When installed in a conventional short/long-arm (SLA) suspension system, an upper and lower ball joint connects connects the steering knuckle to the upper and lower control arms. Each ball joint allows the steering knuckle to move smoothly through a vertical plane while the spindle simultaneously pivots in a horizontal plane. Due to its extreme mechanical simplicity, a MacPherson strut suspension contains only a lower ball joint. This design allows vertical movement in the lower control arm while the steering knuckle and spindle pivots in the horizontal plane.
SLA Suspension Systems
SLA ball joints are manufactured in load-bearing and non-load-bearing versions. The load-bearing joint in an SLA suspension system is located on the load-bearing control arm, which supports the vehicle’s weight via a coil spring or torsion bar. The non-load-bearing or “follower” ball joint is located on the control arm that holds the wheel in correct alignment. The major difference between the two is that the load-bearing ball joint is not preloaded while the follower joint is preloaded.
Wear in the load-bearing ball joint is measured in thousandths of an inch vertical and horizontal play. Because a ball joint is self-centering, manufacturers allow a small amount of play to exist in load-bearing ball joints. Wear in the preloaded follower joint is measured by the amount of preload torque required to turn the ball joint stud in the ball joint socket. The wheel’s alignment angles will be most affected when the upper ball joint wears out.
To take the place of the upper control arm in a conventional SLA suspension system, the spring and shock absorber are built as one assembly in the MacPherson strut design. The upper part of the strut is held in place by a bearing plate assembly that bears the vehicle weight and allows the strut assembly to pivot. The strut is bolted to the steering knuckle and the lower part of the steering knuckle is connected to the lower control arm by the lower ball joint. The lower ball joint is a non-load-bearing design that maintains the camber and caster alignment angles. As with the SLA design, wear in the lower ball joint is generally measured by the amount of torque needed to turn the ball joint stud.
The spherical ball joint used in a tie rod end is spring-loaded to keep the pivot ball centered with the tie rod joint socket. But, since every steering system contains four tie rod ends, a small amount of wear in each can result in enough cumulative wear to affect the toe angle of the wheel alignment, which is the most critical angle for controlling tire wear.
While an individual ball joint or tie rod end might suffer premature wear, most wear at the same rate. Since labor is the larger portion of any suspension repair cost, it’s therefore more cost-effective to sell ball joints and tie rod ends in pairs or sets. On lower-mileage vehicles, load-bearing ball joints are generally replaced in pairs. On high-mileage vehicles both the load-bearing and follower joints should be replaced as a set. And, while the steering knuckle is removed, suggest replacing the rubber control arm bushings on high-mileage vehicles as well.