| An article in the November 2005 TechLink explained how to measure truck front suspension Z height, using the SI procedure and specifications. It explaned how to use a carpenter's level to extend the ball joint reference plane directly underneath the appropriate control arm attachment bolt for accurate measurement. That procedure is still valid for most current and older trucks. Always consult SI for appropriate procedures and specifications for the vehicle you are working on. |
A new procedure is now available for measuring Z height on the Colorado, Canyon, Hummer H3 and 2007 full-size trucks. It should be used for these vehicles as well as future products, as indicated in the appropriate vehicle Trim Height Inspection Procedure in SI for those vehicles.
One of the required preliminary measurements before performing a front end alignment is checking the front suspension Z height. This height setting is critical for proper camber, caster and toe settings as well as overall vehicle ride height.
TIP:Â In addition to alignment issues, extremely incorrect Z height can contribute to a third order wheel tire vibration on vehicles equipped with front drive axles.
In Trim Height Inspection Procedures, SI provides a specification from the center of one of the lower control arm attaching bolts to an imaginary plane extending from the bottom edge of the steering knuckle.
Here's a method of measuring this dimension. It will give accurate results using a 4 footÂ
(1.2 m) carpenter's level and a tape measure.
Prepare the vehicle:
- Sitting on an alignment rack
- Suspension fully supporting the vehicle
- Pins out of the pivot pads
- Tire pressure adjusted to certification label specification
- Doors, hood, trunk all closed
- Full tank of fuel
TIP:Â It is necessary to measure Z height on each side of the vehicle. Do one side, then repeat the procedure for the other side.
Jounce the front and rear suspension to obtain at least 1.5-inch (3.8 cm) deflection and allow the vehicle to settle.
Refer to (figure 22). Set the top edge of the level on the reference surface of the steering knuckle (C) and extend the level directly under the rear end of the front attachment bolt of the lower control arm (A). While keeping the level in contact with the knuckle reference surface, adjust the level up/down until the bubble indicates it is horizontally level. Then extend the tape measure straight down from the center of the attachment bolt. The point where the tape measure and level intersect is your first measurement.
Again, refer to (figure 22). Now keep the top edge of the level on the reference surface of the steering knuckle (C) and move the level directly under the front end of the rear attachment bolt of the lower control arm (B). Repeat the measurement as described earlier.
Average the two readings (add A and B together, then divide by 2) and write down the result. This is the vehicle's Z height. Compare this number with the published Z height specification.
TIP:Â You can obtain statistically greater accuracy if you repeat the procedure described above a total of 3 times and average the results (add them together, then divide by 3). The result is the vehicle's Z height. Compare this number with the published Z height specification.
TIP:Â It may be necessary to cut a notch approximately 3 inches long by 1 1/2-inch deep (7.6 cm by 3.8 cm) in one end of the level to clear the wheel rim and still maintain contact with the level working surface against the steering knuckle reference surface.
- Thanks to Dan Stress and Ken Peacock
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