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20th Annual Technical Forum: Chassis Parts



Q: How does a Short-Long Arm (SLA) suspension differ from a strut suspension?

 A. SLA suspensions use a pair of control arms on each side to support the steering knuckle and wheel. The pivot points of the arms are connected to the chassis with bolts and bushings, and the arms are connected to the steering knuckle with an upper and lower ball joint. The spring is usually placed between the lower control arm and vehicle subframe, but on some applications, it may be located on top of the upper control arm. SLA suspensions are typically used on larger, heavier vehicles such as full-size pickup trucks, SUVs and luxury rear-wheel drive passenger cars.

Alignment problems can occur if the ball joints or control arm pivot bushings are worn. This may cause a steering pull, suspension noise or accelerated or uneven tire wear. Replacing the ball joints and/or bushings will usually cure the problem. However, this isn’t always easy.

On a growing number of late-model cars and trucks, “unitized” aluminum control arms are used where the ball joint is an integral part of the control arm assembly. On most of these applications, the entire control arm assembly must be replaced with a “loaded” control arm if the ball joint is worn out — unless the arm uses a pressed-in ball joint that can be removed and replaced with an aftermarket ball joint.

Most strut suspensions don’t use upper control arms or upper ball joints. These parts and the spring are replaced with a MacPherson strut assembly. The strut attaches to the upper part of the steering knuckle and the bearing plate on top of the strut allows the wheel to steer in either direction. The spring is positioned around the strut for a more direct acting suspension.

Q. What do tie rod ends do?

A. Tie rod ends are flexible ball sockets that can pivot or swivel. Outer tie rod ends connect the tie rods to the steering knuckles. Inner tie rods connect the tie rods to the steering linkage center link. Rack and pinion steering systems only use outer tie rod ends. The inner ends of the tie rods are connected to the rack with sockets.

Left and right side tie rod ends are usually different and may have reversed threads. Most original equipment late-model tie rod ends have a low-friction polished ball stud inside that rides in a polymer bearing. The tie rod end is lubricated and sealed for life and requires no maintenanc.

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