Lots of people wheel with them, they're stable, predictable, easy to setup, tried and true technology that's been borrowed from the horse and buggy days. Just because the technology is old, doesn't mean it's bad, so let's dive into leaf spring setups. How to do it correctly, how to make it flex well, ride well and design a setup that will perform well off road.
Without getting too vehicle specific, let's discuss some common upgrades to a "stock" leaf sprung setup.
1. Shackle Reversal. This is pretty self explanatory and done to the front springs. You take the swing shackle mount from front of the leaf spring and move it to the rear. Now the hard mount is up front. This is done for a couple of reasons.
a. The front mount of the leaf spring is now ridged, so when you ram it into a rock or trail obstacle, it doesn't move. With the swing shackle up front in a stock setup, the spring takes the hit and you could bend or brake the leaf spring.
b. When approaching a ledge and you crawl up the obstacle, the spring compresses. When the spring compresses, the axle moves in the direction of the swing shackle. If the shackle is up front, your axle is literally moving toward the obstacle you're trying to climb and binds the suspension. Once you move that swing shackle to the rear of the spring, the axle and tire moves back away from the obstacle and letting the vehicle climb up more naturally.
Is a shackle reversal right for you? The general rule of thumb is the reversal is for trail only applications and if your rig see's the street, you should probably leave the shackle up front for various handling issues at speed.
So, now you've decided if a shackle reversal is worth it in your case. Now before we jump into the actual leaf spring, we need to understand the shackle purpose, how angles and design effect ride quality spring rate, etc. The truth of the matter is you can have any shape shackle you'd like and it doesn't make a bit of difference. It can be straight, boomerang, S shaped, or shaped like a lighting bolt. The only thing that matters is the angle from mounting hole to mounting hole. The rest is all about packaging and preventing "shackle inversion" which i'll get into later. Here's some pics to illustrate what i'm talking about. The last image is them all overlaid.
*all pics were made by 87jeepwrangler over on the jeepsunlimited forum*
The angle of the shackle however, can soften or stiffen the spring's normal rate. You can find the angle of a shackle by drawing a line through the middle of both spring mounting eyes, and a line through the shackle pivots. Then find the angle made by the two lines. You can increase the effective rate of a leaf spring by decreasing the shackle angle. An increase in shackle angle will produce a decrease in the effective leaf spring rate of a leaf spring. Still with me?
Here's 4 more images of the same spring in a compressed state. The last one is them all overlaid.
This brings us to "shackle inversion". What the hell is that? Shackle inversion is when the leaf spring droops past it's maximum point. This happens when the axle is fully drooped out and something hits the shackle forcing it to compress in the opposite direction. It usually leads to a bent or broken leaf spring, and most people choose a "boomerang" style shackle to avoid this.
In this pic, you can see the shackle closest is almost inline with the leaf spring eyes, while the further shackle is pushed out. This is a good example of how far shackles travel to allow axle articulation.
Yep, i'm still talking shackles here. Now that you understand shackles, what they do and how they do it, we can move on to the some info you can actually use on your rig. Where do I start? What angle should I be searching for while setting my rig up?
A good starting point is 90*. At 90 degrees, the shackle has no bearing on the spring rate of the leaf. Then basically as the leaf spring is compressed, the shackle angle increases and the spring rate stiffens. This is an illustration made by kwrangln on pirate4x4 that helps explain what I'm talking about.
Now that you've got the basics down, you have to decide what you'll be doing with your rig and how you want it to perform. Lot's of angle on the shackle will provide a good amount of droop while off road but will ride very rough on the road. If you've got very little angle on the shackle, you'll have a smoother ride with more up travel in your suspension, but very little droop. There inlays the game of chase and personal preference. Once you've got your setup burned in and leaves installed, the only way to adjust the shackle angle is to get a longer/shorter leaf spring.
Here's a few guys that wouldn't trade their leaf sprung setups for air shocks. Remember the shackle reversal we talked about in the first post? Well, the guy in the last 2 pics really wished he would have done one...
This concludes a VERY basic explanation of the leaf spring setup. If you've got something to add, don't be shy, post up.