Since we have so many threads about different suspension geometries and the discussion is scattered and off-topic in a lot of them, I've started this thread to discuss the analysis of various suspension geometries and their differences. Please avoid off-topic posts here.
I want to focus on the C4 suspensions, stock tri5 suspensions, and a stock suspension with dropped spindles or cut springs, and taller balljoints. The claim has been made in the past that a stock suspension can be made to act and perform like a C4 suspension so we've been debating that for a long time. I want to prove or dispel any statements made to that effect. Clearly the stock tri5 suspension has deficiencies and it can be made better.
During this discussion I want to define "camber gain" as negative camber gain to avoid confusion. I want to define "spindle height" as the distance between balljoint centerlines. "A-arm length" is the distance from the center of the shaft to the center of the a-arm. A-arm "pivot" means the center of the a-arm shaft. "Level" a-arm means the a-arm pivot and balljoint are at the same height.
Early C4 suspensions are '84-87 and late are '88-96. The suspension geometries, both front and rear, were changed in 1988. Upper and lower a-arms and spindles (knuckles) are different in the front. The late front is about 1" wider than the early front. In the rear, the location of the strut rods at the inboard attach point was lowered in 1988. All link lengths are the same in the rear, early and late. The caliper mount was changed on the late suspensions making them a bit wider. The late rear is about 1" wider than the early rear.
To start this off, I decided to take some fairly careful measurements off of a tri5 clip I had cut off of a frame to do a C4 conversions. This allowed me to take things apart so I could take careful measurements. I even cut the balljoints apart to measure them so there would be no question of where the centers were, and it verified prior measurements.
Here's what I found, and should be the basis for any calculations or CAD work to discuss these suspensions. If someone has what they think is better measurements, feel free to prove it.
Stock Tri5 suspension measurements:
Upper a-arm shaft mounting width 29 3/4" front
Upper a-arm shaft mounting width 29" rear
Upper a-arm shaft centerlines (no shims) 28 3/8" (average front and rear)
Lower a-arm spacing 20 1/2"
Vertical spacing between upper and lower a-arm shafts, 10 1/2" front, 10" rear (at upper a-arm mounting holes)
Vertical spacing between upper and lower a-arm shafts, 10 1/4" (average at center)
Upper to lower balljoint centers 9 3/4"
Lower a-arm length 14 3/4"
Upper a-arm length 10 1/4"
Since the front and rear of the upper a-arms are not the same width or height, we should take an average of the dimensions as shown.
So with this information, we can lay out the stock suspension and analyze it. Right off the bat we can see that with the lower a-arm level, the upper a-arm is pointing down at the outboard end and is lower than the upper a-arm pivot by 1/2". We can also see that the upper a-arm pivot is 5" outboard of the lower a-arm pivot which is further than what has previously been reported here.
The question remains as to where "ride height" is as it pertains to a stock suspension, specifically the angle of the lower a-arm. There is a lot of information on the internet that indicates a lower a-arm should be level or slightly down at the outboard end.. The GM assembly manual shows the inner pivot to be at the same level as the lower balljoint, and with the a-arm level. Clearly to avoid tire scrub during suspension movement, the movement should center around a level a-arm. In fact, a level a-arm with some instantaneous negative camber gain would counteract that scrub as the suspension is compressed.
I'm assuming most of the discussion will be around static geometry, but you can infer dynamic effects from that to some extent. If anyone has suspension analysis software and can participate in this discussion, that would be awesome. So let's get to it.
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Originally Posted by Rick_L
. The stock SAI is specified at 3 1/2 to 4 1/2 degrees. So something was wrong with one or more of the dimensions above. 
I did notice the shaft centerlines are NOT centered between the bolt holes. 
measurements again on the C4 suspension dimensions. I actually dissected an upper balljoint to get the spindle lengths...I assumed the lower balljoint was the same length since I didn't have one to dissect. The bare spindles are 9 5/8" tall for the early suspensions, and 11 1/8" tall for the late suspensions (they're probably metric). I measured the upper balljoint center at about 1 1/8" above the spindle. I also re-measured the pivot centers as carefully as I could. The upper pivot center measurement is off of my recently assembled Nomad frame. I measured the shaft centers in front and back, and averaged them to get the center distance. The upper to lower measurement was taken off of my frame too, using the floor as the datum and measuring up to the center of the upper a-arm shaft (averaging front and rear measurements) and measuring from the floor to the lower pivot holes. I put rods across the a-arm shaft holes on an early and late k-member and found the difference in height to be exactly 3/4", which is what I measured before.