Most suspension systems have the same basic parts and operate basically in the same way. They differ, however, in the way the parts are arranged. The vehicle wheel is attached to a steering knuckle. The steering knuckle is attached to the vehicle frame by two control arms, which are mounted so they can pivot up and down. A coil spring is mounted between the lower control arm and the frame.
When the wheel rolls over a bump, the control arms move up and compress the spring. When the wheel rolls into a dip, the control arms move down and the springs expand. The spring force brings the control arms and the wheel back into the normal position as soon as the wheel is on flat pavement. The idea is to allow the wheel to move up and down while the frame, body, and passengers stay smooth and level. There are four basic types of springs used in suspensions: coil, torsion bar, leaf spring, and air spring. The coil spring is the most popular type of spring in both front and rear suspension systems. It is simply a round bar of spring steel that is wound into the shape of a coil. Usually, the top and bottom coils are closer together than the middle coils. The advantages of the coil spring are its compactness, lack of moving parts, and excellent weight supporting characteristics.
The disadvantage of a coil spring is its weakness in supporting side-to-side or lateral movement. When coil springs are used at the drive wheels, heavy traction bars or torque tubes are often required to maintain axle housing alignment.
A number of vehicles use a torsion bar spring. It is a long, solid steel shaft that is anchored at one end to the suspension's control arm and at the other end to the vehicle's frame. Torsion is the twisting action that occurs in the bar when one end is twisted and the other end remains fixed. When a vertical impact on a wheel is transmitted through the control arm to the torsion bar, the bar twists to absorb the impact. The bar's natural resistance to twisting quickly restores it to its original position, returning the wheel to the road.
A torsion bar can store a significantly higher maximum amount of energy than either an equally stressed leaf or coil spring. The torsion offers important weight savings and it is adjustable. In addition, it requires significantly less space than a coil spring.
The leaf spring is made of several layers of spring steel stacked one upon the other. Usually, there is one main leaf that uses spring eyes for locating and fastening the spring to the frame or underbody. Several other progressively shorter leaves are placed on the main leaf, and the assembly or leaf pack is held together in the middle by a center bolt and on the ends by rebound clips. Some spring packs use fiber or plastic pads between leaves to reduce internal leaf friction. Some vehicles use a single leaf instead of a buildup of multiple leaves. One manufacturer is using a leaf spring manufactured from a nonmetal composite. Leaf springs are usually arched so that the ends are higher than the center when viewed from the side.
The leaf spring is usually mounted in three places. A bushing is installed in each of the spring eyes. A bolt through the bushing in the rear spring eye attaches the rear of the spring directly to the vehicle frame. A shackle assembly is attached to the front spring eye and bushing and is then mounted through a shackle bushing to the frame. The shackle assembly allows the leaf spring to pivot up and down. A pair of U-bolts (one shown) and a tie plate are used to clamp the front or rear axle assembly to the leaf spring.
The main advantage of leaf springs is their ability to control vehicle sway and lateral movement. For these reasons, leaf springs are often used on the rear suspension of rear drive vehicles.
Many late-model luxury cars use air springs. The spring is essentially a rubber bag or bladder full of air. A piston is attached to the lower control arm. Movement of the lower control arm causes the piston to move into the air bladder and compress the air in the bladder. Air pressure is used to regulate how easy or hard the bladder can be compressed. The air bladder is usually connected to an air compressor, which regulates the action of the air spring based on road conditions.
All suspension systems use a shock absorber at each wheel. When the coil, torsion bar, leaf spring, or air spring is deflected, it can oscillate (bounce up and down) uncontrollably, possibly causing the tires to lose contact with the road. This could cause the car to bounce up and down without any control. To prevent this from happening, shock absorbers are used, not to absorb shocks, but to control spring rate and dampen spring oscillations.
The shock absorber is a hydraulic device. One end of the shock absorber is attached to a wheel assembly and the other end is attached to the vehicle frame. Shock absorber movement is limited by forcing fluid inside the shock absorber through passages or orifices. This causes the shock absorber to compress or extend at a slow rate.
When the wheel goes over a bump, the shock absorber compresses. When the wheel goes into a dip, the shock absorber extends slowly. This action dampens spring rate and controls spring oscillation.
As we mentioned previously, the suspension system is designed to provide comfortable, safe ride control. For safety, especially in cornering, the suspension system must keep the wheels upright, or nearly upright, under all conditions of driving; a tire can deliver maximum force to the ground only when its tread is flat on the road. Therefore, the tire should be upright when the car is accelerating and cornering, especially the outside wheel that is carrying most of the load; when braking as the front of the car dips and the rear rises; and finally, when it is deflected up or down by road irregularities.
Inspect Suspension Components for Wear
SERVICE TIP: One of the best ways to diagnose a suspension problem is to go for a ride with the owner. Observe what the car is doing. Ask yourself if the problem occurs during braking, steering, or over bumps. Your observations will help you a great deal when you inspect the suspension system.
Anytime you have a car on a hoist to lubricate the suspension components, you should also check the suspension parts for wear. A car with poor steering control, rapid tire wear, noise during stopping and driving over bumps, or poor steering stability may have a suspension, steering, or wheel alignment problem. You should follow a systematic step-by-step procedure to determine the condition of the parts in each of these areas.
To begin your inspection, raise the vehicle on a hoist so you have plenty of room to make an inspection. You can test for loose wheel bearings by grasping the front tire top and bottom and rocking it in and out. Any noticeable looseness is too much. To make sure any looseness detected is in the wheel bearing, check for relative movement between the rotor or brake drum and backing plate. Upper and lower ball joints on most vehicles perform different functions. They are designed differently and require different inspection techniques. The design differs because one pair, called the weight carriers, always supports the front half of the vehicle weight. The other set, called the friction or follower joints, supports no vertical load, but must maintain a firm, quiet connection between the spindle and the control arm. On most American cars with short, long arm suspension, the lower ball joint is the weight carrier. But on many import models with short, long arm suspension, the upper ball joint carries the load.
To apply the proper checking procedure, identify which ball joints are the weight carriers and which are the friction joints. The weight carrier is always mounted on the control arm to which the coil spring or torsion bar is mounted. The friction joint is always mounted on the unloaded control arm. After identifying the ball joints by function, continue the inspection.
Friction ball joints have a heavy coil spring or rubber insert inside that preloads the ball stud and enables the spring to dampen the vibration and road shock of the wheels. Unfortunately, the spring also makes the ball joints difficult to check because the preload keeps the ball joint tight and makes a simple wheel shake test unreliable. The most accurate way to check a friction joint for wear is to disconnect it from the spindle, put two nuts back on the stud to act as a lock, and turn the stud with an inch-pound torque wrench. If you get a reading between 24 and 96 inch-pounds (2-8 newton-meters) for most cars, the ball joint is in satisfactory condition. When the reading is above or below these limits, something is wrong in the socket and the ball joint should be replaced. This checking procedure should be followed when the vehicle has very high mileage, has performed in especially rough service, or appears not to have had proper maintenance.
Weight-carrying ball joints are constantly twisting and turning in response to the steering and up-and-down motion of the front wheels. This wearing motion continues while the two ball joints are loaded with the vehicle weight.
To check a weight-carrying ball joint, first visually check for the general condition of the ball joint and seal. Then check to determine the amount of wear on the inside of the part. Look for broken seals that let contaminants into the part, which shortens the life of the joint. When possible, check the throat area under the seal for cracks or distortion.
The next step is to unload the ball joint and measure the amount of clearance. Lower the car to the ground and position a jack to unload the suspension. For lower weight carriers, place the support under the lower control arm near the wheel.
For upper weight carriers, place the support under the frame crossmember and support the upper control arm in its operating position with a wedge between the body and upper control arm. Check for loose wheel bearings and adjust if necessary. Wheel bearings must be snug to avoid adding their clearance to that of the ball joints.
The most accurate way to measure internal wear is by using a dial indicator that can measure vertical movement right at the ball joint socket. This eliminates the chance of adding other clearances, such as those created by loose wheel bearings, to the ball joint check measurement. The dial indicator is mounted on the lower control arm, near the ball joint to be checked, with locking pliers or a clamp. The dial indicator plunger is then placed against the ball joint steering knuckle (depending on the type of car being checked) so it can read the vertical movement of the parts when the ball joint is unloaded and the wheel is moved through its full vertical range with a pry bar.
Lock the indicator in place by tightening the flexible coupling. Set the dial indicator to zero. Place the pry bar between the tire and ground and pry up on the bottom of the tire. Then, when the wheel is moved vertically through its full range, the reading shown on the dial indicator will equal the amount of internal ball joint clearance. Compare your readings to specifications in the shop service manual to determine if the ball joint should be replaced.
The lower ball joint is the only ball joint on MacPherson strut units. To check it, grasp the lower arm near the ball joint and force the arm up and down to check for looseness, or push and pull on the tire near the ball joint. If there is any movement, the ball joint should be replaced.
Many vehicles use ball joints with built-in wear indicators. Wear indicator-type ball joints must remain loaded to check for wear. The vehicle should be checked with the suspension at curb height. The most common type of wear indicator has a small diameter boss that protrudes from the center of the lower housing. As wear occurs internally, this boss will gradually recede into the housing. When it is flush with the housing, the ball joint should be replaced.
You are ready now to visually check each of the control arm bushing assemblies. Control arm cross shafts and bushings provide the inner hinges for the independent front suspension. They attach the control arms to the vehicle frame in a way that permits the wheels to move up and down independently. The control arm bushings provide the bearing for this hinge. The bushings must be in satisfactory condition to perform their primary function of keeping the control arms in the proper position to maintain alignment settings. The rubber torsion type also has an important secondary function of helping dampen road shock from the vehicle body.
Visually inspect each bushing assembly. The first sign of failure in the rubber torsion-type bushing is when cracks appear around the bushing edges. Small cracks on the outer surface are not harmful, but the bushing should be checked very closely when they are present. Look for severe compression of the rubber on one side or the rubber extruding out of the bushing. Also, check for torn rubber and frayed edges. When you examine control arm bushings, also look carefully at the end nuts. They can work loose and allow the bushing to pop out of the mounting.
Coil springs become weakened through constant twisting and flexing in normal service. This allows them to sag, lowering the vehicle out of its normal curb height range. When the curb height varies by even a fraction of an inch from the original specifications, there are problems. Suspension parts, such as control arms and ball joints, may be extremely overloaded when there is not enough suspension travel.
Coil springs are checked by taking measurements at specific points on the vehicle. The measuring points and specifications can be found in the vehicle shop service manual. The springs can also be checked by comparing measurements taken on each side of the car. Measure dimension A; this is the distance between the lower control arm and the frame. There should be a difference of no more than 1/4 inch (6 mm) between the right and left side of the car. Measure dimensions B and C (from ground level to the center of pivot points). The difference between B and C on either side should not be more than 3/4 inch (19 mm). Make sure the bumper is not bent or deformed; then measure distance D, which is the height from a level floor to the bottom of the bumper. The difference between the D measurements should not be more than 3/8 inch (10 mm). If you find your measurements are beyond specifications, both coil springs should be replaced.
Many vehicles are equipped with strut rods that are bolted to the lower control arm and mounted at the opposite end through the strut rod bushing in the vehicle frame. Strut rods act as a brace for the lower control arm. The strut rod bushing must maintain a firm, flexible shock-absorbing mount.
The strut rod bushings wear due to the constant flexing of the strut rod and rubber deterioration caused by the elements. The results are changeable alignment settings, noise (especially during braking), and pulling to the side during braking.
Strut rod bushings wear from the inside out. A check of the external bushing condition will not help determine if the bushing needs replacing. The best way to check strut rod bushings is to raise the car on a hoist with a helper inside. Spin one of the front wheels and then have the person in the car apply the brakes. Worn strut rod bushings will make a popping noise when the brakes are applied. This noise is due to movement in the bushing assembly as the strut tries to control braking forces on the control arm.
Sway bar bushings anchor the bar to the vehicle frame and the control arm on each side. The purpose of the bar is to reduce body roll and sway. The condition of the bushings will affect the performance of the bar. Check the bar for missing links. Inspect the frame bushings for tightness, distortion, and signs of movement by grabbing the bar with your hand and trying to shake it.
Lubricate Suspension and Steering Components
In years past a "lube job" or "grease job" was a popular and profitable service procedure. Most vehicles had numerous lubrication points that a technician had to locate and then lubricate or grease. Due to advances in lubricant quality and sealing technology, many of the areas that once required lubrication are now permanently sealed. There are, however, some current vehicles that have a few suspension or steering lubrication points.
CAUTION: Air-operated lube guns inject grease through the nozzle at high pressures. Always wear eye protection when using a lube gun. Never point the grease gun at any part of your body because the grease could be injected through your skin.
The ball joints and some types of steering linkages require a grease-type lubricant between the moving parts. Grease is a liquid lubricant that is mixed into a binder material to make it thick.
The grease is injected into a lubrication area with a grease or lube gun. There are two types of lube guns: mechanically operated and air operated. The hand pump-type lube gun is shown below. Grease is stored in a cartridge inside the gun. Pumping the handle causes grease to come out the nozzle under pressure. The air-operated lube gun is often a large roll around unit. Many have a large container of grease that is pressurized by shop air. The nozzle has a trigger, which when activated, allows air to force grease through the grease hose and out the gun nozzle. The grease comes out the nozzle under high pressure.
The nozzle on the lube gun is made to fit on lubrication fittings. The fitting is a small part that screws into the part requiring lubrication. It has a small round valve inside that keeps dirt out of the part. The lube gun nozzle fits tightly over the end of the fitting. When the grease is pumped through the nozzle under pressure, it forces the fitting valve open and allows grease to enter the part.
When you do a suspension and steering lubrication, first look up the location of all the lubrication fittings on the car. Most shop service manuals have a chart that identifies the lubrication points on the suspension and steering. . There are usually two fittings on each side of the car. One on the lower ball joint and one on the steering tie rod. To do the lubrication job, raise the car on a hoist or support it on safety jack stands. Wipe each lubrication fitting clean with a rag. Inspect the grease seals for leaks or tears. You will have to remove these plugs and temporarily install lubrication fittings.
WARNING: If you do not clean a lube fitting before inserting the lube gun, dirt can be forced through the fitting and into the part. Dirt will cause the part to fail early.
Push the lube gun nozzle over the fitting. The nozzle must fit over it completely or the grease will not enter the fitting. Make sure your gun does not deliver lube at too high a pressure because this could rupture the ball joint seals. Work the pump or squeeze the trigger slowly. The grease should enter the fitting and not leak out around the end of the nozzle. If the grease leaks around the end of the nozzle, you probably have one of two problems:
You do not have a good fit between the nozzle and fitting. Clean the fitting and try again. The fitting may be plugged. Install a new fitting and try again. One or two hand pumps or squeezes of the trigger is enough to lubricate the typical ball joint. You should not see evidence of lubricant escaping past the seals. Remove the nozzle from the fitting. Wipe off any excess grease from the fitting area. Repeat this procedure for each of the lubrication fittings on the car.
WARNING: Do not overfill a lubrication area with grease. If you force too much grease into a ball joint, you can blow out the seal. If you rupture the seal, the ball joint assembly will have to be replaced.