It acts as the primary pivot point for the front wheels, allowing the suspension to move up and down while the wheels steer left and right. Inside the metal housing, a steel ball stud sits in a lubricated socket protected by a rubber dust boot. When the internal socket wears out, the joint develops play, which ruins alignment and can cause the wheel assembly to collapse entirely if the stud pulls out.
Heard on the job
Grab the pry bar and check that lower ball joint for vertical play before we put it on the alignment rack.
Insider tip
A torn dust boot is an automatic fail on a safety inspection because the grease washes out and road grit rapidly destroys the socket.