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Leaf Spring BasicsMore Simple Driveway Tech From the September, 2008 issue of 4Wheel & Off-Road By Kevin McNulty Photography by Kevin McNulty
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Leaf springs represent a proven technology that works exceptionally well. When they're properly designed and installed, they can produce a highway ride as smooth and as comfortable as a coil-sprung vehicle. The off-highway ruggedness, durability, and performance of leaf springs, especially for rockcrawling, have been proven time and again, and they are able to endure extreme punishment under harsh conditions while offering exceptional articulation. Compared to modern vehicles and trick coilover suspension systems, leaf springs don't look as high-tech and as fancy, but there's a great deal of science behind their design. The leaf spring dates back hundreds of years. The modern leaf spring was invented in the early 1800s, automobiles were manufactured with them until 1989, and they are still used today on trucks, trains, and heavy-duty applications. Just about anyone with moderate mechanical skills can replace a set of leaf springs. It's a swap that can easily be completed in the driveway, and we could fill volumes with leaf-spring science, progressive rates, tuning, axlewrap, and do's and don'ts. While we were at T&J Performance Center in Orange, California, we watched as they replaced a set of leaf springs on an old worn-out Jeep CJ.  The worn out suspension on...  The worn out suspension on this jeep CJ allowed room enough room for dinky 28 inch tires, and the ride wasn,t very comfortable, especially of -highway. With the new springs installed the old jeep's ride was much more stable on the highway and allowed the jump up to 33-inch tire's , a 5 inch boost in ground clearance.  When installing new leaf springs,...  When installing new leaf springs, the spring and shackle bushings should be replaced as well as the shocks, U-bolts, and brake lines. In most cases a dropped pitman arm should also be installed on a lift of 4 inches or more. A spring pack made from high-quality steel uses a higher count of thinner leaves. Teflon inserts and tapered leaf ends will offer a superior ride over a cheap set of springs.  Swapping your vehicle's leaf...  Swapping your vehicle's leaf springs is relatively easy in theory, but it's labor-intensive. The springs are hung by hangers and shackles and both the chassis and the axles must be supported separately when changing the leaf springs. The differences between vehicles may be shackle style and spring-under or spring-over axle designs. The CJ's shocks were unbolted from the lower spring plate, the U-bolts were removed, and then the springs were unbolted.  The old leaf pack losing its...  The old leaf pack losing its spring rate and flattening out, bad rubber, or polyurethane bushing can greatly degrade ride quality. It's easy to periodically check the bushings, as over time they become hard and brittle and will crack and tear. Greaseable bushing and shackle bolts are available, which makes it easy to maintain the bushing with a fresh squirt of grease. If your vehicle's ride quality has degraded and the springs seem to still be in good shape, it may just be time to replace the bushings.  When installing the new bushings...  When installing the new bushings in the leaf springs, make sure they are well greased, as this will prevent potential squeaks and extend their service life. Polyurethane bushings will give the vehicle a firm ride, while rubber bushings will make it smoother. The difference in ride quality between the two is not overly dramatic, and choosing between the two basically boils down to personal preference.  Leaf springs mount on the...  Leaf springs mount on the axle at the spring pad or perch, and a pin in the leaf pack centers the spring like this spring-under design. In a spring-over design the leaf pack is on top of the axle. Both are secured to the axle by U-bolts or bolts. Most replacement leaf packs will bolt right in, but you have to be careful as some applications use spacers or shims between the axle and the spring, and on some front applications left and right leaf springs are used.  This leaf-spring swap took...  This leaf-spring swap took about two hours with the right tools. The CJ already had aftermarket shackles in the front and rear, and they play an important role in suspension performance because they can either stiffen or soften the spring rate. Shackles also reduce leaf-spring stress and increase spring durability. Also a taller shackle will increase the vehicle's ride height. Most Jeep shackles are mounted up front, but shackle-reversal kits are available which help smooth the ride of the Jeep when it hits a bump.  A properly tuned leaf-sprung...  A properly tuned leaf-sprung suspension can offer exceptional on- and off-highway performance. They have also proven to be extremely tough and very dependable under the harshest four-wheel conditions.
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