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
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.
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.
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.
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. 3). 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
leaf spring 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 shackleºs 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)
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
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.
The AFCO leaf spring sliders (SEE PHOTO) can eliminate the up and
down movement of the rear spring eye caused by shackles moving through
their arcs during suspension travel. Consequently, the rear suspension's
loading points displace less during suspension movement. Sliders,
depending on their installed angle, generally reduce the shackle
effectæ and provide more consistent spring rates than shackles.
There is no shackle effect to the rate of the spring when the slider
is mounted to point directly at the front spring eye center(this
is the preferred mounting position). Since sliders are usually more
rigid than shackles, they have less tendency to bind laterally during
cornering. In general, sliders improve the handling consistency
and predictability of leaf spring equipped race cars.
ROLL CENTER HEIGHT
The rear roll center is the point around which the back of the chassis
rolls. Generally, raising the rear roll center tends to loosen handling.
A lowered rear roll center tends to tighten handling. The lateral
location of the rear roll center is difficult to determine and not
necessary to this article. However, you should know how the height
of the roll center is affected by the mounting position of the leafs.
(see illus. 5)
The free arch of a leaf spring is correctly measured by drawing
a line (datum lineæ) 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. 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
• 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)
• Decreases lateral stiffness of the rear suspension (may
improve side bite but could make the chassis feel loose)
B. You can expect handling changes opposite to the above when using
leafs with less arch.
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 and deceleration
(may tighten initial corner entry handling and may reduce initial
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 slidesæ 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.
Lowering blocks are generally used to change the ride height of
the chassis. Lowering blocks can also be used to adjust wedge when
multi-leafs are used. The addition of lowering blocks can cause
less tightæ roll steer which will help the chassis to turn.
If lowering blocks drop either or both spring eye positions relative
to the axle then the rear roll center height will be lowered and
chassis roll, along with rear side bite, will increase.
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.
*The distance between the bottom of the axle tube
and the top of the leaf should not exceed 4 1/2". What to look
for in a quality leaf spring: 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
and rear eyes should be parallel in all directions).
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