How Car Control UnderArms works

[DMSG]

WHY YOU NEED TO REPLACE YOUR WORN UNDERARMS

Although not a common repair, there are times when it is necessary to replace a car’s control arm. This is an important structural part of the vehicle’s front suspension, attaching the wheel hub and steering knuckle assembly to the vehicle’s frame.

Many vehicles can go through their entire life without ever having a control arm replaced. However, some situations necessitate the control arm’s replacement. There are three basic reasons why a control arm might need to be replaced:

 

If the control arm bushings are worn, they will allow the control arm to move out and away from the vehicle’s frame when the wheel hub is moved in and out. This may be a very small movement, but there should be no movement at all.

While checking, also look at the steering tie rod end, where it attaches to the steering knuckle. Just like the ball joint, there should be no side-to-side movement. Wear in the tie rod end can cause problems with steering, although it will not create any thumping sounds.

The sway bar also connects to the control arm, but this is mounted with a stud and bushing arrangement that is intended to allow flexibility. The sway bar deals with upward and downward movement, so any movement in it is immaterial.

Car Front-End Anatomy

The front end of a car has many parts, which can be confusing to those who are not used to working on them. Three different systems are in place, all of which have to work together and pass through one another. These are the suspension system, the steering system, and the drivetrain system.

Steering System

The steering system does just what it sounds like it does: helps the driver steer the vehicle. When looking at the front of a vehicle with the wheel removed, the only part that is easily visible is the tie rod. This will connect to an arm sticking out from the steering knuckle and wheel hub. The connection is through a small ball joint. This connection should not be removed while replacing the control joint. If it is removed, it must be replaced in exactly the same position, or the vehicle alignment will be off.

Drivetrain System

The drivetrain consists of the engine, transmission, and drive shafts. On a rear-wheel drive vehicle, this is not visible from the front wheels. On a front-wheel drive vehicle, the driveshaft goes from the transmission to the front wheel hub. In the midst of it, there is a constant velocity joint (CV joint) that allows it to be flexible. Check the rubber boot over this joint to ensure that it is not turned.

Suspension System

Everything else that is visible in the front wheel area is part of the vehicle suspension. The most visible part is the wheel hub and steering knuckle assembly, which is what the wheel was mounted to. The control arm is attached to the bottom of this assembly, which goes back to the vehicle frame. On vehicles that have McPherson struts, the struts are attached to the upper part of the wheel hub and go up to the top of the engine compartment. On vehicles with shocks and springs, there is a second control arm, which is attached to the top of the wheel hub and the vehicle’s structure. The last important component is the sway bar, which crosses the vehicle, connecting one control arm to the other.

Preparing the Vehicle for Removing the Control Arm

Replacing the control arm requires getting under the vehicle. Therefore, it is extremely important that the vehicle is properly raised and supported. Failure to properly support the vehicle can allow it to fall while working under it.

The vehicle must be on a flat surface (not on an incline) that is hard enough to support a jack and jack standwithout them sinking into the surface. A concrete driveway is best. If the work is being done over dirt, the dirt needs to be dry and well compacted so that the jack stands cannot sink into it. A 1-foot square piece of plywood under the jack stands can help prevent any sinking.

Break loose the wheel lug nuts before jacking up the vehicle, but do not remove them. Block the rear wheels before jacking. This can be done with a stone, a scrap piece of dimensional lumber, or wheel chocks. The block or chock should be placed tightly behind the wheel so that the car cannot roll backward.

Using a floor jack or scissors jack, raise the vehicle high enough to allow working under it. Be sure to locate the jack on a frame member, or in the case of a car with a unibody construction, at a jacking point where there are several thicknesses of metal together to support the jack. Most modern cars have reinforced jacking points located right behind the front wheel wells for this purpose.

With the vehicle raised, the lug nuts can be removed and the wheel taken off. Place a jack stand under the vehicle, at a point where the frame or unibody construction is sufficiently strong to support the weight of the vehicle. Most cars, even those with unibody construction, have a structural member for the control arm to connect to. This is an excellent location for a jack stand, as long as the stand can be placed in such a way as to not interfere with removing the control arm.

Please note that it is not safe to work under a vehicle that is only supported by a jack. Hydraulic floor jacks bleed off their pressure through time, slowly dropping the vehicle. Scissor jacks and bottle jacks are unstable. If the car is bumped or tries to roll slightly, it can fall off these types of jacks.

Facing worn arms or unsure, make a trip to DM and we will help you to diagnose. Cheers!

Edited March 18, 2015 by DMSG