A ball joint is used for allowing free rotation in two planes at the same time while preventing translation in any direction,including rotating in those planes. Combining two such joints with control arms enables motion in all three planes, allowing the front end of an automobile to be steered and a spring and shock (damper) suspension to make the ride comfortable.

Detail Information
In an automobile, ball joints are spherical bearings that connect the control arms to the steering knuckles, and are used on virtually every automobile made. They bionically resemble the ball-and-socket joints found in most tetrapod animals.

A ball joint consists of a bearing stud and socket enclosed in a casing; all these parts are made of steel.The bearing stud is tapered and threaded, and fits into a tapered hole in the steering knuckle. A protective encasing prevents dirt from getting into the joint assembly. Usually, this is a rubber-like boot that allows movement and expansion of lubricant. Motion-control ball joints tend to be retained with an internal spring, which helps to prevent vibration problems in the linkage.

The "offset" ball joint provides means of movement in systems where thermal expansion and contraction, shock, seismic motion, and torsional motions, and forces are present.

On modern vehicles, joints are the pivot between the wheels and the suspension of an automobile. They are today almost universally used in the front suspension, having replaced the kingpin/link pin or kingpin/trunnion arrangement, but can also be found in the rear suspension of a few higher-performance autos. Ball joints play a critical role in the safe operation of an automobile's steering and suspension.

Many currently manufactured automobiles worldwide use MacPherson strut suspension, which utilises one ball joint per side, between the lower end of the strut and the control arm, with the necessary small amount of articulation at the top of the strut being usually provided by an elastomeric bearing, within which is a ball bearing to allow free rotation about the steering axis. So, there are commonly only two ball joints in the suspension, however there will be at least four (track rod ends and rack ends) in the steering linkage.