Learning the Terminology is Key to Selecting the Correct Chassis Part

Just about everybody remembers the story of the Tower of Babel, in which people began speaking different languages. Unfortunately, the terminology connected with chassis parts often becomes its own Tower of Babel when selecting the correct part for a repair job.

In many cases, term “tie rod” often becomes interchangeable with “tie rod end” and “drag link” is substituted for “connecting link” or “center link.” Many cataloging systems similarly confuse chassis part terminology, which often leaves techs in a Babel-like state of confusion.

The best method of selecting the correct chassis part is to use the correct chassis terminology. In many cases, parts professionals and automotive technicians should consult illustrations that are included with most cataloging to ensure the part will perform the desired function. To better understand chassis parts terminology, let’s separate chassis parts into various categories, beginning with chassis types:

Chassis Types
Most modern passenger cars are built with unitized bodies that have sufficient rigidity to eliminate the need for a heavy frame. Most modern trucks have their bodies mounted on separate frames to provide the structural strength needed to haul heavy loads and tow heavy trailers. The difference between vehicles equipped with unitized bodies and frames is that each design generally requires a specific configuration of steering, suspension and ride control parts.

Steering Rack Systems
Unitized bodies easily accommodate rack-and-pinion steering systems that are connected directly to the right- and left-hand steering knuckles by inner and outer tie rod end assemblies. The only wearing parts rack steering systems are tie rod ends, lower ball joints, upper strut support bearings and the steering rack itself.

The main advantages of rack steering systems are simplicity and durability. Occasionally, a power steering rack will develop a leaking oil seal, which fills the accordion-style dust boot that covers the end of the rack cylinder with oil. A knocking noise on rough roads usually indicates that the inner tie rod ends attached to the steering rack are worn.

Parallelogram Steering Systems
Most vehicles with frames generally use a “parallelogram” steering linkage consisting of a conventional steering gear mounted on the driver’s side of the frame and an idler arm mounted on the passenger side of the frame. A Pitman arm is attached to the steering gear shaft, which allows the steering gear to convert the circular motion of the steering wheel into the lateral motion required to turn the front wheels right or left. The function of the idler arm is to support the opposite end of the center link and help create the steering geometry needed for crisp, accurate turns.

When the front wheels are in the straight-ahead position, the geometry of the Pitman arm, idler arm and center link form a rectangle. When the steering wheel is turned right or left, the Pitman arm, idler arm and connecting link form a parallelogram, thus the term “parallelogram” steering linkage system.

The Pitman arm is connected to the idler arm by a center link, which is erroneously and often referred to as a “drag link.”  To be technically correct, a drag link is used on vintage vehicles to attach the steering gear directly to the steering knuckle. Drag links are usually found on 1950s and earlier vintage vehicles and various hot-rod applications.

The center link is attached to the steering knuckles by the right and left-hand tie rod ends and the tie rod adjusting sleeves. The upper and lower ball joints are also mounted to the steering knuckle and are designed to let the steering knuckle travel vertically to absorb bumps as well as pivot as the front wheels are turned.  Cumulative wear in the idler arm, tie rod ends, ball joints and control arm bushings will cause steering wander and tire-wear complaints. While most of these parts are replaced individually, ball joints and control arm bushings are usually replaced in sets.

The front brakes, wheel bearings and wheels also are mounted to the steering knuckle. The steering knuckle also establishes a critical wheel alignment angle called “steering axis inclination” (SAI), which allows the vehicle to be easily steered around a corner. If the steering knuckle is bent, excess tire wear and poor handing will result.

Can vehicles equipped with frames incorporate a rack-and-pinion steering system? Yes, indeed. Can vehicles with frames incorporate a MacPherson strut suspension system? Likely not, because any such arrangement would not be a true MacPherson strut suspension in which the strut combines the spring, shock absorber and upper control arm functions.

Suspension Systems
Unitized bodies also can easily accommodate independent rear suspension and axle systems because they can be easily mounted to a unitized body.  On frame-equipped vehicles, a short and long-arm (SLA) control arm configuration is generally used to support the front coil or torsion bar springs, shock absorbers, upper and lower ball joints  and steering knuckle assembly. The SLA control arms pivot on rubber or lubricated metal bushings.

Coil springs are used to suspend the chassis. All coil springs are rated by the pounds of force required to compress the spring a specified distance. Variable-rate springs are generally found in light-duty applications. The force required to compress a variable-rate spring a specified distance increases as the spring is compressed. Leaf springs are generally used in four-wheel and rear-wheel drive applications. Because springs work 24 hours per day supporting the weight of the chassis, they eventually begin to sag, which causes a loss of suspension height. When suspension height is less than specified height or if the vehicle isn’t level, the springs should be replaced in pairs or sets.

Ride Control
Shock absorbers are used to dampen rebound in the springs as the wheels travel over bumps and other irregularities in the road surface. Most modern shock absorbers are charged with nitrogen gas to help prevent the fluid inside the shock absorber from foaming during extreme driving conditions. Shock absorbers used in MacPherson strut applications are built into the strut assembly itself. Conventional shock absorbers are mounted between the body or frame and lower control arm or between the body or frame and axle assembly. As with springs, shock absorbers should be replaced in pairs and preferably in sets.

Chassis Tips
If a power steering rack is worn, it’s more cost-effective to purchase a “loaded” rack with the inner tie rod ends attached.

Ball joints also require similar expertise. The load-carrying ball joint in an SLA suspension is mounted on the same control arm as the spring or torsion bar. The “follower” ball joint is mounted on the non-load carrying control arm. Because the follower joint must maintain accurate wheel alignment angles, it’s preloaded and requires a specific amount of torque to rotate the tapered mounting stud. In contrast, the load-carrying ball joint generally loosens up in normal use and will exhibit a small amount of play when tested according to specified procedures.

Worn Pitman arms can be responsible for excessive steering wheel play. Worn idler arms generally cause excess tread wear and cupping on the right-front tire. Wear in the tie rod ends will cause steering wander. Worn Pitman arms and tie rod ends can be extremely dangerous because either can separate and cause a complete loss of steering control.

Last, no steering or chassis part should be straightened, heated or welded. Such procedures create a “stress riser” in the steel that can later develop into a fracturing of the metal part. That’s why the golden rule of repairing chassis parts is, when in doubt, replace.