is Spring Rate?
rate refers to the amount of weight needed to compress a spring
an inch (Example:500# per inch) To understand and properly check
a spring for rate you need to know the factors that determine the
rate of the spring. Fortunately, there are only three things that
affect spring rate, so there's not that much to remember!
1. Wire diameter.
This affects rate since greater diameter wire is stronger than
lesser diameter wire. So, when wire diameter is increased, spring
2. Mean diameter of spring. Mean diameter is the overall outside
diameter of the spring less one wire diameter. When mean diameter
increases, the spring rate decreases.
3. Active coils. Determination of the number of active coils varies
according to spring design. Count the total coils minus two for
springs with both ends closed (includes all AFCOILS). Count the
total coils minus one for springs with one end closed and one
end open. As the number of active coils increases, the spring
If a spring's rate is linear (most racing springs have linear rates)
its rate is not affected by the load put onto the spring. For example,
a linear rate spring rated at 500#/inch will compress 1" when
a 500# weight is placed onto the spring. If another 500 pound weight
is put onto the spring the spring will compress another inch. At
this point the load on the spring has increased to 1000 pounds.
The rate of the spring, however, remains constant at 500#/inch.
If the load
put onto a spring increases the rate of the spring, the spring is
said to have a progressive rate. Progressive rate springs are sometimes
used on torque arms to absorb engine torque. Keep in mind that the
load (or preload) put onto a progressive rate spring can greatly
increase the rate of the spring.
rate springs are made by varying the spacing between the springs'
active coils. During compression the close coils bottom out and
deaden. This reduces the amount of active coils and spring rate
increases as a result.
are designed to include coils of different diameter or are wound
using a tapered wire will also produce a progressive rate.
Most coil springs
are actually progressive to some degree -- as we will learn later!
of Coil Springs:
There are basically
three different spring designs presently used in race cars. They
TYPE I: Closed
and ground on both ends (Coil-overs and rear conventional springs
are this type).
TYPE II: Closed both ends but ground one end only (Conventional
front springs are normally this type).
TYPE III: Closed and ground on one end and open on the other end
(Similar to a conventional spring that has been cut).
The 3 springs types are used in different situations and provide
different effects to rate. Since the designs are so varied, it only
follows that the dynamics of each design are also varied (more later).
You must remember, however, the only factors that affect spring
rate are wire diameter, mean diameter, number of active coils.
Spring Rates Change Dynamically:
Keep in mind
that as a coil spring compresses, the inactive (dead) end coils
gradually contact adjacent, active coils. The contact causes the
active coils to deaden which increases the rate of the spring. The
rate creep that results usually stops after the first inch of spring
travel and does not appear again until spring travel approaches
coil bind. Generally speaking, this type of rate creep is of little
consequence with springs softer than approximately 500#/inch. When
you use springs stiffer than 500#/inch rate creep becomes more pronounced.
It is important
for you to realize that springs will pick up rate during compression.
Consequently, the rate marked on a spring can differ from the rate
as seen by the chassis. This is especially true whenever a spring
manufacturer rates springs based on the first inch of compression.
springs used for this type of application are designed with one
end coil that closely matches the lower arm helix spring seat, a
serious amount of rate creep can result. To minimize this type of
rate creep, a conventional front spring should be wound with its
bottom end closed so that it sits squarely in the helix seat. No
active coil should touch the seat (just like the original production
spring for which the control arm was designed -Type 2 spring).
When built in
this manner, a coil springÂs only contact with the lower control
arm is through an inactive (dead) coil (just like the spring's contact
with the weight jack). Consequently, as the spring compresses, the
number of active coils in the spring is not affected by the lower
control arm. Therefore the spring's rate remains constant throughout
normal suspension travel. Some rate creep still occurs due to contact
between the dead end coils and the adjacent active coils as was
explained earlier, but the amount of rate creep is miniscule compared
to the rate creep produced by an open end coil spring.
If a spring
has an open end coil(type #3), the open end coil is active but gradually
deadens as the lower control arm moves against the spring. A considerable
increase in spring rate occurs until the open end coil is completely
seated in the helix.
during a test a 1500# open end coil spring gained 464 lbs. of rate
after 2 inches of spring travel. By comparison, a 1300# closed end
coil spring gained only 48 lbs. of rate after the same travel.
of a series of open end coil springs produced rate creep so inconsistent
that at some points of spring travel the springs did not remain
in the same rate order of softest to stiffest! It would be very
difficult to make predictable handling adjustments using springs
that exhibit such inconsistencies!
Keep in mind
that any load change to an open end coil spring (via static weight,
wedge, chassis roll, bumps, etc.) usually causes the spring's rate
to change and, consequently, handling to change. If you are using
open end coil springs you should chart their rates from static loaded
height to fully loaded height weight(in one inch increments). You
should compare this information before making spring changes. By
now you should realize the importance of using springs that are
designed to keep rate creep to a minimum.
is Spring Stress?
As was pointed
out earlier, the rate of a spring is determined by its diameter,
the number of its active coils, and the diameter of its wire. Since
most racing springs are built to a fixed diameter, a spring designer
must decide on the diameter of wire and the correct number of active
coils needed to produce the desired rate.
If the designer
chooses a smaller than normal diameter of wire (which tends to soften
rate), he will have to compensate by using fewer active coils (which
tends to stiffen rate) to achieve the desired rate. There are two
possible reasons for a spring designer to use a smaller than normal
wire diameter for a specific rate spring:
1. The ideal
diameter wire may not be made and using the next larger wire (which
requires more active coils) would produce a spring with insufficient
spacing between its coils. This could cause the spring to bind during
2. Cost could be the prime consideration and by using a smaller
diameter wire and fewer coils (shortening the length of wire used)
material cost is reduced. Unfortunately, many racing springs are
built this way and these springs can cause a multitude of problems
for the chassis tuner that we will cover.
mistakenly believe extra spacing between the coils of a spring indicates
a preferable spring. While a spring must have sufficient stroke
capacity it also must have sufficient material to absorb the load
put onto it. If the spring's material is not sufficient for the
load put onto the spring, the material will become over-stressed
and the spring will take a set (lose height). Handling, of course,
is affected and the reason is not always apparent to the racer unless
he pays close attention to his springs.
Example: A typical
asphalt late model set-up calls for a tremendous amount of load
on the left rear spring (upwards to 600 pounds more weight than
on the right rear spring). When the chassis sees normal spring travel,
the cumulative load on the left rear spring produces a tremendous
amount of stress in the spring. If the spring does not have sufficient
material to handle the stress (as many don't), it will take a set
(as many do) and the car will lose crossweight and tend to become
loose off the corner. Excessive spacing between the coils of a spring
is usually an indicator of a potential problem with spring stress.
Consideration in Spring Design
because of the long stroke requirements for certain rates of racing
springs, material strength must be sacrificed to achieve significant
stroke. Couple this with the fact that the ideal wire diameter is
not always made and you can see why some springs have a real potential
to take a set. We have seen some brands of springs lose as much
as 15/16" of free height during normal operation. To eliminate
any set from occurring at the race track, it is good manufacturing
policy to pre-set (press to solid height) all racing springs during
If done correctly,
pre-setting will generally eliminate any potential for additional
set, even when springs are designed with smaller than ideal wire.
Shot-peening will further enhance a spring's durability. It should
be pointed out that all Afcoils are pre-set and shot-peened during
if a Spring "Sets"?
When a spring
takes a set it will normally stabilize at its new height. The rate
effectively remains the same since no appreciable changes have been
made to any of the three factors that determine the spring's rate.
Other than creating a need to readjust the chassis (to restore the
original set-up and ride heights) the spring should provide satisfactory
performance. It is not uncommon for even well designed and properly
manufactured springs to settle up to 1% of their free height. It
needs to be pointed out, however, that in cases where a poorly designed
spring is subject to extreme over-stressing, the spring's height
may not stabilize. The spring may continue to change height (both
shortening and lengthening) as the spring is worked. As a result,
the set-up on the race car changes every time the spring's height
changes. This can cause major chassis tuning headaches!
that you monitor the free heights of your springs on a regular basis.
This is so important that some Indycar teams measure their springs'
heights to the thousandth of an inch. Be sure to always measure
height at the same point on the end coils(mark your springs to indicate
the measuring point). You should suspect that a spring is setting
whenever wheel weights continually change. Under no circumstances
should springs be used that change more than 2% in height or do
not stabilize in height.
At the least
you should inspect all springs for free height changes after racing
on a very rough track or if your race car was involved in a wreck.
By now, you should realize there is much more chance for a spring
to change its height than its rate. Consequently, you should spend
your time monitoring your springs' free heights and not their rates!
is Coil Bind?
Coil bind occurs
whenever a spring is compressed and one or more of the springs active
coils contacts another coil. The rate of the spring increases whenever
a coil binds since the bound coil or coils are no longer active(this
changes one of the three rate-determining factors). Of course, handling
is affected whenever a coil binds. If the spring is compressed to
solid height (all coils touching) during suspension movement, the
suspension will cease to work. You can, and should, check for evidence
of coil bind by examining the finish between the active coils. If
any coils have bound the finish between them will show contact marks
that appear as though they were drawn with a lead pencil. Normally
any spring that is binding should be replaced with a taller spring.
Be aware, however, there are racing springs on the market that are
built with wire that is heavier than what's needed. These springs
will coil bind before others that are built with the proper size
Under very extreme
conditions, coil binding can cause a spring to unwind slightly.
This can cause the mean diameter of the spring to increase and reduce
rate of the spring. You should realize that the potential for coil
bind is increased whenever short springs are used. Always match
the spring to the job.
have lengths greater than 4 times their diameter will have a natural
tendency to bow when loaded. Consequently, tall springs tend to
bow more than short springs, and small diameter springs tend to
bow more than large diameter springs. Generally, the more a spring
is compressed the more it will tend to bow. Keep in mind the rate
of a spring will increase if an active coil rubs another part of
the race car. Here are some tips to minimize bowing:
correctly fitting coil-over hardware or install weight jack assemblies
so that the spring mounting surfaces are kept as parallel as possible
during suspension travel..
• Use springs that do not lean excessively (when positioned
on a flat surface). This indicates that the ends are ground parallel
to each other. This reduces the tendency for a spring to bow.
You should check both ends.
• If a coil-over spring is rubbing the shock, try reversing
the spring so the bowed part of the spring is around the shaft
where there's more clearance.
• Use coil-over springs that have straight sides rather
than an hour glass shape. This maximizes the clearance between
the shock and spring.
• Use springs that are wound straight. You can roll the
spring on a flat surface to check for straightness.
we know of no reasonably priced spring checker that will accurately
measure a spring for rate. We have tested most brands of checkers
and cannot give recommendation to any. However, there are steps
and procedures that can increase the reliability of the spring rate
checkers commonly sold to racers. The accuracy of a spring checker
should be monitored. This can be done through the use of a checking
spring. A checking spring can be any spring that has been accurately
rated at one inch (or smaller) increments up to a load close to
the total capacity of the checker. It is important that the free
length of the checking spring remain constant. The rates given by
the checker can be compared to the known rates of the checking spring
(at each increment of compression). Any rate discrepancies between
the checker and the checking spring should be noted and taken into
consideration when checking for rates of other springs.
of a spring rate checker should also be monitored. Simply put an
old spring in your checker and preload it to at least 20 lbs. Then
compress the spring and note gauge readings at 1" increments
(or less) for the next three or four inches of spring travel. Tag
the spring with this information and use it occasionally to check
for repeatability. Make sure the free height of the spring remains
constant. Do not use the spring if any change in free height occurs.
A checking spring can also be used to check for repeatability. A
rate checker should consistently repeat rates to within 2.5%.
Some Final Points on the use of Spring Rate Checkers:
use similar preloads when checking different brands of springs.
It's best to preload springs to a height equal to their loaded
height (as installed in the race car) before checking for rate.
This simulates what the race car sees for spring rate.
• Use a dial indicator to measure travel.
• Take dial indicator readings as close to the spring's
center line as possible. Readings taken very far from the springs
center may not allow for any rocking of the spring seat which
distorts the actual amount of spring travel.
• Realize travel indicated stiff springs can flex the framework
and fixtures of portable checkers. Consequently, the spring compresses
less than its indicated & the rate shows softer than actual.
• The dial indicator should hold steady whenever rate readings
are being taken. If the indicator moves, suspect the units framework
is flexing or there is a problem with the units jacking device.
• Checkers equipped with load cells tend to be much more
accurate than checkers equipped with hydraulic gauges.
• Avoid checkers that allow the spring seats to rock in
any manner or amount.
• Always use the proper spring seats.
• When using a helix type spring seat make sure the spring
is positioned against the stop in the helix.
We have pointed
out the more important features you need to consider when choosing
and using coil springs. You should now have some basic understanding
of the differences between springs and how those differences affect
By knowing more
about springs you will be able to confidently select springs that
suit your application and expect that they will give consistent
and trouble free performance.