacturer’s specification.

Load-Carrying Type, continued Wear Indicator Ball Joints Some lower load-carrying ball joints are equipped with wear indicators. The most common type of wear indicator has a small boss which protrudes from the center of the lower housing. As wear occurs this boss will recede into the housing. When it is flush, or receded into the cover plate, the ball joint should be replaced. When inspecting, wear indicator ball joints must remain loaded with the vehicle resting on the tires to check for wear. The vehicle should be checked at curb height (normal running position).	Normal – Boss protruding (above)
	Worn – Boss recessed (above)

Follower-Type
Some ball joint replacement recommendations for follower type ball joints state "no perceptible movement". To check for perceptible movement, be sure that the ball joint is unloaded. Then, grasp the tire at the top and vigorously push and pull on the tire while watching for signs of movement. If movement is suspected, an assistant may be required to confirm the movement is in the ball joint. Not all follower ball joints use the "no perceptible movement" specification. Always check the appropriate manual for OE specifications. 

To check follower ball joints on MacPherson type suspensions, raise the vehicle by its frame or unibody. Allow the suspension to hang in the maximum rebound position. Grasp the tire and move it vigorously while looking for signs of movement. 

NOTE: Some MacPherson suspension vehicles have a type of wear indicator ball joint. With the vehicle in the normal running position (load on tires), grasp the grease fitting and attempt to move it. If movement exists between the grease fitting and the housing, the ball joint should be replaced.

 
 
Ball Joint Replacement and Installation
Replacement of the ball joints should be suggested when tire wear or poor handling can be attributed to ball joint looseness. When ball joint looseness is excessive or exceeds specification, replacement is normally required by the vehicle manufacturer.

One should replace ball joints in pairs. Both ball joints (upper or lower) are the same age with the same mileage. When one needs replacing, the other may fail soon. It is recommended to replace them both now to save time, prevent future problems and keep the system in balance. Also, the control arm should be inspected for damage before installing the replacement. 

Ask about the upper ball joints. The catalog will list vehicles that have four ball joints. If your customer is replacing the lower ball joints, it’s a good idea to check the uppers.

There are three basic ways of mounting ball joints to control arms, which dictates their design:

Rivets / Bolts
Press Fit 
Threaded

The most popular is press fit, which requires special tools to service (as shown).

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