Most suspension systems utilize a spring and shock absorber. Suspension systems differ in the type and arrangement of the linkages used to connect these elements to the frame and wheel. The unequal length control arm or short, long arm (SLA) suspension system has been common on American vehicles for many years. Because each wheel is independently connected to the frame by a steering knuckle, ball joint assemblies, and upper and lower control arms, the system is often described as an independent suspension. The short, long arm suspension system gets its name from the use of two control arms from the frame to the steering knuckle and wheel assembly. As shown, these two control arms are of unequal length with a long control arm on the bottom and a short control arm on the top. The control arms are sometimes called A arms because in the top view they are shaped like the letter A.
The short, long arm suspension is designed so that the wheel and tire assembly tends to rise and fall vertically as it goes over bumps in the road. The unequal length control arms compensate for jounce (upward movement of the wheels) and rebound (downward movement of the wheels). For example, if the front wheels hit a bump, as shown below, the wheel and tire assembly move upward in a jounce movement. To offset this movement, the upper control arm is made shorter so the arc it travels is shorter than that of the lower control arm. This causes the top of the wheel to lean inward as the wheel rises. Any side-to-side movement of the tire that could cause scuffing and result in tire wear is eliminated by the short, long arm design.
If the control arms are of equal length, the wheel will move in an arc as it passes over irregular road surfaces. The short, long arm suspension is designed to allow each wheel to compensate for changes in the road surface while not greatly affecting the opposite wheel.
The upper control arm is attached to a cross shaft through two combination rubber and metal bushings. The cross shaft, in turn, is bolted to the frame. A ball joint, called the upper ball joint, is attached to the outer end of the upper arm and connects to the steering knuckle through a tapered stud held in position with a nut. The inner ends of the lower control arm have pressed-in bushings. Bolts, passing through the bushings, attach the arm to the frame. The lower ball joint is usually pressed into the control arm and connects to the steering knuckle through a tapered stud that is held in position with a nut. A ball joint is used on the control arms because it allows movement in more than one direction. It allows the up-and-down motion required as the wheels pass over dips and bumps. This type of joint also allows side-to-side motion as the wheels are turned back and forth for turns.
The ball stud in the ball joint is a tapered stud at one end with a ball-shaped end. The ball end is supported in a similarly shaped housing called a socket. The shape of the housing allows the ball stud to turn around or move side to side. A plastic or sintered iron bearing is positioned between the ball and socket. The bearing allows the ball stud to turn in relation to the housing for steering. The tapered stud and nut hold the ball joint in position in the steering knuckle.
Grease is used to prevent wear between the ball stud and bearing. A rubber seal is held in position around the ball stud by a seal retainer. The seal holds in the grease and prevents the entrance of dirt or moisture. The steering knuckle assembly is used to mount the wheel and wheel bearing assembly to the control arms. A spindle is usually forged in one piece with the steering knuckle. The wheel spindle is the unit that carries the disc rotor hub and bearing assembly. Through the wheel bearings, it carries the entire wheel load. The wheel bearings have a large inside bearing and a small outer bearing. The disc rotor hub is designed so that the center plane of the wheel is closer to the center plane of the larger inside bearing. The inside bearing supports most of the wheel load.
A coil spring is commonly used on the short, long arm suspension system. The most common spring mounting position is between the frame and the lower control arm. Some cars have the spring mounted from the frame to the upper control arm. In either case, the shock absorber is mounted through the center of the spring. Most short, long arm systems use a stabilizer bar between the two sides of the suspension. The sway bar connects both lower control arms to the frame crossmember. Movements affecting one wheel are partially transmitted to the opposite wheel through the frame to stabilize body roll. The sway bar is attached to the frame crossmember and lower control arms through rubber insulator bushings to reduce noise and vibrations. Sway bar end bushings and crossmember bushings are permanently installed on the sway bar.
MacPherson Strut Front Suspension
The MacPherson strut suspension has become very popular on both imported and American vehicles. The MacPherson strut suspension uses a single lower control arm connected to a long, tubular assembly called a strut. The shock absorber, strut, and spindle are a combined unit that is supported by the coil spring at the upper end and by the lower control arm at the bottom. A ball joint is attached to the lower part of the spindle. The lower control arm is sometimes referred to as a track control arm, or transverse link. The lower arm is held in position by a sway bar and frame-mounted rod called a strut rod, or by a stabilizer bar that functions as a combined strut rod and sway bar.
The shock absorber is called a cartridge and fits inside the strut housing. A metal dust cover is used on some units to protect the strut cartridge assembly. A coil spring is held in place by a lower spring seat welded to the strut housing and an upper spring seat bolted to the shock absorber piston rod. The upper mount is bolted to the vehicle body, through two or three studs that go through the vehicle shock tower or fender well. A rubber bumper fits on the piston rod and protects it in case the shock absorber is compressed to its limit.
Some vehicles use a suspension system described as a modified MacPherson strut. This suspension, shown below has a lower control arm and coil spring similar to that used on the short, long arm suspension. Instead of an upper control arm, a strut assembly connects the top of the steering knuckle to the body. In this case, the strut does not have a spring attached to it. The strut acts as the upper control arm and shock absorber.
Q & A: Front suspension
1. My mechanic says my car needs ball joints. Please explain.
Ball joints are a part of your vehicle's suspension that connects the steering knuckles to the control arms. A ball joint is essentially a flexible ball and socket that allows the suspension to move and at the same time the wheels to steer. Cars and trucks without strut suspensions typically have four of them (one upper and one lower on each side). Cars and minivans with strut suspensions have only two (one lower ball joint on each side). Some front-wheel drive cars also have ball joints on the rear suspension.
Ball joint locations in front end
Like any other suspension component, ball joints eventually wear and become loose. Excessive play in the joint can affect wheel alignment and tire wear. Loose joints can also cause suspension noise (typically a "clunking" sound when hitting a bump).
WARNING: If a ball joint fails, the suspension can collapse causing a loss of control. So don't put off having a bad set of joints replaced.
Joints should be inspected before they're greased (since grease takes up some of the slack in the joint). Ball joints are pretty easy to check, but each type requires a different inspection procedure. Use the wrong procedure and you'll get misleading results. The procedure that needs to be used depends on the location and loading of the joint:
* LOWER LOAD CARRYING ball joints are found on front- and rear-wheel drive vehicles where the coil spring or torsion bar is on the lower control arm. You'll also find them on the rear suspension of 1985 & up FWD Buick, Cadillac, Pontiac & Oldsmobiles, too.
Joints with built-in wear indicators (most GM and Ford RWD cars, rear joints on the FWD GM cars, and GM RWD vans, S10 & S15 Blazer) must be checked with the full weight of the vehicle on the tires on the shop floor or on a drive-on style ramp -- not with the wheels up or the suspension supported by jack stands.
No measurements are required if a joint has a wear indicator because internal play is indicated by the position of the grease fitting boss. The boss protrudes about .050 inches on a new joint. As the joint wears, the boss recedes into the housing. The joint is considered "good" as long as you can see or feel the edge of the boss protruding from the housing. But if the top of the boss is flush or below the housing, it's time to replace the joint.
On lower load carrying ball joints without a wear indicator, the joint is checked in the unloaded condition with the wheel raised off the ground and the lower control arm supported by a jack stand. A dial indicator is then used to measure play in one of two directions: sideways (horizontal or radial play) or vertically (axial or up-and-down play). The direction to measure depends on the application (refer to a manual for the exact specs).
Sideways play is measured with the indicator positioned against the inside of the wheel rim near the joint. The wheel should be pushed in and out by hand to check sideways play, and lifted with no more than 25 lbs. of force to check vertical play. Many joints allow up to .250 in. of sideways (radial) play, but some allow no play or only .015 in. of play. Always refer to the vehicle manufacturer's specs.
Vertical play is measured with the dial indicator positioned against the knuckle stud nut (Ford & GM) or the joint housing (Chrysler). A joint that has more than .050 in. of vertical play doesn't necessary require replacement because the specs range from zero play to as much as .125 inch of play.
The most common mistake that's made here is to use too much pressure on a pry bar or to insert a pry bar between the control arm and knuckle rather than under the wheel. Pry hard enough and any joint may appear to be bad.
* LOWER FOLLOWER NONLOADED ball joints are found on two kinds of applications: RWD cars where the spring is over the upper control arm, and vehicles with MacPherson strut suspensions. On both applications the lower joint is checked with the wheel raised off the ground hanging free (no stand under the lower control arm). Rock the wheel in and out by hand. A good joint should show no movement.
One exception here is 1978-80 Omni & Horizon which allows up to .050 inch of sideways play. Another exception is Chrysler FWD minivans and FWD cars ('81 & up). On these applications, the lower joint has a wear indicator grease fitting. Joint play is checked with the wheels on the ground rather than raised. If the grease fitting can be twisted with your fingers, the joint needs to be replaced.
* UPPER LOAD CARRYING ball joints are found on vehicles where the spring or torsion bar is on the upper control arm. Like the lower follower nonloaded ball joints, the upper joints are checked in the unloaded condition with the wheels off the ground -- but with a wedge or block between the frame and upper control arm to support the upper arm. On most applications, any movement calls for replacement. But on some Fords, up to .250 in. of radial play is allowed.
* UPPER FOLLOWER NONLOADED ball joints are also checked with the wheels off the ground but with the lower control arm supported. Any movement usually calls for replacement.
Any joint that exceeds the vehicle manufacturer's maximum allowable wear needs to be replaced. The greater the amount of wear, the greater the urgency to replace it.
Ball joints are often replaced in complete sets, or at least in matched pairs on both sides (both lowers or both uppers). This is because the joints on both sides of a vehicle usually have the same amount of wear. If one is bad, the other usually is too. Load carrying ball joints usually wear out before ones that don't carry a load, so it may only be necessary to replace the loaded joints instead of the complete set.
Replacing a set of ball joints requires separating the control arms from the steering knuckles, a job which can be difficult depending on the design and age of the vehicle. At the very least, it usually requires a special "ball joint fork" tool to loosen the ball joint stud from the knuckle. If this sounds like more of a a job than you want to tackle, let a professional do it the work.
2. I feel a high speed shimmy in the steering wheel. What's causing it?
A high speed shimmy is usually caused by a wheel that's out of balance or a bent wheel.
The first thing to check for would be a bent wheel. Raise the front of the vehicle off the ground and rotate each wheel by hand. If you see any sideways or in and out movement of the wheel, it is bent and needs to be replaced.
WARNING: Although some people claim they can straighten bent wheels, doing so is risky -- especially with aluminum alloy wheels. Replacement is the safest option (but also expensive).
If you don't see any sideways movement in the wheel, it doesn't necessarily mean the wheel is straight. There may be just enough sideways runout to cause a shimmy, but not enough to see. To find this kind of problem, you'll need a dial indicator. More than about .050 inch of sideways runout can be enough to cause a problem.
If the wheels seem to be straight, have the balance of both wheels checked (or rebalanced). If that fails to cure the shimmy, you may have some kind of tire problem due to defective belt alignment or tire construction. Other causes may include loose or improperly adjusted wheel bearings, insufficient caster alignment (check and readjust alignment as needed), or a worn steering damper (on trucks or other vehicles equipped with a steering stabilizer).