Diagnosing bent steering knuckles using alignment data

Jan. 1, 2020
An alignment angle diagnostic (AAD) chart can be used to diagnose camber problems.
When performing steering angle diagnostics, it's not easy to verify if steering knuckles are damaged due to their size and design. Steering knuckles are heavy-cast parts made of steel or aluminum. They are not repairable and should be replaced if damaged.

A damaged steering knuckle could be discovered anytime during the collision repair process. Many times a damaged knuckle escapes replacement up to the point of the vehicle getting an alignment. We'll examine this situation.

This article looks at the alignment data for a vehicle with a strut-type suspension. While trying to adjust camber on the left-front wheel, the suspension cannot be adjusted enough to move camber to the specified value. Several problems could cause this so more diagnostics are needed.

Here are steps to diagnose the problem using an alignment angle diagnostic (AAD) chart and alignment printout data. The AAD chart uses the steering axis inclination (SAI), camber and included angle measurements to narrow down the possible list of causes for an alignment problem. To use the AAD chart, locate the correct suspension type. Then look at the actual (before) SAI measurement on the alignment report. Determine if SAI is greater than, less than or equal to the specified angle. If the specification and actual angle are 12.7 degrees, mark the AAD chart in the multiple areas that indicate correct SAI.

Next, do the same for camber, and make marks on the AAD chart that correspond with the condition of the camber angle. Let's say the specification for camber is -1.2 degrees, and the actual measurement is -0.2 degrees. This makes the camber greater than specification. Mark the AAD chart in the multiple areas that indicate greater camber.

Lastly, compare the actual included angle reading to the specified included angle, and make a check on the AAD chart that corresponds with the current position of the included angle. Let's say the actual included angle is 12.5 degrees, which is greater than the specification of 11.5 degrees. Mark the AAD chart in the multiple areas that indicate greater included angle.

Looking at the AAD chart for a MacPherson strut suspension, in the row where SAI is correct and camber and included angle are greater, the listed possible causes are a bent strut and/or a bent knuckle.

To determine which part is bent, additional diagnostics are needed. A visual inspection may help uncover signs of damage. Look for signs of flaking coatings or corrosion on the steering knuckle with an LED flashlight. When a cast part is bent, it may disturb and loosen any coatings or corrosion that have built up on the part. Minor damage may not show visible signs and will require measuring.

Measurement checks

Typically, there is no measuring data available for steering knuckles or struts. Therefore, measuring quick checks used to identify damage to these parts must be done comparatively to a known good part, such as the opposite side of the vehicle.

To test the strut cartridge for damage, a straightedge may be placed along the side of the strut cartridge to look for areas where there are gaps between the straightedge and the strut. Another test is to measure from a straightedge placed on the hub face to symmetrical locations on the strut. When measuring the strut in this manner, make sure struts that have camber adjustments (where the strut and steering knuckle are connected) are adjusted to the same position on the left and right side of the vehicle.

The strut also can be tested by doing a strut-rotation quick-check. To do this, loosen the lock nut at the upper strut bearing one-half to one full turn. Don't completely remove the lock nut. Then, using a wrench or locking pliers, rotate the strut rod shaft a minimum of 360 degrees while observing the top of the tire for in-and-out movement or camber change. Camber changes as the strut rod is rotated indicate a bent strut rod shaft.

Steering knuckle measuring quick-checks are done very similarly to those for a strut cartridge. A straightedge is placed against the hub or brake rotor and measurements are made from the straightedge to points on the steering knuckle. If the brake rotor is used, ensure that it is held tightly to the hub with lug nuts. Measure to at least two different points, one where the steering knuckle and strut are connected. Differences in the measurements between the part being tested to a known good part indicate a bent steering knuckle.

Though not identified by SAI, included angle, or camber, the steering arm on the knuckle should also be measured to ensure it is not bent. To do this, the steering arm distance can be measured between the end of the steering arm, where the tie rod connects, and a symmetrical part attached to the knuckle, like the axle joint or bearing assembly. The steering arm position also can be measured to a straightedge placed against the hub or rotor. This measurement helps determine if the steering arm is moved in or out in relationship to its intended location. Also, the steering arm should be measured to a symmetrical location on the upper or lower control arm. This measurement checks for vertical movement in the steering arm.

When measuring a steering knuckle or a steering arm for movement toward or away from the vehicle centerline, where the straightedge is placed is critical. Placing the straightedge on the face of the hub, or against the rotor may not provide a true measurement if the rotor or hub assembly is damaged. Ensure that parts being used as a measuring reference are not damaged.

If the diameter of the rotor or dust shield impedes access to the steering arm, a small tram bar can be used to measure around obstacles or the steering arm can be measured with the straightedge placed on the knuckle front if the rotor, hub and bearing assembly are removed.

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