Leaf springs are one of the oldest forms of suspension. For hundreds of years, leaf springs have been used on automobiles, carriages, trailers, and so much more—and they’re still in use today. On the surface, leaf springs appear to be extremely simple. Compared to many other suspension designs, the flat pieces of steel fixed on one end and a pivoting shackle on the other are straight forward and simple. However, leaf springs are much more complex than what meets the eye.
Leaf springs are comprised of one or more lengths of material, almost exclusively steel, that have two rolled ends. Most modern leaf springs use a rubber or polyurethane bushing in the rolled ends to help reduce road vibrations. The larger rolled end, known as the main eye, goes to a fixed point on the vehicle. The smaller end attaches to a shackle that creates a pivoting point and allows the leaf spring to cycle through the compression and droop of the suspension.
There are many types of leaf-spring suspension designs. Auto manufacturers have used them in transverse arrangements (spring is set perpendicular to the framerail), quarter-elliptic (looks like half a leaf spring bolted to the frame and differential), elliptic (looks like a spring inverted and bolted to another) and semi-elliptic, which is the most common.
Semi-elliptic springs are more than just the bouncy parts of your suspension. They are the suspension. Each unit plays an integral part in keeping the differential connected and centered below the vehicle.
Leaf Springs In Today’s World
Coil springs have been slowly gaining ground for decades but they haven’t taken over entirely. Leaf springs are used in the majority of new pickups coming from auto manufacturers, because they’re simple and can carry heavy loads. Companies like Deaver Suspension of Santa Ana, California, have made leaf springs the center of their business.
Since 1892, Deaver Suspension has been building and repairing leaf springs in the United States for any application. Horse-drawn buggies, classics, and hot rods, work trucks, Jeeps, and desert racers of all sorts. In that time, they have learned a few things so we spent some time with them in their busy shop to get the lowdown on leaf springs. We talked with Jeff Crosby, owner of Deaver Suspension and his right-hand man Scott Born, who handles business operations and many other facets of the company.
What goes into leaf spring design?
: We start with the OEM specs and then do a full analysis of the intended application use, ride height, load, and shackle position. We then start building with old-school R&D techniques and build a prototype. After we get something close, it gets drawn up using CAD software to be fine-tuned.
Leaf springs are used in countless applications from desert race trucks, horse-drawn buggies, locomotives, and heavy equipment. They all work the same way, just on different scales.
How do production springs and custom springs differ in design?
: For production springs, we try to target the middle of the range between performance and load carrying capacity while maintaining the desired ride height. For the majority of people, this will give a good balance of ride and weight bearing strength. On custom springs, we look at the one specific vehicle, the intended use of that exact vehicle for a much more focused design.
Spring under or spring over?
: Spring under allows you to get more compression out of the spring because of the available arch at ride height, allowing you to get a better balance between up-travel and down-travel. Most trucks start with a flat OEM spring at ride height and as a spring over, the vehicle must be lifted much higher to get the same effect or suffers from limited up-travel.
Leaf springs start off as flat bar stock cut to specific lengths. The ends are then tapered as needed and go through a rigorous process of shaping into a usable spring.
Is there an optimum angle for the shackle?
: Each vehicle has a different optimal angle. It all depends on the growth rate of the spring based on the compression travel as well as how much room the shackle has to be able to move. It is best to have the shackle at a neutral position (centered in its travel) when at ride height.
What is axlewrap, and how is it reduced?
: Axlewrap is when the axletube tries to rotate due to the torque of the powertrain getting power to the tires. The rotation of the axle housing twists the leaf springs into an S shape. There are many factors that go into reducing axle-wrap. A flat spring is much more likely to twist as is a spring over design. Proper spring rate plays an important role and we like to add a Rebound Leaf to many of our applications. The Rebound Leaf, also known as a Snail Wrap, gives added rate on the forward section of the spring to control torque from the axle.
What is a military wrap, and why is it good to have?
: Military wraps are a spring eye configuration that has a secondary leaf that wraps around the main leaf. It gives an extra layer on security and safety. If a main leaf breaks, the main connection of the differential to the chassis of the vehicle is broken and the axle is now able to move in ways it shouldn’t, which will allows other things to get damaged. With a military wrap, if the main leaf breaks, there’s an additional layer that will keep the suspension connected and the keep the differential where it needs to be. It is an insurance policy.
Specialized machinery heats up the ends and wraps them into the necessary eye formation with exacting specs.
What are some of the other spring eye styles?
: There are three common spring eye configurations out there. Regular, reverse, and Berlin. The reverse eye routes over the top of the eye and then travels down whereas a standard eye routes under the main eye. A reverse eye allows the spring to create more compression travel and help keep the spring from going negative, where it is weaker and under more stress. A Berlin eye centers the eye to the main leaf and is very common for shackles, because it gives a good balance for movement. The application is a major determining factor on which eye is used.
What are the benefits of using multiple thinner leaves instead of a few thick leaves?
: High leaf counts of thinner material using quality high-carbon rate steel allows you to utilize that thinner material without compromise of sag or fatigue. In the simplest of terms, take 10 pieces of notebook paper and see how they can flex and move compared to four pieces of cardboard. The staggered leaf lengths allow us to get a much more progressive spring rate that increases as each leaf is engaged. This allows a smoother cycling of the suspension that doesn’t get the bucking reaction or rebounding.
Not just a leaf in the wind
Leaf springs may be losing ground to coil springs in OEM designs but they still have a place and use in the automotive world as well as off-road. Not many other 100-year-old technologies are still used on a daily basis, and Deaver Suspension can use their expansive experience to help give your vehicle a softer ride while performing better both on-road and off.
The steel goes through a heating and rolling process and is then checked for proper thickness. A measurement off here would change the overall spring rate of the leaf spring. Using a quality high-carbon steel allows Deaver Suspension to use thinner leaves, improving compression and ride.
Each spring goes through a heat-treating process to further strengthen the material.
Leaf-spring formation is done with the spring glowing hot. Adjustable roller systems allow the process to be customized for each application.
The military wrap gives your spring an insurance policy. If the spring is pushed beyond its limits and the main leaf breaks, the military wrap makes sure the differential is still located where it needs to be and keeps things from getting much worse.
Reverse eyes allow the spring to get more curvature. More curve means more compression. Spring clamps are used consistently to keep each leaf in its place.
Deaver uses a rebound leaf to help reduce axlewrap. The rebound leaf wraps the main eye and extends just beyond the center pin, giving the spring more strength against the twisting forces of axlewrap.
Overload springs are still used in certain heavy load applications. Even the overloads can be staggered in size to give a dual-rate effect when loaded down.
Using multiple thinner leaves allows the spring to have a consistent and progressive spring rate as the spring flattens out under compression. Each leaf is engaged more and steps up the rate, avoiding the hard rebound of an instant jump to a high spring rate of a thick overload.
Deaver Suspension’s Mini Packs are a great way to get some extra performance from a stock leaf spring. Depending on the application, the Mini Pack replaces the stock overload and/or fits between existing layers of the stock leaf spring.