Due to popular demand I'm reposting this.
I'll leave it open for your comments and conversations with each other. I'm out after this first post.
If your Jeep is pictured, and you don't like it. Find a picture to replace it fulfilling the purpose of the picture.
It's frequently discussed on here about why go to a long arm suspension.
I'll try and explain what I know.
The stock TJ suspension is extremely well designed from the factory. It articulates well as is. Giving it a little lift (2-3") it'll still function quite well. When you start increasing the lift much over 4", the angle of the control arms suffers. This is because of the short length. Not only does the axle move up and down. It moves front to back and side to side thanks to the arms and track bar. The short arms allow the axle to move around the circumference of a 32" (roughly) circle. This is because of the 16"(roughly) control arms. and the pivot point at the frame. As the axle goes up and down the track bar is rotating on it's 32" (ish) axis and pulling the axle side to side. As you increase suspension lift. The axle is pushed further away from the frame and further down the arc of the 16" control arm.
Here's a pic of a Jeep on stock length arms and about 4" of lift. See how the tires are pulled together.
This pic shows how the axle pulls back at full droop. Look at the control arms and curve to the spring.
Those were both short arm suspensions. With a long arm, you're increasing the radius of the circle the axle travels. This makes lift height changes impact the control arm angles less severe. Here's an assumption to help explain.
Lets say a 4" lift will push a 16" short arm down (inverse of lift) 2". Then a 4" lift will push a 32" long arm down 1". Engineers, I'm trying to K.I.S.S. here, don't take me to task on my math.
So changing to LA's will soften the ride at an equal ride height to SA's. And it will allow a larger amount of lift then SA's. let's face it the SA's run out of travel quicker than the LA's do.
Here's a great pic from Superlift of an entry level LA suspension. This system is similar to Rusty's and Rough Country's.
There are entry level LA's that only replace the lowers with a LA and keep the uppers SA. yes it will help the ride on the street and light trail riding. But it also causes the pinion to rotate down during heavy flex. This is because of the two different arcs the control arms are following. Watch the yellow line. It represents the pinion angle.
Thanks to OmanJeep for the excellent graphic of the axle travel.
They're pretty mandatory for the front. Mostly because we have an engine to work the suspension around. For the rear, they are totally useless.
In the rear we can use a single or double triangulated 4 link set up. This will remove the track bar and allow the axle to pivot and move more freely.
This is a single triangulated 4 link. See how the uppers go from frame to center of the axle, and the lowers are parallel with the frame.
This is a double triangulated 4 link rear. The lowers go from the outer ends of the axle to the center under the transfer case. The uppers go from the frame to the center of the axle.
Neither rear 4 link systems use track bars. This is because the angle of one of the sets of control arms locate the axle side to side. There is a characteristic of rear suspension called rear steer. This happens at full flex when one side of the axle is forward of the other. It's not a great amount of steering but to some it's noticeable. A 4 link minimizes this and a double triangulated 4 link nearly negates it.
Tri Link systems using a single upper center point follow the idea of a 4 link.
Front long arms have historically been more difficult to build around the engine. People have used standard 4 links just to find the ride height is too tall due to clearance of the oil pan. People have used inverted 4 links with the uppers parallel and the lowers converging at the diff. This help to lower the truck and keep good geometry.
3 Link fronts are becoming increasing popular. These use 2 long arms on the diff side and one long lower on the non diff side. Then a track bar for lateral control.
Radius arm or Y-link fronts use a long lower on each side. The uppers are connected to the lower arms. Still retaining the track bar for lateral control. This allows the pinion angle to track with the lower arms. But it'll still travel side to side.
Greg Volkamn brings up a good point I didn't address well, control arm angles.
On the front axle more than the rear. The steeper the angle the more road harshness is transmitted to the truck. This is due to the angle of the control arm. In a perfect world the arms would ride at about a 10deg down angle to the axle. This way when the tire hits a bump and, the axle is pushed back and up. The springs then absorb all the energy. As you lift the Jeep upward, the control arm angle increases from 10deg to 20, 30 or more degrees. Think of it like your jaw. When your mouth is slightly open and you get a glancing blow to the lower edge. It's going to close easily. If your Jaw were wide open and you took the same hit. Your jaw would try to open more.
Well with our front arms this happens at higher lifts. The force of a speed bump, instead of being absorbed by the springs, gets sent up the control arm. The only dampening then is the rubber bushings at each end of the control arm. This also puts severe strain on the bolts and brackets. Take a look at Jeeps with 6"+ of SA lift. Most have wallowed out bolt holes in the frame brackets.