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Flex Joints for Suspension Movement

Posted in How To: Suspension Brakes on January 9, 2017
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Suspension systems are comprised of components that allow parts to rotate and flex as they cycle through travel and articulation. Some type of flex joint is needed to allow this movement whether it’s on an IFS a-arm or some form of linked suspension.

Each of these joints contains some material for movement and inserts may consist of rubber, urethane, or steel spherical joints, depending on application. Vehicle manufacturers tend to use rubber joints where they can to minimize noise and vibration transmission into the vehicle. Urethane bushings are a step up in performance over rubber due to their higher material density, but often need to be kept lubricated to prevent squeaking. Some aftermarket and race suspension arms use rod ends (heim joints) at the arm hinge points, providing greater strength and durability, especially under severe use conditions. These joints transmit even greater levels of chassis noise into the cab, but don’t suffer from flex and play as bushings do.

From the factory, suspension arms typically use rubber bushings at the hinge point and studded, greased ball joints of some type at the spindle end. The rubber provides some flex to prevent suspension harshness and helps isolate noise transmission into the chassis. Aftermarket replacement arms such as the one shown at top typically use urethane bushings in place of rubber, and use spherical uniballs. The urethane provides more precise and firm suspension control. The uniball provides greater strength than the OEM ball joint and can be mounted in double-shear with a bolt that can hold the joint captive even if it wears and breaks.

High quality rod ends use a Teflon or similar material liner between the rod end body and the spherical ball, and are recommended for off-road use. Metal-on-metal rod ends can be lightly greased to extend their useful service life, but this is generally impractical in an off-road application due to the dirty environment. The impact loading on a suspension link causes rod end loads that will exceed the film strength of any lube used, and metal-to-metal galling will occur. Once a rod end starts to wear and loosen over time, it may not make a large difference in the vehicle behavior. However, the resulting noise created by the sloppy joint can be annoying, and may eventually lead to body failure.

Spherical uniball assemblies may be used at the spindle end of a front suspension arm instead of OEM ball joints. In the case of uniballs in suspension arms, they tend to be tighter and produce more noise than a standard ball joint. Preventive maintenance to keep the uniball assembly clean of dirt can prolong their life and prevent accelerated wear to the liner.

A number of joint types are available for suspension applications. Determining which type is best is often a question of strength, available mounting area, tolerance for transmitted noise in the cab, and the need for tight, precise suspension operation.

Rod ends (sometimes called heim joints) are very common on aftermarket suspension links. They come in many grades with the best typically having heat-treated chromoly or stainless steel bodies, plus Teflon or similar liners between the body race and the spherical ball inside. The liner serves to lubricate and prolong the life of the joint, especially under off-road use. Over time, however, even the highest quality joints will start to push the liner out of the joint as shown here and start to develop looseness. Once this occurs, they need to be replaced. When significant angular flex is needed, misalignment spacers can be added to a rod end, at the expense of a decrease in mount bolt diameter.
Various flex joints may be used on custom or aftermarket suspension links. These joints are often constructed from forged steel bodies with threaded shafts. A spherical ball is captive between two synthetic cupped races inside the joint body and held captive with a threaded collar or retaining clip. Most offer zerk fittings for grease to be injected into the races.
Rebuildable flex joints are also available in weld-on versions for those looking to make their own links. Most have replaceable synthetic race inserts and some threaded means of preload adjustment. These joints are often larger than equivalent rod ends so usually require greater mounting area for use. However, with their races made from non-metallic material, they transmit less noise through the suspension links into the chassis than a rod end would. In some suspensions, it’s possible to use a firm rubber or urethane bushing at on link end and a spherical flex joint at the other end to get some benefits from both joint materials.

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