Control Arm Information

Purpose of Control Arms

To connect the axles to the frame
- On a JK there are four controls arms per axle for a total of eight arms
  - Two upper arms per axle (one driver side, one passenger side)
  - Two lower arms per axle (one drive side, one passenger side)
To provide longitudinal (up and down) axle control by pivoting at both mounting points (axle and frame)
To work in coordination with the shocks and coils springs to control the axles
To prevent forward and backward movement of the axles
To determine wheelbase by setting position of the axles within the wheel wells
To manage the caster (front only) and pinion angle (both) of the axles

Note: Control arms do not provide lateral control of the axles. Lateral control of the axles is provided by the Track Bars.

Types of Control Arms

1) Fixed Length Control Arms

Fixed Length Control Arms

The ends of the control arms are fixed to the control arm tube and therefore the overall length of the control arm is “fixed” based on the manufacturer’s specifications

Adjustable Length Control Arms

The ends of the control arms (either one or both) are adjustable where they can be rotated to allow the control arm to be lengthened or shortened within the manufacturer’s specifications.
Through their adjustability, they provide the ability to properly calibrate caster and pinion angles of the axles if they become affected by adding a lift kit to the Jeep
They also provide the ability to re-position the axles (providing both upper and lower are installed) in cases were the wheelbase may have been shortened due to the installation of a lift kit.

Factory Control Arm Lengths for a JK

· Front Upper – 18 13/16”

· Front Lower – 22 5/8”

· Rear Upper – 17 7/16”

· Rear Lower – 19 3/4”

Why Do I Need New Control Arms When Installing a Lift Kit?

Since stock control arms are a fixed length, when a jeep is lifted, the axles will 1) move toward the center of the vehicle, 2) rotate upward increasing the pinion angle and 3) on the front axle, the upward rotation will decrease the caster angle. As a general rule of thumb, for every inch of lift to your JK you can decrease the caster angle by as much as a half a degree. For example, if you put a 4” lift on your JK your caster angle may be decreased by approximately 2 degrees. This would put your caster angle at 2.2 and your pinion angle at 3.8. For sure your drivability will be affected.

Properly Reestablishing Wheelbase after a Lift

In the case of moving the axles forward or backward to reestablish wheelbase the conventional wisdom is to lengthen both upper and lower arms to set the axle location and then use the upper control arms to set the caster and pinion angles. The reality is that setting caster and pinion angles can be done by adjusting either uppers or lowers based on their direct relationship. Uppers will move the top of the axle forward or backward and lowers will do the same for the bottom of the axle. So if you want to increase the caster you can either make the upper control arms shorter or the lower control arms longer.