Spring Over Axle (SOA)
First off, I must give anyone reading this a disclaimer. SOAs are a huge change to your Jeep's suspension and steering. I do not recommend anyone performing an SOA conversion unless they fully understand the ramifications of this type of lift. I could tell you about how raising your center of gravity will increase the possibility of a roll over and other such safety rhetoric, but if you're far enough along to consider doing an SOA on your rig, you already know this.
This is only my experience with Wrangler (YJ) SOA conversions. Besides my own, I have participated in a few other SOA conversions. Most every YJ I wheel with, runs SOA on their YJ. I have never participated in an SOA conversion on any other vehicle. This is not to say they cannot be done, just that what you read here may or may not apply to your particular situation or vehicle. Before you get started, fabrication skills are a must for performing an SOA. In particular, you'll want a good weldor at your disposal.
Driveline Angles - The driveline angles are very critical part of setting up an SOA. On a YJ, the front is not as crucial since the YJ driveshaft is long enough, the u-joints do not have to operate at severe angles. But the rear driveshaft on a YJ is extremely short. Close attention must be given to this area to prevent vibrations and u-joint failures.
On the front perches, you'll simply want them welded parallel to the stock perches underneath. Two reasons, one the front is a standard driveshaft so the transfer case output shaft and the front axle pinion shaft should be parallel. The second reason is, changing the angle of the front perches will affect your caster. Caster is what causes your wheel to return to center after a turn and track in a straight direction. I've heard Rubicon Express recommends changing the caster for their SOA kits. I'd contact them for information. All the SOAs we've ever done kept the caster the same and we've never had a problem in this area.
On the rear perches, it will depend on your driveline setup. With a standard driveshaft, like the front driveshaft, the transfer case output shaft the pinion shaft should be parallel. While you can operate a standard driveshaft on an SOA YJ, it is not ideal nor recommended. Generally, a transfer case drop is used to lower the angle of the transfer case output shaft and the rear axle is shimmed about 2 degrees up to compensate for the change in the transfer case angle.
The best solution is to install a Slip Yoke Eliminator (SYE) in the transfer case and use a Constant Velocity (CV) driveshaft. When you run a CV driveshaft, you basically want the pinion shaft and driveshaft operating in the same angle (in practice, drop the pinion 2 degrees to compensate for the lifting of the pinion when torque is applied). On my first SOA setup, I used an SYE on the NP-231 and a CV. Today, I'm using an Atlas II which has essentially the same setup on the rear output. So I must essentially point my pinion at the CV. Below left is a picture of the NP-231 transfer case with the MIT SYE and the CV driveshaft. Below right is a picture of the pinion and driveshaft angle. Notice the pinion is pretty much inline with the driveshaft.
If you still don't understand driveline angles,
here's a write-up done by John Nutter that explains it pretty clearly.
Steering Geometry - Another problem to overcome with SOAs is the steering geometry. The angle of the draglink is of specific concern. When the draglink is positioned in severe angles it creates bumpsteer when going over bumps. When the suspension compresses and the axle moves up so does the axle end of the draglink. This in effect, increases the horizontal length of the draglink and pushes on the pitman arm. This translates to the steering wheel as movement. It can be a frightful experience at highway speeds. Although, some get used to it and adjust their driving style.
On my first SOA, I used a stock pitman from a ZJ. This is identical to a drop pitman for a YJ that a 4x4 shop will sell you (except about $30 cheaper). I also used a drop draglink from MIT. This is not the ideal setup. It works fairly well, but there are better solutions. The picture below is my old steering setup which is currently installed in my son's Jeep.
Another steering solution, if you're willing to spend the dough on your stock Dana 30 is from M.O.R.E. Basically the M.O.R.E. kit adds bracketry to the Dana 30 so you can mount the axle end of the draglink above the springs. This kit is pictured below. Not a cheap solution, but a very good one.
Now the ideal SOA steering setup is known as "high steering". This setup not only moves your draglink above the springs, as is the M.O.R.E. setup above, but it also moves the tie rod up over the top of the springs. Not really a steering geometry issue, but more an issue of clearance and protection for the tie rod. This is accomplished by adding steering arms on top of the knuckles to connect the steering. Below left is a close up underneath. Below right, you can see the tie rod is almost at no risk of being damaged due to its height.
Spring Pack - Stock springs, with an add-a-leaf was the first spring pack I used in my SOA. I used the stock front YJ spring packs (front and rear) with Rancho 2" full length add-a-leaves. The Rancho full length add-a-leaf is very stiff. This aids in preventing spring-wrap and also helps to fight premature spring sag which is also common among SOAs. I ran this pack for about 3 years, then gave the springs to my son and he is currently using them on his SOA YJ. He's had them about 2 years.
My friends have tried other add-a-leaves with varying degrees of success. National sells an add-a-leaf that's full length like the one from Rancho, but a much softer spring. MIT used them in their SOAs. None of us have had much luck with them because they're so soft they sag prematurely. The Procomp add-a-leaf just plain doesn't hold up. It's too short so it does nothing to fight axle wrap. A Jeep with us on the Rubicon broke both rear add-a-leaves on that single trip.
Another spring pack configuration I am aware of is stock springs with an additional stock spring added in the pack. The eyes of the longest spring are cut off and added to the pack. I mostly hear of premature sag with this configuration. Although I have to admit, I have one friend who has been running this configuration for a long time without issue.
Another option is to use lift springs. Generally 2-2.5" lift springs work well. I do know of people using 4" lift springs SOA. I myself have no experience using springs that tall on an SOA conversion. Currently I run custom 2" lift springs from Alcan. Very good springs. National now makes a spring especially designed for SOA conversions. These springs are arched more than a normal lift spring and the eyes are reversed to remove some of the lift from the arch. This arch will dramatically increase the life of the spring.
Perches - To help fight axle wrap, I used the RE anti-wrap perches in my first SOA. If you could get your hands on a set to look at them and measure them, they'd be real easy to fabricate instead of the $75 they'll cost you. The perches are very beefy and the fronts are offset so you can clear the pumpkin and axle disconnect housing. If you go into a 4x4 shop and ask for perches, they will give you an identical set to what you currently have on your rear axle. We have used these, but on the front axle you will have to grind the arch on one side of each perch to make them sit level on the axle because of the pumpkin and axle disconnect housing. Two the the SOA YJs I wheel with have even re-used the front perches. They cut them off and swing them up to the top. Better be real good with a torch if you plan on reusing perches.
Shocks - Naturally you want as much travel as possible, but you don't want to eliminate clearance under the axle. I cut off all of the stock lower shock mounts and fabricated my own shock mounts. Then they were welded as low as possible on the axle housing without hanging below the axle and taking away clearance. On the front shock towers, I installed the upper shock mount conversions from Rubicon Express (pictured below). These convert the top mounts to an eye instead of the stud that is stock on YJs. This allowed me to mount my shocks upside down. We've had a problem with some of the SOAs beating the shock canister on the springs during articulation.
Brakelines - I use extended brakelines and have re-routed the front lines under the frame. You must use very long brakelines in the rear. For the emergency brake you can buy extended lines (although I have no idea where) or you must re-route them. For the passenger side this is simple. Just disconnect the line from the mount near the exhaust, move it below the mount and that pretty much takes care of the passenger side. The drivers side doesn't have enough existing length or slack for anything that simple. But there is a little trick. Disconnect the driver side emergency brake line at the adjustment contraption. Drill a new, lower hole for the break line mount and reconnect it. I would recommend gusseting this bracket. I haven't and as you can see below, it is bending.
Probably one of the biggest problems with an SOA conversion is spring-wrap (a.k.a. axle-wrap). Spring-wrap is where the axle, torqued by the driveshaft, is attempting to rotate and twist the springs around it.
Normally the springs would prevent the axle from rotating and instead the tires would move forward or rearward. Spring-wrap can occur on SOA or Spring Under Axle (SUA) vehicles, but appears to be more prevelant on SOAs. One symptom of spring-wrap is wheel hop. There are a few ways to cure this problem. A good spring pack, anti-wrap perches or traction bars are a few. This doesn't seem to be real problem with 33" tires or less. Once you get to 35" tires or larger, a traction bar is just about mandatory.
Misc - Check everything going to your axle for adequate length. In particular the differential breather hoses and also the 4WD axle disconnect on the front axle. There is without a doubt, not enough slack in these lines if you do nothing. However, it's just a matter of a little re-routing to solve the problem.
Other sources of SOA info: