Chassis parts are all the components that attach the wheels to the rest of the vehicle, and help support the vehicle’s weight. These include such things as ball joints, control arms, bushings, sway bars, springs, shocks and struts. Most of these components are supposed to last a long time, but often don’t because of the pounding they take from everyday driving. Corrosion also can shorten the service life of chassis parts. Some chassis parts can also be damaged by hitting a curb or pothole with sufficient force.
Tie-rod ends, tie rods and idler arms are other chassis parts that can wear out, too. But these parts are technically part of the steering linkage, not the suspension. Even so, most people lump them into the general category of chassis parts.
Worn chassis parts often pass unnoticed until they cause noticeable problems such as road noise, uneven or rapid wear on one or more tires, or a steering or handling problem. Worn tie rods are the most common culprit for causing rapid tire wear, but worn control arm bushings or ball joints can also contribute to tire wear, too.
Suspension noise such as clunks, squeaks or groans when passing over bumps or dips in the road, can often be traced to loose ball joints, bad control arm bushings or even worn upper bearings plates on MacPherson struts.
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.
Worn ball joints upset camber alignment, causing a wheel to lean in. This can cause a steering pull to one side, and/or increased tread wear on the inside shoulder of the tire.
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 is also 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. But once 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.
If one ball joint is worn out, chances are the ball joint on the opposite side is also near the end of the road, even if it is still marginally within specifications. Many technicians recommend replacing both joints at the same time.
Tire wear, steering and handling can also be affected by ride height. If the chassis is sagging on one side, or in the front or back, weak springs are the likely cause. Weak springs can affect both camber and caster, which may result in a steering pull, a change in steering effort or return, and/or uneven tire wear.
Weak springs are also more likely to fail. The springs on many late model vehicles are thinner to reduce weight, and have an outer plastic coating to protect the metal from corrosion. If this outer coating is cracked or damaged, corrosion can form a hot spot that eats into the spring, weakens it and eventually causes the spring to break.
Vehicles today use a wide variety of spring types, including coil springs, tapered springs, variable rate coil springs, flat springs, composite springs, even air springs. The latter often develop air leaks after five or six years of service due to pin holes or cracks in the rubber air bladder.
This can cause the compressor to work overtime and burn out. Leaky air springs can be replaced with new rubber springs, or they can be removed altogether and replaced with less expensive and simpler conventional springs. Bushings are another chassis part that often fail with age. Most original equipment control arm bushings are made of synthetic rubber or urethane.