Everything You Ever Wanted To Know About Leaf Springs

The Simplest Suspension Is More Complicated Than You Think

While new Jeeps and SUVs these days typically come with coil springs, leaf springs still exist on the rear of most pickup trucks (Ram 1500s and 2500s being the exception). Leaf springs are the most simplistic and inexpensive suspension system for your 4×4. The basic technology has been around since the days of covered wagons. In a coil-sprung suspension, the coil’s job is to hold the vehicle up and comply with the terrain. Links position the axle fore and aft and keep the axle from rotating. Even more links position the axle side to side. In a leaf spring suspension, the leaves do all of these tasks. Leaf springs may be more simplistic, but there is a lot to know about them. Read on to find out.

Many thin leaves mean that each leaf can have a lower spring rate, which results in a softer ride. A spring pack with many thin leaves is thicker overall than one with a few thick leaves. That is a good thing in a spring-over application since the main leaf is flatter giving a better ride. In a spring-under application, the thicker spring pack can eat up ground clearance.

Some springs feature military-wrap spring eyes, where the second leaf wraps around the main leaf to form a double-wrap. This design element transmits less stress to the main leaf during extreme articulation. It also lessens the odds of spring separation in the case of main leaf failure and provides greater strength, support, and durability as a whole.

Details Matter

Leaf springs are made up of a main leaf, with eyes on each end that hold bushings and bolt to the vehicle. Multiple other leaves carry the weight of your rig. There are numerous options for how to configure these packs of spring steel to perform under your vehicle. They vary greatly not only in quality, but in price and performance as well. When we purchase aftermarket leaf springs, we are generally looking for springs with increased arch to allow the fitment of bigger tires. How this is accomplished will dictate the ride quality and amount of articulation.

Longer leaf springs don’t have to deflect as much as shorter leaf springs to provide the same amount of wheel travel. But, unless you plan to cut the existing mounts off of your vehicle’s frame and add new ones, you are generally stuck with the same length spring that came from the factory. Springs made up of many thin leaves will offer a more supple ride than springs that use just a few thick leaves. The problem with many thin leaves is that there can be friction between each of those leaves. High-end spring packs solve this issue by tapering the leaves, so they are thinner at the ends. They also use Teflon pads between the leaves to reduce friction.

Other features to look for when shopping for leaf springs include bolt clamps, shot-peened leaves, and military wrapped eyes. A military wrap is when the second leaf wraps around the main leaf at the eye to retain the spring in place in case the main leaf breaks. Shot peening the individual leaves makes them stronger and more resistant to fatigue. Bolt clamps are preferred to regular crush style clamps since they cause, you guessed it, less friction.

Clamps prevent the leaf plates from “fanning out” laterally. Springs can utilize bolted clamps, as shown here, or clamps that wrap tightly around the spring. Bolted clamps allow for less restrictive movement and more leaf separation to keep from inhibiting articulation.

There is inherent friction in leaf springs as leaves slide against each other. Features like diamond-cut leaves, Teflon sliders between the leaves, and even Teflon paint are used on high-end leaf springs to minimize this friction. The Teflon sliders do eventually wear out but can be replaced.

Over Or Under The Axle

Some vehicles, such as Jeep CJs and Toyota Land Cruisers, come with the leaf springs under the axle tubes. Most trucks use leaf springs that mount to the top of the axle. Swapping springs from under the axles to the top used to be popular. The benefits are numerous as you gain lift height equal to the thickness of the axle tube and the spring pack without having to buy new springs. Using the soft factory springs also offers a smooth ride and gobs of articulation. And, you gain ground clearance where the leaf springs and axle plates used to reside. Does it seem too good to be true?

Well, it is. You need a lot more than just new spring perches when performing a spring-over conversion. In the rear axle wrap from the soft leaf springs is common, so adding an anti-wrap bar is a must. If left unaddressed, axle wrap can not only ruin your leaf springs but result in broken U-joints or driveline yokes as well. In the front, care must be taken to have enough caster to keep the vehicle from wandering as it goes down the road. This can often be a balancing act between the caster angle (the angle the steering knuckle leans back at) and the pinion angle.

Additionally, the springs’ new home on top of the axle tubes is often in the same location where the drag link used to reside. To resolve this usually requires a high-steer conversion, where the drag link connects to a steering arm on top of the knuckle. The benefits of a spring-over still apply, but cost savings isn’t necessarily one of them. Correctly done, a spring-over can cost more than a set of high-quality lift springs.

Leaf springs use rubber or polyurethane bushings at each end with a metal sleeve inside that the mounting hardware rides on. These bushings are wear items and should be checked (and replaced if needed) regularly. If you get a new clunk in your leaf spring suspension, this is an excellent place to start looking.

Jeep CJs, Toyota Land Cruisers, and Suzuki Samurais all came with leaf springs under both the front and rear axles. This suspension design is rugged, simple, and straightforward. However, it does leave plenty of room for improvement with regards to ground clearance and articulation.

Lift Blocks

Lift blocks can be used between your axle and leaf springs in a spring-over configuration. They increase the distance between the axle and the spring pack. Blocks are an inexpensive way to make room for larger tires, only requiring longer U-bolts. Lift blocks should never be used in a front axle application, since the forces generated from steering put side loads on the blocks. Should the block break or the U-bolts loosen, there is the potential to lose all steering. We don’t have to explain to you why that is a bad idea.

In the rear, some trucks come from the factory with lift blocks. These should never be stacked with additional lift blocks because they can fold and fall out on the street or the trail. The added distance between the spring and the axle created by lift blocks results in additional leverage on the leaf springs, which can result in axle wrap. Depending on your rear springs, you may need a traction bar to battle the axle wrap, or it may not be an issue at all. As noted above, axle wrap is a more significant issue with long, soft leaf springs than it is with shorter, stiffer leaf springs.

Spring-over suspensions provide excellent ground clearance and can yield gobs of articulation. Notice the anti-wrap bar that greatly reduces spring wrap. An anti-wrap bar may be necessary to keep wrap under control. 

Shackles allow the leaf spring to extend and contract in length as the suspension cycles. Some shackles use greasable bolts or a crossbar for more stability. They can be as simple as two flat pieces of metal, though.

Shackles

As leaf springs compress and extend, the arch changes causing the spring to get longer and shorter as they cycle. One end of the spring is fixed to the chassis, while the other end uses a shackle to account for this change in length. On rear axles, the shackle is always at the rear. The shackle can be above the spring (compression style) or below the spring (tension style). Tension style shackles require the spring to have more arch for the same ride height. Sometimes a “shackle flip” is used to gain lift by converting the shackle from a tension style to a compression style using the same springs.

On front axles, the shackle can mount at the front or the back of the spring. There are advantages to both. Having the shackle in the front means that the axle doesn’t move back when you encounter obstacles on the trail, increasing contact pressure and climbing ability. The flip side is that having the shackle at the back of the spring does allow it to move back when hitting obstacles providing a smoother ride.

A shackle angle around 45-degrees will let the suspension compress more easily than a shackle at a 90-degree angle to the frame. This ease of compression improves ride quality as a result. Shackles should be long enough to allow the leaf spring to cycle without the shackle inverting. Longer shackles can be used to gain some lift, but they only lift one end of the leaf spring. This changes important parameters such as the pinion angle and caster angle when you alter the shackle length.

Final Thoughts

It doesn’t matter if you have leaf springs front or rear, on top of the axles, or under them. There are a variety of lift options for leaf springs to fit any use and budget. Armed with the necessary information, you should be able to find the best spring for your specific needs. But if you are looking for the best ride quality and articulation, go with the highest-quality springs you can afford.

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