Suspension Lift Kit Knowhow - Part 2

by Jim Allen

Leaf Springs

The best way to lift most leaf spring four-bys is with new springs. Lifted leaf springs have more camber (arch) and are longer along the main leaf, so the mounted eye-to-eye distance at ride height is roughly the same as the original. Among other things, this maintains the correct shackle angle. Add-a-leaf kits can be used with leaf spring suspensions to add a small amount of lift, but are best used to bring sagging springs back up to stock height. If used just for lift on good springs, you will likely experience a noticeable decrease in ride quality.

Lift blocks are a reliable, inexpensive lift alternative for the rear of trucks with a spring-under axle configuration. Just as it sounds, the axle mounts under the spring and blocks placed between the axle and spring result in lift of an amount equal to the thickness of the block. Lift blocks can provide 2-8 inches of lift over stock. The downside is that blocks, or taller blocks since some trucks have blocks from the factory, can increase the tendency towards spring wrap.

Spring wrap occurs when traction and torque combine, causing the axle housing to rotate. The leaf springs resist, but the extra leverage of a lift block can multiply the forces against which the springs are fighting. With spring wrap comes temporary changes in u-joint angularity (hopefully small changes). Many times, spring wrap will be released slowly, but if the tires break traction it will happen suddenly and that’s hard on the components. The heavy rate springs in pickups don’t usually have problems with wrap using blocks at or below four inches and often only minimal trouble over that. Much depends on the combination of engine torque and traction available.

One thing to keep an eye on with regards to lift blocks are the u-bolts. Longer ones tend to stretch and loosen up, so you should regularly retorque them for safety. That’s one reason why you never, ever stack lift blocks because they can collapse more easily if the u-bolts become even slightly loose. Most of the time, you can combine blocks with lifted leaf springs but you should never install blocks up front. You might survive a rear lift block collapse but, since the steering is up front, it makes for a more critical situation if it happens there.

Shackles are an important consideration with leaf spring suspension lift kits. A longer shackle will lift a vehicle by half of the extra length. For example, a shackle that is three inches longer than stock will lift the vehicle by 1.5 inches. It will also alter the pinion angle, tipping the diff snout up on the rear axle and moving the shackle angle more vertical (forward tilt). Up front, a longer shackle will tilt the diff snout down if the shackles are mounted at the rear of the leaf springs and up if they mount at the front of the leaf springs (as with many Jeeps). Significantly longer shackles can decrease lateral stability and increase the loads on the spring bushings and chassis eye. On CJ and YJ Jeeps, broken shackle hangers are notorious and made worse with longer shackles. Sometimes a change of shackle length can be used for corrective purposes, such as a slight lift to level the vehicle or to correct pinion angle.

Poly (polyurethane) spring bushings are the way to go with any lifted leaf spring rig. The factory rubber bushings don’t usually hold up when subjected to lots of flexing because the center sleeve is thermally and chemically bonded to the rubber insulator and the extra movement tears the sleeve away from the rubber. The sleeve can rotate independently inside a poly bushing, so it both flexes better and lasts longer.

Stay tuned for part 3 of our series where we'll cover the highlights of coil spring and independent suspension lift kits.

Leaf spring anatomy. This CJ is upgraded with wider Wrangler springs but remains in spring under configuration and is lifted approximately 4.5 inches. Note the dropped Pitman arm and the very nice drag link angle. The steering box is braced and the u-bolts are protected on the bottom with skid plates. This CJ likely rides well on the highway and has very good street manners. On the trail, it flexes well, rides well, and does not have axle wrap.

  • 1. Wrangler style 2.5-inch spring
  • 2. Adapter shackle, 2-inch to 2.5-inch
  • 3. Steering box brace
  • 4. Drag link
  • 5. Tie rod
  • 6. U-bolt skid plate
  • 7. Dropped Pitman arm
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