The control arm assembly normally includes the control arm shaft and bushing and the ball joints. It may also include a stabilizer link kit.
Front Control Arm Assembly
The upper and lower control arms on the traditional independent front suspension (IFS) function primarily as locators. The outer ends are connected to the wheel assembly with ball joints inserted through each arm into the steering knuckle.
Ball Joint Locations. Courtesy of Moog Automotive, Inc.
There are two types of control arms: the wishbone or double-pivot control arm, and the single-pivot or single-bushing control arm.
The single pivot control arm is lighter and requires less space than the wishbone type.
The wishbone offers greater lateral stability than the single-pivot arm, which is lighter and requires less space than the wishbone but also requires modifications in suspension design to compensate for the reduced lateral stability.
The unequal length control arm or short-long arm (SLA) suspension system has been common on domestic-made vehicles for many years.
A typical SLA front suspension.
Each wheel is independently connected to the frame by a steering knuckle, ball joint assemblies, and upper and lower control arms. The SLA system is often called the A-frame wishbone system.
The following describes the procedure to remove a lower control arm on an SLA suspension with a spring on the lower arm. A similar procedure is used to remove an upper arm from this same suspension or a lower arm from a strut suspension; there is no need to compress or remove the spring for these other styles of control arms. A similar procedure to that described here is used on an upper control arm with a spring mount on it.
To remove a lower control arm, you should:
- Raise and support the car on a hoist or jack stands.
- Remove the wheel.
- Remove the shock absorber and the stabilizer bar end link.
- If the car has a tension-loaded lower ball joint, break loose the ball joint stud taper.
- Install a spring compressor and compress the spring enough to remove the spring load from the control arm.
- You can be sure the spring load is removed when the spring becomes free to move around or when the control arm can be moved upward.
- If the car has a compression-loaded ball joint, break the ball joint stud taper at this time.
- Disconnect the ball joint stud and separate the steering knuckle boss from the ball joint stud.
- It is often necessary to pry between the lower control arm and the brake splash shield, rotor, or steering knuckle to keep the end of the control arm from catching on the splash shield.
On some cars, the lower control arm needs to be guided past the brake splash shield as it is lowered; a screwdriver or small prybar can be used (Courtesy of General Motors Corporation, Service Technology Group).
- Inspect the tapered portion of the stud for any bright, worn metal, which would indicate a worn steering knuckle boss.
- The upper control arm, with the steering knuckle attached, can be raised and a block can be inserted between the control arm and frame or body bracket to hold them up and out of the way.
A hammer has been placed under the upper control arm to hold the steering knuckle and brake assembly out of the way. Courtesy of Moog Automotive.
- The lower control arm can be pivoted downward to remove the spring.
- If desired, the lower ball joint can be replaced.
- The lower control arm can now be removed by removing the inner bushing/pivot bolts.
The upper control arm can also be removed at this time by disconnecting the ball joint stud and the inner pivot bolts.
- If this is done before replacing the lower control arm, it will be necessary to support the steering knuckle with the brake rotor or drum and caliper or backing plate, or disconnect the brake hose and completely remove the steering knuckle and brake assembly.
- Many front-end technicians prefer to service the parts on the lower control arm, reattach the lower control arm to the steering knuckle and frame, tie the control arm in place (if necessary to keep it from falling and stressing the brake hose), and then service the parts on the upper control arm.
- The steering knuckle and brake assembly can be removed from the car, but the tie-rod end will have to be disassembled from the steering arm and the brake hose from the caliper or wheel cylinder.
- If the brake hose is disconnected, it will be necessary to bleed the air from that brake after reassembly.
Many General Motors cars with SL A suspension mount the upper control arm on a pivot shaft, which is bolted to a bracket on the frame; the pivot shaft is positioned inward of the bracket.
- This control arm must be slid inward and off the bolts to remove it.
- The steering shaft or exhaust manifold often prevents enough inward movement to allow the shaft to be slid off the mounting bolts.
- Use a socket, extension bar, and long socket handle or air-impact wrench to turn the mounting bolts; this will loosen them.
- The bolts should work their way out of the frame bracket.
- It helps to pry outward on them as they are rotated.
- A special tool is available to press these bolts out of the frame bracket.
- With the bolts out of the way, the control arms can be lifted out.
- For replacement, the bolts are merely driven back in place after the control arm is repositioned.
- New bolts will be required if their serrations become too badly worn; If they do not fit tightly, the bolts may turn when replacing shims during a wheel alignment.
The steering shaft interferes with the removal of this control arm; if the control arm mounting bolts (arrows) are removed, the control arm can be easily removed.
To install a tower control arm with a spring on an SL A suspension, you should:
- Place the control arm in position in the frame or body brackets and install the inner pivot bolts.
- Replace the nuts and tighten them finger tight.
- Turn the ball joint stud to align the cotter pin hole lengthwise to the car so the cotter pin will be easy to install.
- Clean and inspect the ball joint stud hole in the steering knuckle boss.
- Replace the compressed spring and align it to the correct position. Swing the control arm upward so the ball joint stud enters the steering knuckle boss and thread the nut onto the ball joint stud.
- Tighten the ball joint stud nut to the correct torque and install the cotter pin.
- If a prevailing torque nut is used, it is sometimes difficult to tighten the nut because the stud tends to rotate.In this case, it is usually necessary to either pretighten the stud using a plain nut, use a stud holder, or tap the control arm upward to try to seat the stud taper.
- Remove the spring compressor, making sure the spring seats correctly.
- Install the shock absorber and stabilizer bar end links.
- Install the tires and wheels and lower the car onto the tires.
- Tighten the inner pivot bolts to the correct torque.
Rear Control Arms
To remove the upper rear control arms from the vehicle:
- Remove the bolts passing through the control arms at the frame and at the axle ends.
- Usually the rear coil spring does not have to be removed for this.
- Service one side of the vehicle at a time. This simplifies realigning the parts during assembly.
- On a serviceable control arm, replace the control arm bushings by removing the defective bushing with an appropriate puller.
- Properly position the new bushing and press it into place in the same manner as is done on front suspensions.
- Position the repaired control arm on the vehicle and loosely install the bolts.
- Repeat the service on the other control arm if necessary.
- Properly torque the nuts and bolts once the vehicle's entire weight is on the springs again.
To remove the lower control arms from the vehicle:
- The coil springs must be removed to service the lower rear control arms.
- Again, one side of the vehicle should be serviced at a time.
- Once the vehicle is properly supported and the springs are dismantled, remove the nuts and bolts that pass through the control arm.
- Remove the control arm from the vehicle and service it in the same way as the upper control arm.
- Check the service manual to see if there is an adjustment for the driveline working angle.
- If none is specified, torque the control arm bolts to specification while the full vehicle weight is on the rear axle. This sets neutral bushing tension at normal curb height.
- When there is a driveline working angle adjustment, adjust the angle before torquing the control arm bolts.
- After the rear suspension has been serviced, always check the working angle of the universal joints on the drive shaft. This minimizes the possibility of driveline vibration.
- Some independent rear-suspension systems have ball joints that perform a function similar to the front ball joints, and they should be inspected in the same way.
- Although very few cars have rear wheels that steer, some independent suspension systems have components that would normally be seen only on vehicles with four-wheel-steering.
- Components such as tie-rod ends may appear to serve the same purpose as if they were on the front suspension.
- They are used to adjust the angle of the wheels for stable straight ahead performance.