In our rush to sell ride control products like MacPherson struts, shock absorbers and springs, we often forget the importance of inspecting and replacing rubber suspension bushings. Unfortunately, worn or hardened suspension bushings can cause many of the mysterious squeaks, knocking noises and vibration complaints that come through the door of a typical repair shop.
Even worse, worn suspension bushings can cause excessive tire wear because they can’t maintain accurate toe angles during braking and acceleration. On the bright side, the most opportune time to inspect and replace suspension bushings is when ride control and other suspension parts are being replaced.
Because a rubber bushing is designed to operate within a limited range of motion and requires no lubrication, rubber bushings work well in control arm and leaf spring applications. To eliminate wearing surfaces, the rubber bushing is bonded to inner and outer metal sleeves. The outer sleeve is usually pressed into the control arm or spring while the inner sleeve is clamped to the frame by a retainer bolt. (See Photo 1)
Because the rubber bushing absorbs the torsional movement of the suspension component, no frictional or rotational wear takes place. Wear problems occur only when the rubber bushing separates from its inner and outer sleeves.
Rubber bushings are also designed to insulate the chassis against road vibration and noise. Ozone, ultraviolet light, extreme temperatures and other atmospheric issues tend to harden rubber bushings and cause them to transmit noise and vibration.
Worn suspension bushings can easily be detected by checking the lower and upper control arms for excessive movement when the suspension is disassembled for other repairs. In most cases, the rubber bushings should tend to hold the control arm in position and limit control arm travel. If the control arm can be easily moved beyond its normal range of travel, the rubber bushing has deteriorated or the pivot bolt has loosened and is no longer holding the inner sleeve in position. (See Photo 2)
During a normal vehicle inspection, rubber bushings should be inspected for hardening and stress-related cracking. In many cases, a disintegrating bushing can easily be detected by the blackened ring of rubber particles forming around its circumference when the bushing separates from its metal sleeve. In other cases, the bushing can deform and allow the control arm to move off-center from its normal pivot point. When the bushing deforms, positive camber angle is reduced.
Let’s review how vehicle load affects rubber bushings during normal vehicle operation. Looking from the front of the vehicle, the thrust on the driver’s side lower and upper control arm or strut support occurs in a counter-clockwise rotation. The passenger side thrust occurs at a clockwise rotation.
During acceleration on a front-wheel-drive vehicle, the forward thrust of the front wheels tends to increase the toe angle. Loose lower control arm bushings aggravate that tendency by allowing more toe increase than normal. Even though the toe angle meets specification at rest, the symptoms of worn control arm bushings will be more tire wear on the outer edge of the passenger side front tire and a lesser amount of wear on the outside of the driver’s side tire.
Toe angle wear can be differentiated from camber angle wear by stroking your fingers lightly across the tread. Toe-in angle wear will cause the treads to be “sharp” or “saw-toothed” on the inside of the tread bar and smooth on the outside of the tread bar. Toe-out wear will feel the opposite.
During braking, the thrust on the suspension assembly is in the direction of wheel rotation. Clunking noises associated with braking can be caused by worn upper control arms or by worn or hardened strut rod or radius arm rubber bushings. Worn lower control arm bushings will also allow the front wheels to toe out during braking, which causes a wandering sensation when the brakes are applied.
Upper Strut Bushings
Because most vehicles are now equipped with MacPherson strut suspension systems, it’s important to understand that the strut experiences the same dynamic forces as a conventional upper control arm assembly. The rubber-mounted strut rod support bearing essentially supports the vehicle weight and counters both braking and acceleration torque.
For the above reasons, if the upper strut bearing assembly appears deteriorated, it should be replaced.
When replacing the load-bearing control arm bushings, it’s also important to inspect the condition of non-load-bearing rubber bushings. Non load-bearing bushings include sway bar, radius arm, shock absorber and steering rack mounts. Check the sway bar bushings for severe cracking, looseness and loss of compression. (See Photo 3)
Because radius arm bushings are designed to absorb the fore-and-aft thrust of a lower control arm, they must have enough flexibility to compress under load. If the radius arm bushings are hardened, severely cracked or oil-soaked, they should be replaced. Worn shock absorber bushings usually cause a squeaking or knocking noise. On the other hand, loose steering rack mount bushings normally cause steering wander complaints.
Many technicians don’t like to replace control arm bushings because they are usually firmly rusted in place. One solution is to replace the complete control arm. The other, more obvious, solution is to obtain the correct tooling or use an effective substitute.
One tool is a set of bushing tools that can be used to press bushings in place or used with a hammer to drive bushings into a control arm. (See Photo 4)
A good substitute tool is a common ball joint and universal joint pressing tool with a full complement of attachments. If the control arm can be mounted in a hydraulic press, the bushing can, in most cases, be easily removed and replaced.
An air-operated impact hammer with bushing attachments may also be used in some applications to remove and replace suspension bushings. In some cases, an exhaust cutter bit can be used with an impact hammer to peel the steel sleeve from rusted control arms. (See Photo 5)
When used with skill and common sense, an acetylene torch can be used as a “last resort” to remove a badly rusted bushing from the control arms. The key point is to avoid weakening the control arm by applying excessive heat to the suspension bushing or by cutting into the base metal.
After control arm bushings are replaced, they continue to conform to the vehicle’s weight and to braking and acceleration thrust. In most cases, it’s a good procedure to recommend bringing the vehicle back for an alignment check about 5,000 miles after the control arm bushings have been replaced.