Auto Service World
Feature   March 1, 2006   by Auto Service World

Counterperson Training: Suspension Systems

A suspension system’s performance can be affected by any number of factors, all with the common fact that the alignment, or geometry, of one area can affect another, usually leading to poor handling characteristics or excessive wear of tires or other components.

One issue that many counterpeople come across is that many components go by more than one name, which can lead to confusion. Care should be taken to ensure that the part being requested is the part that is needed.

Suspension Systems can use coil springs, leaf springs, or torsion bars to support the weight of the vehicle. There are a variety of arrangements in use. Here we will deal only with the common front suspension arrangements.

Each corner of a common Front Coil Spring Suspension is made up of an upper control arm, a lower control arm, a steering knuckle, a spindle, an upper and a lower ball joint, bushings, a coil spring, and a shock absorber. The most common style is the Short-Long Arm (SLA) Suspension.

On this type, the Control Arms are attached to the vehicle frame with bushings (metal or rubber) which prevent the wheel assembly from moving side to side but do allow up and down movement. Upper and lower control arms are of different lengths, so that a slight camber change occurs as the wheel moves through jounce and rebound. This minimizes track change, which would cause the tires to scrub sideways.

Ball Joints are used to connect the steering knuckle to the control arms and form the steering axis. One ball joint is called the load carrier; the other is called the follower. If the spring’s upper mount is on the frame and the lower mount is on the lower control arm, vehicle weight is transmitted through the spring to the lower control arm, and then through the control arm to the lower ball joint, making it the load carrier ball joint and the upper ball joint the follower. If the spring is mounted to the frame and the upper control arm, the opposite is true.

Shock Absorbers act to control the up-and-down movement of the vehicle suspension. They do not support any vehicle weight. Shocks have a single job: to control bounce, roll or sway, brake dive, and acceleration squat.

MacPherson Strut Suspensions operate using the same basic principles, and have lower control arms and ball joints as well as steering knuckles and spindles. The coil spring encircles the strut, with the upper and lower spring seats as part of the strut assembly. However, in a Strut Suspension, the struts perform all of the functions of a shock absorber but also provide structural support, taking the place of the upper control arm and ball joint. This is why a strut shaft is much larger than a shock’s. The strut mount is a critical part of proper service. Since many struts are only replaced long after they have deteriorated, the strut mount is often also worn, and failure to replace it can result in poor ride control and noise.

Modified Strut Suspensions are arranged so that the spring is located separately from the strut, rather than encircling the strut.

Double Wishbone Suspensions combine the space saving of a strut suspension with the ability to ride low to the ground (a shortcoming of strut suspensions). This allows for a more aerodynamic hoodline. On a Double Wishbone system, the lower portion of the strut forms a wishbone shape where it attaches to the lower control arm. The wishbone does not rotate as the strut does in a strut suspension. Instead, the spindle rotates on upper and lower ball joints (similar to a SLA system).

A Strut Rod is used in a variety of suspensions and provides bracing to the (usually lower) control arm to limit front and rear movement. Some also allow for adjustment of caster during alignment.

Anti-Sway Bars, also known as Anti-Roll Bars, Stabilizer Bars or (incorrectly) “Sway Bars,” can be found at the front or rear suspensions. Each is constructed so that the unequal movement of the left and right suspensions causes them to twist, exerting torsional forces.

Alignment terminology is important to understand when communicating with a customer who has a difficult-to-solve tire wear or steering issue. There are a few key alignment parameters.

Camber is the degree to which the wheel varies from the vertical. It is adjustable through the addition of either shims or cam bolts. In positive camber, the top of the wheel and tire leans out (generally undesirable); negative camber describes the top of the wheel and tire leaning inward, usually only by a few degrees.

Caster is the angle that the suspension moves up and down relative to the plane of the road, and adds stability. Positive caster describes the top of the shock or strut mount being ahead of the centre of the wheel; negative caster has the upper mounting point behind the wheel centreline and is the common construction. It is generally not adjustable, but can be out of spec through wear or collision.

Toe in and toe out describe the angling in or out of the leading edge of the wheel and tire as compared to the trailing edge of the tire. Front toe is adjustable through threaded tire rod ends; rear sometimes only through the addition of shims.

Steering Axis Inclination (SAI) is the angle formed by a line that runs through the upper and lower steering pivots with respect to vertical, and adds on-centre feel for the driver. On a SLA suspension, the line runs through the upper and lower ball joints. On a MacPherson strut suspension, the line runs through the lower ball joint and upper strut mount or bearing plate. Viewed from the front, SAI is also the inward tilt of the steering axis. Like caster, it provides directional stability. But it also reduces steering effort by reducing the scrub radius. SAI is a built-in nonadjustable angle and is used with camber and the included angle (SAI angle plus the camber angle) to diagnose bent spindles, struts, and mislocated crossmembers.

The key for a counterperson is to continually remember the importance of having the whole suspension system operating properly, with tires, ride control, and steering performance hanging in the balance.

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