Even though it is the oldest type of automotive suspension, leaf springs continue to be a popular suspension choice among racers. Though simple in appearance, a leaf spring suspension involves many intricacies understood by few racers. The following information should help you to better understand and consequently, better tune a leaf spring suspension. The information applies only to unsymmetrical leaf springs with shackles mounted above the leafs (typical of most race cars), unless noted differently.


When a leaf spring is checked for rate, its ends are first attached to rollers. A leaf spring produces fairly consistent rates when checked in this manner. However, a leaf spring can produce an installed rate stiffer or softer than the checking mechanism readings. It is important to understand the installation factors that affect a leaf spring's rate.

For instance, the axle mount deadens part of the leaf and an increase in spring rate results. The increase is proportional to the amount of spring that is deadened by the mount. You can expect an increase in spring rate after replacing "factory type" rubber lined axle mounts (which deaden relatively little of the leaf) with solid axle mounts and lower plates.

Any lengthwise twisting of a leaf spring will also cause an increase in spring rate. The increase is proportional to the degree of the twist. The twist results from stagger, wedge, bent axle tubes, misaligned chassis mounts, etc. that cause the axle mount and spring to be at an angle to each other from a rear view.( see illus. 1) The leaf twists whenever it is tightened flat to the unparallel axle mount.

Illustration 1.

Leaf twist is more pronounced with solid type leaf eye bushings than with the more pliable rubber bushings. Practically all potential for twisting a leaf spring is eliminated by using an AFCO front eye pivot instead of a bushing.

Your leaf will become softer as twist in the leaf is reduced. At the least, you should eliminate any static twist present in your leaf springs whenever your chassis is at ride height. Otherwise, you may unknowingly change rear spring rate whenever you replace or adjust rear suspension components.

A static tension in the leafs is present whenever the leafs and axle mounts are not parallel from a side view(see illus. 2). Any static tension will cause an increase in spring rate. You can check for this tension by placing jack stands under both leaf springs directly below the axle and then unbolting one side of the axle from its leaf (use this same procedure on both leafs to check for lengthwise twist). The axle mount should seat evenly to the leaf as viewed from the side. If necessary, reposition the axle clamp on the axle tube to eliminate any twist. AFCO clamp-on axle brackets will facilitate corrections.

Illustration 2.

Illustration 3.

Shackle length is another factor affecting the rate of a leaf spring. A short shackle will change its angle (and the effective rate of the leaf spring) quicker than a long shackle upon deflection of the leaf. There is a second shackle effect on the stiffness of the rear suspension that counteracts and sometimes exceeds the shackle�s effect on spring rate. This second effect occurs whenever the shackle swings in its arc and moves the rear spring eye vertically.(see illus. 4)

Illustration 4.

The vertical movement of the rear spring eye causes a jacking effect. If the shackle movement forces the rear spring eye downward, the leaf will deflect and exert an upward force on the chassis that will add stiffness to the rear suspension. Conversely, the shackle will reduce suspension stiffness if t causes the rear spring eye to move upward during suspension travel.

The stiffening effect occurs during suspension deflection whenever the rear spring eye is ahead of the upper shackle pivot and the shackle is moving rearward (see illus. 4, example B). In this position, however, the shackle also produces a softening effect by reducing the effective rate of the leaf spring (due to the large shackle angle). The overall effect to the stiffness of the rear suspension is determined by the greater of the two shackle effects. Under opposite conditions, you can expect a reversal to the above effects. If the rear spring eye is located behind the shackle pivot (illus. 4 example A) the shackle effect will tend to reduce suspension stiffness whenever the shackle moves rearward. However, the small shackle angle will tend to stiffen the spring's rate. The overall effect to the suspension's stiffness is determined by the more dominant of the two shackle effects. Keep in mind that the movement of the rear spring eye (from its static position) is mostly forward under racing conditions.

If a leaf goes into negative arch the travel direction of the shackle changes and the shackle effects change. Handling is not consistent under these conditions.

The second effect of the shackle can be enhanced by increasing the length of the shackle. Generally, the second shackle effect (jacking)is dominant.

Illustration 5.

You can expect results opposite to the above when you raise the front eye of the leaf and adjust the chassis back to its original ride height. Mounting the leafs so that the front eyes are slightly inboard of the rear eyes will cause the leafs to have more lateral stiffness. This can make the chassis feel tighter and may help prevent the rear suspension from binding due to excessive lateral deflection of the leaf. However, if the leafs are offset too much, the suspension becomes too stiff laterally and rear side bite is lost. Whenever the body slides� over the rear end during cornering, the splayed leafs can cause rear steer that will help the car to turn. Also, if the right front spring eye is mounted more inboard than the left eye (measured from the corresponding tires), the right rear tire will tend to be loaded less than the left rear tire during acceleration. As a result, the chassis will tend to be tighter off the corner. Corner exit handling tends to be loose under opposite conditions Generally, moving the front spring eye 1 1/2" laterally will produce a noticeable effect to corner exit handling.

Too much lowering block* can cause the forward thrust of the rear axle to prevent the leafs (or torque arm) from wrapping up and absorbing engine torque. Consequently, forward bite is diminished. A symptom of this problem shows up on torque arm equipped cars as very little 5th coil/shock movement.