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Suspension Lift Kit Knowhow – Part 3

by Jim Allen

Lifts for Coilers

Coil spring suspension lift kits start one of two ways: as a longer coil spring or as a spacer above or below the spring. A spacer lift is the more economical of the two. In the 2-3 inch range (for most rigs), the negatives are minimal and few corrective actions are required. You can retain the factory spring rates and maintain good ride quality. Sometimes you can even retain the stock shocks (using extensions), which leaves you with most of the factory ride tuning intact. A moderate coil lift may deliver many of the same things, albeit at a higher cost. New springs also offer the opportunity to increase spring rates for a higher load capacity or to compensate for the weight of accessories.

The downside to all coil spring suspension lift kits is that they alter the operating angles of the various links (or radius arms) and track bars in the system, changing the designed geometry of the system. In small amounts, that’s no biggie. In larger amounts, it leads to expensive corrections. With typical four-link suspensions that have upper and lower arms on each side plus a transverse track bar (which isn’t counted as a “link”), a lift decreases the mounted eye-to-eye distance of the links at ride height. That changes the axle location (axle moves to the rear in front, and forward in the rear), changes the pinion angle on both axles, and changes the caster angle up front. On a two-link radius setup (one link per side, such as the older solid axle Fords or the Twin Traction Beams), the same thing happens but the pinion angle and caster are even more affected. Changing the angle of the links also changes the way torque and ride height affect steering and directional stability.

Beyond the basic 2-3 inch lift, you need a corrected system. By “corrected,” we mean one that has new links adjusted for length and possibly modified in shape to correct some of the geometry problems. Offset eye bushings or dropped mounting brackets are also used. Corrected suspension lift kits usually include a corrected track rod because the angularity of the transverse bars is also adversely affected by lift. Sometimes that comes in the form of a longer track bar, with or without changes in its shape, and sometimes drop brackets are used to correct angularity. A good corrected coil spring lift will often deliver near-stock drivability into the 4-6 inch lift range.

At some point in your quest for large tires and wheels, as well as more lift, if you want to retain good manners and efficient operation you will need to go with a long arm suspension lift. These are the most expensive and difficult-to-install kits, but they deliver the best performance because they are fully engineered to operate at the higher ride height, not just adapted to it. A long arm kit will deliver the best street and trail performance with the most lift and fewest compromises.

Polyurethane bushings are as useful to a coil spring rig as they are to one with leaf springs, but they can sometimes decrease ride quality if used in the wrong spots. Poly bushing manufacturers can vary the softness (called “durometer”), but rubber still rules in the isolation of noise and shock. For this reason, some manufacturers of suspension lift kits use rubber bushings in certain places, such as the chassis end link of many Jeep coilers. Don’t be turned off by that.

Lift Kits for Independent Suspensions

Independent suspension lift kits may be the most complex lifts of all, similar in complexity to solid axle coil spring setups. At lower levels of lift height (2-3 inches), many of the newer strut-type setups are quite simply addressed by a spacer lift. From there, the changes of angularity to the control arms, steering rods, and the CV axle angles are the major concerns.

All these potential troubles, and others, are addressed in lifts at or above 3 inches by relocating the upper/lower control arm or replacing it. A steeper angle on the upper or lower arm moves the steering knuckle inboard and increases the angularity on the ball joints, sometimes maxing out their movement. The longer or relocated arms solve those issues. Another common element is a modified knuckle. The point at which the CV axle enters the knuckle may be altered for some lift, but, more importantly, the distance between the ball joints is increased to decrease their angularity. Many times, the steering arm is relocated to lessen the angularity on the tie rods. Some IFS systems relocate the entire suspension and differential at a lower position to obtain lift.

CV joint angularity is always a problem with an IFS lift, but it’s one usually addressed by the suspension lift kit designer. They calculate the maximum angularity of the inner and outer joints and limit suspension travel to prevent binding. The other consideration is the plunge depth of the inner joint, often called a tripod because it has three pivot points. As the suspension travels up or down, the mounted end to end distance of the CV axle changes. As angularity decreases, that distance grows and vice versa. It’s just like a driveshaft in that way. The “slip yoke” for the CV axle is called plunge and it’s done inside the inner joint, the three legs of the tripod able to move in and out inside tracks. At normal ride height, the plunge depth must be centered or at least positioned so that the joint will neither pull apart on downtravel nor run out of travel on suspension uptravel, causing it to jam and break the diff housing. Plunge depth is usually addressed by a spacer between the inner joint and the drive flange on the differential.

Stay tuned for part 4, the final part of our series on suspension lift kits, where we'll cover pinions, universal joints, driveshaft angles, and more.

Jeep Axles Coil Springs – Lift Kit How To

Anatomy of a common coil spring setup as found on many thousands of Jeeps, though similar setups have been used elsewhere.

  • 1. Axle
  • 2. Coil Springs
  • 3. Shock Absorbers
  • 4. Upper Control Arms
  • 5. Lower Control Arms
  • 6. Track Bar
  • 7. Sway Bar
  • 8. Sway Bar Link
  • 9. Drag Link
  • 10. Steering Damper
  • 11. Tie Rod
  • 12. Rod End
Jeep Shock Absorbers & Track Bars – Lift Kit How To

A rear coil spring setup that is also found on many Jeeps.

  • 1. Axle
  • 2. Coil Springs
  • 3. Shock Absorbers
  • 4. Upper Control Arms
  • 5. Lower Control Arms
  • 6. Track Bar
  • 7. Sway Bar
  • 8. Sway Bar Link
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