Though it's one of the oldest suspension technologies there is, leaf springs are still very relevant in modern-day suspensions. And it's all too often we hear enthusiasts talk about how they have to get rid of their leaf springs because they ride too stiffly, or they don't have enough travel. And lots of times, those same individuals are automatically thinking the next step has to be to a four-link and coilovers. But this isn't the case! In fact, you might not even need a new set of leaves, much less a new suspension setup.
Chances are your existing leaf springs can be modified to work better with your vehicle application. You see, leaf-spring manufacturers make leaf springs for a semi-universal fit to particular models of vehicles. They try to account for the way most customers are going to use their trucks, the height of lift a customer wants, the size of the shackle, if the customer is hauling heavy loads, etc. On top of that, they have to do it at a cost point that is reasonable enough for them to still make a profit. Therefore, the chances are that even if your leaf springs are almost perfect for your truck, you can probably improve them with some custom tuning for your particular truck.
We've had a couple sets of custom springs made by National Spring in the past, and we've had some really good luck, but we've changed the builds of our trucks a little bit over the years and therefore have changed the loads and forces on the suspensions. We called up National Spring to get some advice and found that we could modify our existing springs to work better with our current builds. Travel could be increased, spring rate and load-carrying capacity could be increased or decreased, and the leaves could even be straightened if some of them were damaged or had an "S" shape to them from wrapping up.
How Did They Work?
We were able to get our springs back onto our truck and make a run to a nearby off-road area. Bill had done a great job getting our National springs to work better on our truck, and because he was able to keep every leaf in the pack, we didn't feel like it had any more spring wrap than it did before we had them modified. The rear would stay on the ground and wouldn't lose traction as easily as it did before when the rear was stiffer. And now, we don't have to worry about bending our leaf springs because our shackles were too short.
Re-Arching Leaf Springs
If you haven't had it done yourself, then you might have a friend who has had his leaf springs re-arched before. And you've probably asked yourself, "Isn't that temporary? And won't they just settle back to their original height?" Well, yes and no.
The metal in the spring leaves has a certain grain structure, and the metal definitely has memory that brings it back into a certain shape. The temporary or permanent status depends on the age of the spring, how many leaves are in the leaf pack, the thickness of the leaves, and what the original arch was. If you've got semi-new 10-leaf springs, and you want to gain 2 inches of height, then it can probably be done permanently. If you've got some old factory four-leaf packs that you want to crank up 4 inches, then they'll probably be flat again by the time you arrive home.
This is how leaf springs start. National Spring has many of its off-the-shelf springs built off site, but for custom applications, springs are built in-house. Depending on what type of truck and the type of use, National can stack more or less thick or thin leaves to make the right type of leaf pack for you. And if your springs need maintenance or modification, then National can add, take out, or cut leaves for varied performance. | 
The leaves of the custom springs are cold-arched over an anvil at National. Robert Kressler will spend more time making them this way, but the process produces stronger spring leaves. If your leaves are twisted a little or need more or less arch, then the guys at National can possibly hammer the leaves out on an anvil. This obviously requires complete disassembly of the leaf spring though. | 
This is an example of a heavily arched main leaf for a Super Duty. This is the most important leaf and will dictate how much arch the pack will have and how long the flattened length of the spring will be. |
National will usually put the center pin in the "stock-style" location, but you can request to have the pin moved forward or backward if you are changing wheelbase length or hanger placement. Generally, the closer the center pin is to the front spring eye, the more axlewrap is inhibited upon acceleration. | 
Springs are cut to length but can be shortened to change the ride and spring's rate even after the packs are assembled and used. | 
A military wrap is where the second lower leaf wraps around the spring eye of the main leaf to keep you mobile even if the main leaf snaps. But these springs utilize a snail wrap on the front spring eye. In a snail-wrap spring, the main leaf is actually the second leaf down; the third leaf down is the military wrap; and the top leaf is the secondary wrap (the snail wrap) for the ultimate in failure prevention. It is possible to get a snail-wrap leaf added to your already-built leaf pack. |
This is a 12-leaf front spring pack for a K5 Blazer. The springs have a ton of travel (so much that our 14-inch-stroke shocks are too short). But the spring rate was too soft, and even though the truck was floating at the right height, the front end would bob too much. We had National add another leaf into the pack to stiffen up the front end a bit. Be careful though, as many times bobbing can be due to improper shock valving, so be sure you're addressing the correct problem. | 
On this truck, we had a different problem with leaf springs. They felt pretty good, but they were a bit too stiff, and they felt like they sort of stopped abruptly before reaching the end of their travel.Bill Rounds took measurements of what the flattened length of the leaf springs were, the length of the shackle, and then the total length possible offered by our spring-hanger locations and shackle length. | 
After weighing and measuring our truck, Bill found out we had two separate problems. Our first problem was that we had too much spring rate in our leaves for the rear of our truck. The second problem was a bigger issue: Our springs did not have enough room to flatten out. The shackle was too short with the spring we were using, and there was not enough available length forcing the spring to bind up. This is the number one way springs get broken and bent. It is imperative to match your spring with your shackle. |
Since we hadn't yet damaged our springs, they were no big deal for National to modify. We could go about it one of two ways: a longer shackle or a shorter main leaf.Bill made us a custom shackle that allowed us to use all the available length of our leaf springs without them binding. Using a shorter main leaf is an option but would not give us as much travel. | 
Bill is just way too calm around hot metal. Or maybe we just get a little too excited when we see fire. He threw these leaves in the oven to heat up the ends, so he could modify them. | 
After the leaves were heated, Bill placed them in a spring shear and clipped the ends of the lower supporting leaves (not the main leaf) to make the lower leaves shorter lengths than they were. You have to have a lot of experience to be able to do this, and Bill was able to properly decrease our spring rate while keeping every single leaf in the pack. |
Once the leaves are heated and clipped, the ends are put in an oil bath for tempering. | 
With our spring leaves clipped and smoothed, our leaves were ready to be painted and reassembled. If you ever have your springs apart, you might want to take the time to clean the rust and debris from the leaves and buy special graphite paint that will allow the leaves to slide more easily past each other when the pack is moving.The top leaf is not in this image because Bill was using it for mock-up under our truck, making sure that the flattened length worked in conjunction with our shackle. | |