I get alot of PM's, and I have seen this come up a quite few times in threads.
About a year ago, there was quite a bit of experimentation done with methods for lowering the rears of our cars. There weren't any known, inexpensive, g-body application specific springs that would lower the rear any more than 1" or so. The proposed solutions that come to mind were cutting IROC cargo coils, and using QA1 coil-over springs, without the central strut. There are inherent problems with both of these methods.
The stock rear springs for the G-body are "pigtailed" on both ends. This means that the spring diameter reduces from roughly 8-9" to about 3-4", and the end slip over the spring seat on the frame and on the rear end axle tube. On the upper spring perch, however, there is a "pocket" that will also support a "box" end spring, albeit this was not the intended design. Pigtailed ends (both top and bottom)Tangential on bottom, boxed on top
By cutting a cargo coil or any other spring, you end up with not a pigtail, not a box end, but a tangential end. This leaves a very small area for the spring to contact the upper spring "perch" (it's not really even a perch). The entire corner weight must be transfered through that small contact area, causing high stresses on the frame, and possibly damage. This can be somewhat mitigated with a spring isolator, but the isolator will take quite a bit of abuse because of the stresses. If you want to see an application that the tangential end spring was designed for, take a look at the lower control arms up front. They have the tangential pattern stamped into them, which is why the spring has to be "clocked" correctly.
Another problem associated with cutting the rear springs is that the spring can come unseated rather easily. Again, the frame is not designed for any type of spring other than the pigtailed one. So any type of spirited driving could potentially dislodge the spring and then you have big problems. I was able to unseat a cut IROC cargo rear spring during an autocross run once, and I was VERY lucky that I didn't do any further damage.
Cutting springs also increases the springs "rate" which is a measure of the springs stiffness. The foce (in pounds) required to compress the spring 1 inch is it's rate. The spring rate, and more specifically the distribution of spring rate from front to rear, plays a prominent role in determing how the car handles. So if you cut cargo coils (you will have to cut them ALOT to get down to even a 2-3" drop) then you will end up with very stiff rear springs. The cargo coils are already stiff to begin with, you can see how thick the wire is that is used to make them. The long and the short of it is, you end up with a car that tends to oversteer. It is "loose" in stock car terms, or basically, the rear of the car will lose traction first in steady state cornering. (Search "Roll Stiffness" for more information.) This becomes hard to control at times (think emergency lane change maneauvers) and is not a good thing for racing... especially with V8 torque. Again, I know this from experience. Overall, cut springs are a very bad comprimise for the money saved, and there are much better solutions.
Some have tried running the QA1 coilover springs, as they can be ordered with many different spring rates and dimensions. They fit the pigtail, from what I have read on this forum, and they can provide the ride-height drop that many desire. However, as pointed out by many of the guru's on this site that are much smarter and experience than I, they are designed for coilovers, and as such, using them without the central supporting strut can cause problems. The length to diameter ratio of the spring is very large, which lends well to buckling. Just for understanding's sake, imagine you had a spring that was 1" in diameter, and 2 feet long. Trying to compress it would likely cause it to fold up rather than compress uniaxially. The coilover springs are obviously not this bad of a case, but they are getting to the point where they could potentially buckle instead of functioning properly. I have personally never tried running these springs, so I cannot comment on how they work in practice under all conditions, or how the car handles with them.
Finally, there are the springs that are pigtailed on both ends, and provide a 2 or 3" drop. OPG offers the only 2+" drop springs that I know of that are marketed as "G-body springs", but they cost $170/pair. There is one other option that I know of...
Classic Performance Products sells springs that are marketed for '67-72 A-bodies, and they advertise a 2 or 3" drop. When these springs are used in our cars, the "2 inch" springs provide an actual 3 or 3.5" drop, and the "3 inch" springs are far too short for the G body. The best part is that they sell for right around $100 shipped. They are pigtailed on both ends. Many other companies also sell A body springs, Belltech sells 1.5" drop springs for the G-body, and between all of the manufacturers, you are bound to find a spring that puts you where you want to be, safely.
Now that we know of springs that will drop the ride height to where you want it, there are some other things to consider... at least for drops of around 3+ inches.
The stock frame side bumpstops will begin contacting on a regular basis. I have removed mine completely, but left the pinion snubber.
The big problem with droping the rear 3+ inches is the suspension link geometry. The springs only locate the rear end in the car vertically. The upper and lower trailing arms are left to do the work of locating the rear laterally and longitudinally (left/right and fore/aft). The lower the car is, the more the trailing arms are pointed downward on the front (frame) side and upward on the rear (axle) side.
As you would expect, the trailing arms pivot about their frame mounts, and the result is that the axle mount travels in an arc path. At stock height, the arms are parallel to the ground. When the car makes a turn, the body rolls towards the outside of the turn. The outside spring goes into compression, and the spring on side of the car that is on the inside of the turn goes into droop. So the outside trailing arm travels upwards in its arc, and the inside one goes down. If the arms are parallel to begin with, there is no problem because the axle mounts are still the same distance from the rear end.
With a severely lowered car, and no further modifications to correct suspension geometry, the trailing arms are not parallel to the ground. When the body rolls during a turn, the outside arm travels further
up in it's arch, and the inside arm travels down in its arch, ending up closer to parallel with the ground. This means that the outside arm pulls the rear end further foward, and the inside arm pushes the rear end further back towards the rear of the car. The rear ends up actually steering the car, as it is no longer perpendicular with the centerline of the car. This is called "rollsteer", and it is not something that you generally want, at least not in sever amounts. (See picture- The wheel would travel down with the axle side of the control arm, illsutrated by the heavy grey line above and below the heavy black line. The black line is where the control arm is at static ride height, and the grey lines illustrate the inner/outter arms under body roll. Upper trailing arms have been omitted for simplicity, but the concept is the same.)
There are relocation brackets that will lower the axle side mount of the lower trailing arm, and get it pretty close to parallel with the ground. I am not aware of any solution for the upper trailing arms though. I have the BMR lower trailing arm relocation brackets, and they help, but there is still a very noticable amount of rollsteer in the rear of my car (~3.5" drop). It is enough to make me very cautious when entering and exiting turns. I still have to solve this issue, but I think it is going to entail relocating the frame-side mounts for the upper arms.
Finally... there is the issue of pinion angle. Because of the arcs that the arms travel on, and the fact that the upper arms are shorter than the lower ones (and because they are angled in, further reducing the longitudinal distance between the frame mount and the axle mount) the pinion angle becomes more severe, the lower you go. Adjustable upper trailing arms are needed to remedy this, and with stock arms and a 3.5" drop, the driveline vibrations were absolutely unbearable on the highway. I have the Currie Enterprises Curectrac adjustable upper arms with the johnny joints, and so far, so good. I have also noticed that by using the BMR reloation brackets, and adjusting my upper arms to be longer (to account for the pinion angle) that the rear axle actually sits further back in the wheel well. It looks a bit funny, and I'm sure that it hurts the corner weight distribution a little bit by making the rear axle a hair lighter than it used to be. I even went as far as putting a longer yoke on my driveshaft just to make sure that it was engaged well enough.
I hope that this sheds some light on lowering the ride height on the G-body. As with most mechanical systems, you cannot simply alter one aspect and expect that it will not affect something else.