What is a control arm and how does it work?
A control arm is a suspension component, usually made of heavy gauge steel or aluminum, that links the steering knuckle in the front — or axle carrier in the rear — to the frame of the vehicle. Many vehicles have upper and lower control arms. The inboard link(s) of the arm securely bolt to the frame of the vehicle through rubber bushings, while the outboard link of the control arm connects to the steering knuckle through a ball joint; all of which allow the control of the up and down movement of the steering knuckle or axle carrier. This minimizes the transmission of shock and vibration to the vehicle’s body.
When to consider replacing a control arm:
The structural portion of the control arm can last the life of the vehicle — except for cases of corrosion or crash damage. However, the rubber bushings and the ball joint in a control arm have a limited life. A mechanic should investigate if you notice:
- Wheel alignment difficulties. Should the bushings, or the ball joint, in a control arm be worn, it will be impossible to properly align the vehicle. The worn components, or the entire arm, must be replaced.
- Vehicle won’t track straight or pulls to one side. A car will not track straight and may require constant steering correction if there is a worn, bent, or loose control arm.
- Clunking or squeaking noise. A loose control arm, or a control arm with dried out, rotted, or loose bushings, may move enough to cause detectable noise from the suspension as you go over bumps in the road.
- Vehicle pulls to one side only when braking. Worn bushings or ball joints can allow control arm to shift when braking, causing the vehicle to pull to one side. Although, this could also be caused by brake calipers.
How do mechanics replace a control arm?
- The vehicle is raised and supported on steel jack stands, and the wheel and tire assembly is removed.
- Stabilizer bar links are detached from the control arm, if applicable. ABS wiring to the wheel speed sensor, if routed along the arm, is disconnected.
- Control arm mounting bolts are detached from the frame and from the axle carrier if the control arm is in the rear.
- For front control arms connected to the steering knuckle via a ball joint, the nut is removed from the ball joint stud and the stud is pushed through the steering knuckle to free the arm.
- The arm is lowered from the vehicle and examined by the mechanic to confirm that the noise, or other problem, actually originated from a defect in the arm.
- The new arm is installed using new mounting bolts if required by the service manual. In many cases, the fasteners must be tightened with the vehicle weight on the suspension in order to ensure the bushings are in a neutral, or resting, position.
- The vehicle is lowered and road tested to confirm the problem is resolved. Replacing a control arm will change the vehicle alignment settings and the mechanic will refer you to a professional alignment shop to have the vehicle alignment set to factory specifications.
Is it safe to drive with a control arm problem?
No. If the car wanders on the road, pulls to one side, or noise from the suspension is loud, you need to schedule repair right away. Should the problem be limited to low levels of noise, such as clunking or squeaking, continued use of the car is reasonable until you can schedule a repair. Should the control arms be damaged as a result of a collision, it would be unsafe to drive the car until it is repaired.
When replacing a control arm keep in mind:
- The number of control arm types vary from vehicle to vehicle depending on the design of the vehicle suspension.
- Control arms should be replaced in pairs — arms on both sides of a front or rear axle — if the reason for replacement is worn control arm bushings or a worn ball joint.
- All other suspension components should be inspected when control arms are replaced because looseness, damage, or excessive wear in other parts of the suspension system will make it impossible to perform a wheel alignment after the control arms are replaced.