What a ball joint actually does

A control arm moves in an arc as the wheel rises, and at a steered wheel the knuckle also has to turn. A ball joint lets that connection move in more than one direction while keeping the two parts joined.

What’s inside

MOOG’s overview compares the joint to a human hip. A metal housing holds a stud that can swing and rotate. Bearings sit between the ball and the housing to cut friction and wear. Grease fills the socket, and a rubber boot keeps grease in and dirt out. A cover plate closes the assembly.

Brake & Front End magazine adds a Belleville washer or spring, a cone-shaped spring that keeps tension on the bearings so the joint holds zero vertical (axial) play as it wears. It can’t make up for sideways (lateral) wear, which lets the stud shift and can change camber and tire wear. Brake & Front End: ball joint inspection.

Joints are pressed, riveted, bolted or built into the arm. Brake & Front End notes that pressed joints can seize, especially a steel housing in an aluminum arm. Riveted joints are drilled or chiseled out and the new one is bolted in. A tie-rod end has a ball and socket too, but it transfers steering motion rather than locating the knuckle on an arm, so the two aren’t interchangeable.

Carrying weight or following

Some joints carry the spring load; others are followers. MOOG says that in a short-long-arm (SLA) layout, the lower joint usually carries the load and the upper joint is a follower. In a MacPherson strut layout, the lower joint is a follower and the strut carries the weight. Load-carrying joints usually wear faster.

Follower doesn’t mean unloaded. MOOG’s follower-joint article lists radial loads, impacts, vibration, contaminants, corrosion and heat as causes of wear. MOOG also says many original joints are sealed, and once the boot fails, water and grit wear them quickly. Some replacements add a grease fitting so fresh grease can flush contaminants out.

How a joint is checked

MOOG lists steering pull, uneven tire wear and front-end shimmy as signs of wear. It recommends checking for looseness regularly and before every alignment. MOOG’s inspection tip.

• Wear indicator. On some joints, the grease fitting doubles as a wear gauge. Brake & Front End says that if a grease gun won’t couple to it, the joint needs replacing. Check joints with built-in indicators with the car on its wheels.
• Unload the joint correctly. How you lift the car depends on where the spring sits. MOOG has the tire raised so the suspension is unloaded. Brake & Front End supports the lower arm under a loaded lower joint, and supports the upper arm when the spring sits on top.
• Measure. Pry vertically for axial play and rock the tire in and out for radial play, ideally with a dial indicator.

MOOG adds a caution: play that’s within the manufacturer’s spec can still hurt alignment and handling, especially when other parts are also slightly loose.

The tilted wheel in the last photo shows why this matters; Brake & Front End notes that a failed joint means losing control. A clunk or worn tire alone, though, doesn’t tell a ball joint apart from the other parts that locate the wheel. The inspection has to find the actual play.