WHAT TO LOOK FOR IN A QUALITY SPRING
Remember, the many factors of a leaf spring suspension are interrelated and a change to one aspect of the suspension usually affects others. Consequently, the handling results are not always as predicted! Hopefully, this text will provide you with the understanding necessary to correctly analyze handling and adjust your leaf spring suspension correctly. When looking for leaf springs, watch for the following critical qualities to ensure that you will get a quality product:

1. Smooth, continuous and consistent arch
2. No lengthwise twist
3. High-strength steel
4. Heat-treated
5. Tension side of leaf shot-peened for increased durability (produces a satin smooth finish)
6. Rubbing blocks between secondary leafs
7. Secondary leafs taper cut at ends
8. Proper eye alignment (front & rear eyes should be parallel in all directions)

ARCH
The free arch of a leaf spring is correctly measured by drawing a line (“datum line”) through each leaf eye center and then measuring (perpendicular) from the datum line to the spring surface at the centering pin. The spring should be lying on its side in the free state whenever it is measured.

To accurately check for arch measurement, lay spring on side and pull a straight line from center of front eye to center of rear eye. Measure from line to top leaf at the locating pin. On monoleafs measure at locating pin hole. When experimenting with different arches, you can expect the following effects on handling.

A. More arch:

• raises chassis
• raises roll center (causes less chassis roll and less rear side bite)
• increases wedge when arch is increased on LR only (makes chassis tighter off corner)
• changes roll steer (may help car to turn)
• increases shackle angle (may cause a change in spring rate; see shackle section, page 81)
• decreases lateral stiffness of the rear suspension (may improve side bite but could make the chassis feel loose)

LEAF MOUNTING ANGLES
The geometry of a leaf spring suspension appears to provide the best overall handling whenever the front leaf eye is mounted below the rear eye. If you lower the front eye of the leaf 1˝ or more and readjust the chassis back to its original ride height, you can expect the following:

1. More “tight” roll steer (may tighten handling)
2. Increased rear suspension stiffness
3. Lowered roll center (increases body roll and rear side bite-handling tightens)
4. Less body / tire separation during acceleration (may reduce forward bite)

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 in corner exit handling.

LEAF TWIST
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, twisted leaf springs, etc. that cause the axle mount and spring to be at an angle to each other from a rear view (see illus. 3). The leaf twists whenever it is tightened flat to the unparallel axle mount.

You can eliminate any static leaf spring twist by shimming or angle milling the axle mount or lowering block, or by angling the leaf and shackle frame mounts. However, twist will still develop as soon as the chassis begins to roll! 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 (part #20229P).

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 leaf is present whenever the leaf and axle mounts are not parallel from a side view (see illus. 4). 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.

SHACKLES
The angle of the shackle can stiffen or soften a spring’s normal rate. You can determine the effective angle of a shackle by drawing a line through the middle of both spring eyes and a line through the shackle pivots. Then measure the angle formed by the two lines (measure ahead of the shackle; see illus. 5). You can increase the effective rate of a leaf spring by decreasing the shackle angle. An increase in shackle angle will produce a decrease in the effective rate of a leaf spring.

A good starting point for shackle angle is 90 degrees. In this position, the shackle has no effect on spring rate. Keep in mind that the shackle angle changes (and consequently the spring’s effective rate changes) whenever the suspension moves. Also, the shackle’s angle will change whenever you change the chassis’ ride height, the arch of the leaf, the load on the leaf, or the length of the shackle. Since the shackle direction changes when the leaf is deflected past a flat condition, you should avoid deflecting the right rear leaf to an extremely negative arch condition. This could cause a very large shackle angle at high loads and consequently a very soft spring rate. Excessive body roll and poor handling could result. You can correct this problem by decreasing the shackle angle, increasing the arch, of the spring by increasing the rate of the right rear leaf spring.

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. 6).

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 it 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.