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19th Annual Technical Forum: Chassis Parts


8/10/2011
By Larry Carley

How often should chassis parts be replaced?
 

A. More often than they are now. Many vehicles end up in salvage yards with most or all of their original equipment chassis parts still on the vehicle. Most of these vehicles have worn-out ball joints, control arm bushings, tie rod ends, steering racks, springs and other chassis parts that should have been replaced long ago, but were not.

Worn chassis parts are most often detected when a vehicle is experiencing a tire wear, steering or handling problem, or when a technician is doing a pre-alignment inspection. Many motorists either don’t know the warning signs of worn chassis parts, or they just ignore them, fearful that repairs might be expensive. The consequences of doing so can be serious, if not deadly. If a badly worn ball joint fails, the suspension collapses. This can cause the driver to lose steering control. The same thing can happen if a badly worn tie rod end separates from a steering arm.

Most original equipment chassis parts are built to withstand the rigors of everyday driving, and will usually last well beyond 100,000 miles or more. But as the cumulative effects of potholes, jars and jolts add up over time, even the toughest chassis parts can succumb to wear. The life of the parts depend on the size and weight of the vehicle, and what kinds of roads it is driven upon and the number of miles driven. Pickup trucks and full-size SUVs can be very hard on ball joints.

A classic symptom of worn ball joints is suspension noise when hitting bumps (clunks and rattles), and inner shoulder wear on the front tires due to camber misalignment.

Worn tie rods are the most common culprit for causing rapid tire wear. Play in the tire rods ends also can cause noticeable steering looseness and steering wander when driving on straight roads. On cars and trucks that have short long arm (SLA) suspensions, there are four ball joints: one upper and one lower on each side. The load-bearing joint (which may be the upper or lower depending on where the spring is located) is the one that experiences the most wear.

On cars with strut suspensions, there are no upper ball joints. The weight is supported by the strut and spring. Lower ball joints are still used, but these are “follower” joints that carry no weight.

Many load-carrying lower ball joints on General Motors and Ford rear-wheel drive cars and trucks have a built-in “wear indicator.” This same type of joint also is used in the rear suspension on some of GM’s big front-wheel drive cars (Cadillac, Buick and Oldsmobile). The wear indicator is the shoulder on the grease fitting. As the joint wears and the stud sinks deeper into the housing, the grease fitting recedes into the housing. Joint wear is considered acceptable as long as some shoulder protrudes above the face of the housing. When the shoulder becomes flush with the housing, it’s time to replace the joint.

Wear-indicating ball joints should not be checked with the wheels raised off the ground because the weight of the tire, wheel, steering knuckle, brake rotor and caliper are not supported and push down on the joint. This can push the indicator out, giving the false impression that the joint is not worn.

On ball joints that do not have built-in wear indicators, joint wear and play is measured using a dial indicator. On vehicles with SLA suspensions, the lower load bearing joints are checked with the weight of the vehicle off the wheels while supporting the lower control arms. On vehicles with strut suspensions, the non-load bearing lower joints are checked with the wheels off the ground and the suspension hanging free. If joint play exceeds specifications (which varies depending on the application), the joint is worn out and needs to be replaced.

 

Wipers

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