Ball joints are chassis parts that connect the steering knuckles to the control arms. The ball and socket design of the joint
allows it to swivel so the knuckles can pivot as the wheels are steered, and to arc so the knuckles can follow the vertical
motions of the suspension as it reacts to changes in the road surface.
LOW FRICTION JOINTS
Ball joints were first used back in the 1950s as a replacement for king pins. The joint consisted of a steel stud with a rounded
head that rode against a steel bearing. Grease provided lubrication, and was kept inside the joint by a rubber boot or seal.
In the 1970s and 1980s, a new type of "low friction" ball joint began to appear in many applications. "Low friction" ball joints
have a highly polished ball stud and a polymer rather than steel bearing. The polymer bearing provides a low friction surface
that reduces steering effort and improved steering return. Most such joints are sealed and have no grease fitting. This reduces
the need for maintenance and minimizes the risk of outside contamination. Low friction joints are used on most front-wheel
drive cars with rack & pinion steering as well as many late model light duty trucks.
TYPES OF BALL JOINTS
Vehicles with short-long arm (SLA) front suspensions have four ball joints (two uppers and two lowers), while most strut
equipped vehicles have only two lower joints in the front suspension (except those with "wishbone" strut suspensions such as
Honda that have four). Rear ball joints are also used in some front-wheel drive cars with independent rear suspensions.
Some ball joints are load carrying while others are nonload carrying depending on their position in the suspension. And the
loaded variety come in two basic types as well: compression joints tension joints.
Compression ball joints are those designed to carry loads that bear down on the ball stud. Consequently, most joint wear
occurs where the ball stud presses into its bushing. This type of joint is found in SLA suspensions where the spring is mounted
over the upper control arm.
Tension joints, by comparison, are designed to carry loads that try to pull the joint apart. Wear occurs in this joint at the point
where the shoulder of the ball stud pulls against its seat. The lower ball joints on SLA suspensions where the spring sits on the
lower control arm are tension joints. Tension joints are also found on "modified" MacPherson strut suspensions (Ford Mustang,
Fairmont and T-Bird) where the spring sits on the lower control arm rather than on the strut.
The third type of ball joint is the nonload carrying joint (also referred to as a "dampening" or "follower" ball joint). This type of
joint is used for the upper ball joint on SLA suspensions where the spring seats on the lower arm, the lower ball joint on
suspensions where the spring is over the upper control arm, and the lower ball joint on MacPherson strut suspensions where
the spring is mounted around the strut. Because this type of joint does not carry weight, it is preloaded to keep it tight and to
provide a resistance for improved steering stability. Any play in this type of joint means it should be replaced.
Though most original equipment ball joints today are designed to provide 100,000 mile plus durability, many never make it that
far for a variety of reasons. One is wear. The constant friction created by turning and driving on rough roads creates friction
between the ball stud and bearing. And the rougher the roads and the heavier the vehicle, the faster the rate of wear.
Wear can be further accelerated by contamination and/or lack of lubrication. With a greaseable joint, lubing the chassis
periodically is necessary to maintain a layer of grease within the joint. Lubing the joint also helps flush out the old grease and
contaminants, which extends the service life of the joint. But most OEM ball joints today as well as some aftermarket
replacement joints are "sealed for life" and have no grease fittings.
Load carrying ball joints do tend to wear at a faster rate than their unloaded counterparts because of the weight they carry.
That's why the lower ball joints on an SLA suspension typically wear out before the upper joints.
Some symptoms of worn ball joints include:
* Front wheel shimmy at low speed
* Steering wander
* Clunking noises from the front suspension
* Camber wear on the tires
CHECKING JOINT WEAR
The different types of ball joints require different inspection procedures -- which means first identifying the type of joints on the
Joints (all types) should also not be greased prior to inspection because filling the joint with grease can make the joint appear
to be less worn than it actually is.
JOINTS WITH WEAR INDICATORS
Many load carrying tension type lower ball joints on General Motors and Ford rear-wheel drive applications have a built-in
"wear indicator" to show how much wear has taken place inside the joint. This same type of joint is also used in the rear
suspension on some of GM's big front-wheel drive cars (Cadillac, Buick & 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 also 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.
For an accurate wear indication, the joint must be checked with the full weight of the vehicle resting on the suspension, and the
tires in full contact with the ground. 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.
Something else to keep in mind about ball joints with built-in wear indicators is that the wear indicator only shows axial (up and
down) wear, not radial (sideways) play. Radial play is usually not a factor because of the way in which the joint carries the load.
On certain Chrysler applications, a slightly different type of wear indicator is used. As the joint wears, the grease fitting
becomes progressively looser rather than receding into the housing. If the fitting can be wiggled by hand, the joint is worn and
CHECKING LOADED LOWER JOINTS WITHOUT WEAR INDICATORS
On vehicles that have loaded lower ball joints without built-in wear indicators (SLA suspensions & modified strut suspensions),
the joints have to be unloaded to check for looseness. This is done by raising the wheels off the ground and supporting the
lower control arms. The support stand should be placed as near to the joint as possible. The lower ball joint can then be
checked for axial (vertical) play by wiggling the wheel up and down. Radial (lateral or sideways) play can be checked by
rocking the wheel in and out. Don't use a pry bar between the lower control arm and knuckle because doing so may make a
good joint appear to be loose.
The amount of play that's considered acceptable will vary according to the application, so always use a dial indicator to get an
accurate reading. Then you can compare joint movement to the specifications listed by the manufacturer to determine if the
joint is worn or not. Axial play should be measured between the knuckle and lower control arm, and radial play between the
lower control arm and wheel rim.
The old rule of thumb that says any more than .050 in. of vertical play calls for joint replacement doesn't necessarily hold true
for all applications because some specifications allow no movement while others allow up to several times as much vertical
play! The maximum limit for radial (sideways) play in most applications is 0.25 inch, but again always check the specifications
to make sure.
CHECKING LOADED UPPER JOINTS (SLA SUSPENSIONS)
In suspensions where the upper ball joints are the loaded ones, the upper control arms must be supported to take the weight
off the joints. This can be done by wedging a block of wood between the upper control arms and frame to support the vehicle's
weight when the wheels are raised off the ground. The lower control arms must be allowed to hang free, with the supports
positioned under the frame, not the lower control arms. Both axial and radial movement in the upper joint can be checked by
rocking the tire up and down, and in and out. As with loaded lower ball joints, a dial indicator should be used for accurate
measurements. The limit for lateral play is usually 0.25 inches. Refer to the manufacturer's specs for vertical play. The lower
ball joint in this type of suspension is unloaded, so any looseness would indicate a new joint is needed.
CHECKING UNLOADED UPPER JOINTS (SLA SUSPENSIONS)
The upper nonload carrying ball joint can be checked by grasping the top of the wheel and attempting to rock it in and out. Any
looseness in the upper ball joint means the joint should be replaced.
CHECKING UNLOADED LOWER JOINTS (STRUT SUSPENSIONS)
On vehicles with MacPherson struts, the strut carries the load and the lower ball joints are unloaded. Play can be checked by
raising the wheels off the ground so the lower control arms hang free with the strut fully extended. Rocking the wheels in and
out should produce no horizontal play between the control arms and knuckles if the joints are good. On Chrysler FWD cars or
others with built-in wear indicators, the wheels must be left on the ground as described earlier.
On some vehicles, the vehicle manufacturer may recommend a more involved joint checking procedure. Toyota, for example,
recommends the following procedure for preloaded ball joints: Disconnect and separate the ball joint stud from the steering
knuckle. Then use a torque wrench to measure how much effort it takes to rotate the stud. If the amount of force is required is
less than the specs (typically 9 to 35 inch pounds), the joint is worn. Excessive friction would indicate binding inside the joint,
probably due to dirt contamination.
BALL JOINT REPLACEMENT
The time to replace ball joints is when they've become worn. When the amount of play or movement in the joint exceeds the
maximum allowed by the vehicle manufacturer, alignment, tire wear and handling can all suffer. The amount of wear that can
be tolerated in a joint varies from one vehicle application to another. Some allow no visible play while others can handle up to
.250 inch or more of play and still be considered okay. The only way to know if a joint is worn, therefore, is to refer to the OEM
specs and measure play (both sideways and vertical) or check the joint's built-in wear indicator if one is provided.
If a ball joint is worn, it may be unsafe and should be replaced. But should its counterpart on the opposite side be replaced,
too? The Motorist Assurance Program (MAP) guidelines say replacement is only required for the joint that is worn beyond
factory specifications. Even so, many experts recommend replacing both joints (both uppers, both lowers, or all four) in a high
mileage vehicle even if only one joint is worn because the other joints are likely nearing the end of their service life, too. It's
more convenient and less expensive in the long run to replace all the joints at one time than to piecemeal the repair work.
Some technicians still prefer a greaseable ball joint and the durability of a steel bearing. This type of ball joint is used in most
older rear-wheel drive cars and trucks, and is still offered as original equipment on some newer trucks that are built for serious
As a rule, the same type of ball joint (standard or low friction) should be used to replace a worn ball joint to maintain the same
steering effort and feel. If a sealed low friction ball joint in a newer vehicle is replaced with a greaseable standard ball joint, it
may cause a noticeable difference in steering feel, effort and return.
Another item that should be checked when ball joints are replaced is the stud hole in the steering knuckle -- especially if the
ball joint stud has broken or is loose. An out-of-round hole can allow flexing that leads to metal fatigue and stud breakage. The
new ball joint stud should fit snugly in the hole without rocking, and only the threads of the stud should extend above the hole.
On front-wheel drive suspensions that use a pinch bolt arrangement to lock the ball stud in the knuckle, do not use a chisel to
spread the flange as doing so may damage the flange. Replacing the pinch bolts and nuts is also recommended.