The Purpose of Control Arms, Bushings, & Ball Joints

Team BlueStar Inspections

Control arms, bushings, and ball joints may not be common household terms for many vehicle owners. You may have never even heard of them, but they are critical components of your vehicle's suspension system that allow your tires to smoothly go up and down in a controlled fashion. Without them, your ride probably wouldn't be very enjoyable.

Most vehicles use either one or two control arms per wheel, on both the front and rear suspension. Many front-wheel drive vehicles only use a lower control arm, while trucks and SUVs often have both an upper and lower control arm. A control arm connects the wheel hub and steering knuckle to the frame of the vehicle. They are typically equipped with bushings on the frame side of the vehicle and a ball joint on the wheel side of the vehicle that allow flex and controlled movement according to road conditions and steering input from the driver. Control arms allow wheels to move up and down while preventing forward and rearward movement.

Control arm bushings usually consist of an outer metal sleeve, a durable rubber or polyurethane bushing, and an inner metal sleeve. Control arm bushings are important for driving comfort and handling. They cushion the suspension system which in turn controls noise and vibrations, and also provide a softer ride over bumps. Bushings can flex and move while retaining stiffness and the ability to return to their original shape and position.

Many suspension and steering joints utilize bushings when components need to be mounted together where vibration is a concern. Bushings are used in control arms, shock absorber mounts, stabilizer bars, stabilizer links, engine and transmission mounts, and other suspension and steering components. Bushings have a function similar to cartilage in joints of the body. Worn or damaged cartilage results in bone on bone contact and discomfort. Worn or damaged bushings can allow metal on metal contact, tire wear, discomfort, noises, and vibrations. Bushings deteriorate due to heat, age, exposure, heavy loads, salt, oils, and the stress of frequent movement.

Like the ball and socket joint that connects and holds your leg bone to your hip bone, a ball joint connects and holds the front suspension of your vehicle together. In the same way your leg can move up and down, and side to side, a ball joint enables the wheel and suspension to move together in the same manner.

Ball joints allow a limited range of movement in all directions and are the pivot between the wheels and the suspension. A single ball joint is used to allow free movement in two planes at the same time, including rotating in those planes. Combining two such joints with control arms enables motion in three planes, allowing the front end of an automobile to be steered and a spring and shock suspension to make the ride controlled and comfortable.

There are two kinds of ball joints. Ball joints are classified as either load-carrying or follower types, and their position in the suspension varies depending on the suspension design. Load-carrying ball joints are designed to support the weight of the vehicle while providing a pivot point for the steering system. Follower ball joints are designed to maintain precise dimensional tolerances as well as a pivot point for the steering system. These two types of joints often have different wear and failure rates, with the load-carrying joints usually failing first.

Many currently manufactured vehicles worldwide use a MacPherson strut suspension, which utilizes one lower control arm and one lower ball joint per wheel with the necessary small amount of movement at the top of the strut usually provided by an elastomeric (rubber like) bearing, within which is a ball bearing to allow free rotation about the steering axis. In this design, the lower ball joint is a follower, with the bottom of the strut connected directly to the steering knuckle and wheel. The bearing plate of the upper strut mount carries the vehicle's weight, leaving the lower ball joint to act only as a pivot point. In a non-MacPherson strut automobile suspension, the two ball joints per wheel are called the upper ball joint and the lower ball joint. In the majority of these designs, the coil spring is seated in the lower control arm, supporting the weight of the vehicle. The lower ball joint is the load-carrying joint in this type of suspension, while the upper ball joint is the follower, with no significant load to support. It acts only as a second pivot point for steering.

Most modern ball joints are sealed and do not require lubrication as they are lubed for life. Historically, most ball joints had grease fittings called grease zerks and were designed to have lubricant periodically added. The lubricant was usually a very high-viscosity lubricant. Almost all modern vehicles now use sealed ball joints to minimize maintenance requirements. New technology, especially applied to the internal bearing design and synthetic lubricants along with improved dust boot sealing, has allowed longer ball joint service intervals and better grease retention.

While there is no exact lifespan that can be put on sealed ball joints, they can fail as early as 80,000 miles in modern vehicles, and much sooner depending on the type of driving. If a ball joint fails completely, the wheel could separate from the steering knuckle which will cause a complete loss of control. The tire will be at an unintended angle, and the vehicle will come to an abrupt halt. This could damage the wheel and tire, other suspension components, and possibly even other parts of the vehicle.

There are a lot of possible signs of a failing ball joint or control arm bushings. These may include a clicking, popping, or snapping sound when the wheel is turned and eventually turn into a squeaking sound at the end of a stop, when the gas pedal is used, and/or when turning the steering wheel. Another symptom could be knocking and clunking noises coming from the suspension when going over bumps. The sounds will continuously get louder as the component wears or eventually breaks. Dry ball joints have dramatically increased friction and can cause the steering to stick or be more difficult. Excessively worn bushings or ball joints can cause wheel shimmy, which may cause vibrations that are felt in the steering wheel. Vibrations may increase during acceleration and smooth out at higher speeds.

Another symptom commonly associated with bad or failing control arm components is steering wandering. Excessively worn ball joints, bushings, or a combination of these can cause the vehicle's steering alignment to shift, which may cause the steering to pull to the left or right when traveling down the road. This will require constant driver correction to steer the vehicle straight.

The time for regularly scheduled oil changes is the best time to inspect control arms, bushings, and ball joints. A quick test drive will allow for a wide range of driving conditions. Cornering left and right, hitting bumps, and cruising on straight and level ground while braking and accelerating are all important to get a feel for any noises or performance issues related to the control arm assemblies. Visually inspect the control arm bushings for cracking, splitting, tears, missing parts, and oil saturation.

When inspecting ball joints, first look to see if the rubber boots holding the grease inside the ball joint are in good shape. If the boot is torn, or just plain gone, chances are the ball joint is in the process of failing. If the ball joint is capable of being greased, use only the grease recommended by the vehicle's manufacturer. Most modern ball joints come lubricated for life and are therefore incapable of being greased.

Determining if the ball joints are good or bad depends largely on which type of suspension and ball joints are used in the vehicle in question. If you are lucky, ball joints will have built-in wear indicators allowing easy inspection. If not, relieve the load on the suspension by jacking the vehicle up and inspecting each individual ball joint for play. As there are more than a few types of suspension setups, it is best to follow the inspection procedure provided by the vehicle manufacturer. Severe ball joint failure can cause vehicle suspension separation, so it is best not to take guesses when dealing with suspension issues.

Vehicle control arms, bushings, and ball joints are very important suspension and steering components. When they become worn it can cause problems for the vehicle that may compromise handling, comfort, and may result in catastrophic separation of the control arm and knuckle. For this reason, if you suspect your vehicle's control arms, suspension bushings, or ball joints may be bad or worn, have them inspected by an ASE certified technician.