Ball Joints

The parts of a ball joint labeled.

Ball joints attach the control arm to the spindle. They allow the spindle to swivel as the vehicle turns and provide for up and down movement as the vehicle brakes and travels over bumps in the road. This is because it consists of a ball stud and socket that provide a pivot for the steering knuckle. They’re preloaded and a tight fit as they rotate against each other on a thin layer of lubricant, similar to a person’s shoulder socket. A rubber wedge or a spring keeps the ball stud tight in its seat. As they wear, clearance increases and noise and vibration occur.

Ball joint function.

Conventional systems found on older passenger cars and many of today’s light trucks have two ball joints; a load bearing joint and a follower joint. The load bearing joint is attached to the control arm that seats the suspension coil spring. The coil spring is attached between a control arm and the vehicle’s frame. The other joint keeps things in alignment but does not carry the vehicle’s weight. They wear and cause a clunking or popping sound when excessive clearance develops between the ball and socket.

Most of today’s passenger cars are front wheel drive models that contain struts. The coil spring is wrapped around the strut. These systems contain only one follower ball joint. The vehicle’s weight is transferred through the upper strut mount instead of a load bearing ball joint.

Many replacement ball joints have a zerk grease fitting and must be lubricated after installation. Others are sealed and contain sufficient lubricant from the parts manufacturer to last the lifespan of the ball joint. Some ball joints contain a wear indicator; when the grease fitting becomes flush or recesses into the base of the ball joint, the ball joint is worn and must be replaced. Others require a jack to unload the joints so end play can be recognized. This is because the spring is forcing things together. If the spring is between the lower control arm and the frame place the jack under the lower control arm close to the ball joint. If the spring is between the frame and the upper control arm, place the jack under the vehicle’s frame. Raising the wheel off the ground unloads the ball joint.