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When performing steering angle diagnostics, it is
not easy to verify if a steering knuckle
[sometimes also called a hub carrier]
is damaged. Due to their size and design, even when they are bent, it is
typically not obvious. Steering knuckles are heavy cast parts made of steel or
aluminium. They are not repairable and should be replaced if damaged.
Reading Alignment Data
For purposes of this article, we are looking 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. There are a number of problems that could
cause this condition, so further diagnostics are required to identify what is
damaged.
We will take you through the actual steps of
diagnosing this problem using an Alignment Angle Diagnostic
chart (see below) and the alignment printout data.
The Alignment Angle Diagnostic chart uses these
measurements to narrow down the possible list of causes for an alignment
problem:
To use the Alignment Angle Diagnostic chart, first
start by locating the correct suspension type. Then look at the actual (Before)
SAI measurement on the alignment report. Determine if SAI is greater than the
specified angle, less than the specified angle, or the correct angle. In this
example, the specification is 12.7 degrees and the actual angle is 12.7 degrees.
Therefore, SAI is correct (green boxes above).
Next, do the same for camber, and make marks on
the Alignment Angle Diagnostic chart that corresponds with the condition of the
camber angle. In this example, the specification for camber is -1.2 degrees, and
the actual measurement is -0.2 degrees. This makes the camber greater than
specification. Therefore, mark the Alignment Angle Diagnostic chart in the
multiple areas that indicate Greater Camber (red boxes above).
Lastly, compare the actual included angle reading
to the specified included angle, and make a check on the Alignment Angle
Diagnostic chart that corresponds with the current position of the included
angle. In this example, the actual included angle is 12.5 degrees which is
greater than the specification of 11.5 degrees. Therefore, mark the Alignment
Angle Diagnostic chart in the multiple areas that indicate Greater Included
Angle (yellow boxes above).
Looking at the Alignment Angle Diagnostic 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.
Visual Inspection
To determine which of these parts is bent,
additional diagnostics are needed. A detailed visual inspection may help uncover
signs of damage and should be the next step in the diagnostics process. One way
to look for signs of damage may include using an LED flashlight to look for
signs of flaking coatings or corrosion on the steering knuckle. Some people
prefer the use of an LED flashlight because of the blue-colored light that the
LED bulbs emit. When a cast part is bent, it may disturb and loosen any coatings
or corrosion that has built up on the part if the degree of the bend is severe
enough. Minor damage, however, may not show any visible signs and will require
measuring to positively identify.
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, and more positive, test that can be done
to test for a bent strut cartridge 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 that 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 can also 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. Do not completely remove the lock nut. Then
using a wrench or locking pliers, rotate the strut rod shaft a minimum of 360º
while observing the top of the tyre for in-and-out movement or camber change.
Changes in camber as the strut rod is rotated indicate a bent strut rod
shaft.
Steering knuckle measuring quick-checks are done
very similar 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 can also 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 towards or away from the vehicle centreline, 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. In some situations, the diameter of the rotor or the dust shield
may impede 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 being
placed on the front of the knuckle if the rotor and hub and bearing assembly are
removed.
Conclusion
When trying to determine if a steering knuckle is
bent, multiple steps may be needed to either rule out or confirm whether the
part needs replacement. Keep in mind that replacement is key. Steering knuckles
are made of cast steel or cast aluminium, neither which are repairable or should
be heated. If the part is damaged, replacement is the only option.
