Anting2 atau shackle tak harus lebih panjang atau lebih pendek. Yang standar sudah teruji oleh para insinyur Jepang karena akan berdampak pada gerakan per daun. Simak artikel berikut:
http://www.quadratec.com/jeep_knowledgebase/article-93.htm
Shackle Length and Their Impact on Spring Movement
Lets discuss shackles with leaf spring suspensions:
- What do we need shackles for in the first place
- Pros/cons of longer/shorter than stock
- How longer/shorter shackles affect wheel travel
- How to determine the proper length of shackle for your setup
- Shackle inversion (it’s a bad thing)
- Bracing
- Shackle shape and what it does
- Shackle Mechanics
What do we need shackles for in the first place?
Simple, to allow for length changes of a leaf spring. A
leaf spring suspension is a pretty simple thing, leafs position the
axle under the vehicle, and supports the weight of the vehicle. As a leaf spring flexes up or down, its length from eye to eye changes. Since
one end is mounted solidly, and cant move, all the length change
happens at one end, which has a shackle between the spring and frame to
allow for movement.
What are the pros/cons of different shackle lengths?
In relation to stock length, there are a bunch of things affected when you change shackle lengths.
A longer shackle will move one end of the leaf spring further away from the frame, doing several things.
- It rotates the axle down at the shackle side of the spring.
- It decreases castor angle of the front axle
affecting the steerings stability at high speed, and reduces the
steering ability to return to center after a turn.
- It increases pinion angle
- It increases leverage on the shackle mount
- It increases droop (down) travel
- It decreases approach/departure angle
- It adds lift to the suspension in the amount of half the difference between the new shackle and the original shackle. Example: stock YJ shackle is 4” from bolt to bolt; replacing it with a shackle 6” from bolt to bolt will provide 1” of lift)
While some of these can be good things, and some bad, it depends on your setup which one is which. If
you’re starting a build, then these traits can be accounted for and
made to work to your advantage when it comes to fine tuning a suspension
to get maximum performance from it, which brings us to…
How do longer/shorter shackles affect wheel travel?
Bear with me here, this digs a little deeper into how leaf spring suspensions work. To start with, a shackles length can dictate overall travel, both droop and compression. Let’s start with the leaf spring. Stock
jeep leaf springs sit pretty flat with the load of the jeep on em, not
so with lift springs since they have a larger arch in order to provide
that lift. Since lift springs are designed to
bolt in with no changes to the mounts, the eye to eye dimension of the
spring is the same as stock, but with the additional arch, the main leaf
of the spring pack is actually quite a bit longer. So,
in order for a lift spring to compress all the way to being flat, the
eye to eye dimension is going to be greater than the stock spring. If
the shackle is not long enough to allow this to happen, the spring will
bind before it gets flat, if force is applied after this bind point,
the spring pack will flex into a W shape, fatiguing the springs and
shortening their lifespan.
How a shackle affects droop travel is a bit easier to picture. When
a spring droops, it will do so until its eye to eye dimension is
limited, it hits this limit when the shackle is in line with the arch of
the main leaf of the spring. A longer shackle
will allow for more droop travel since when it reaches the max droop
angle, the shackle end of the spring will be further away from the frame
then it was with the stock length shackle.
In
the this picture you can see the shackle closest to the camera is
almost at its maximum droop travel, its almost in line with the leaf
spring.
You can also see just how much a shackle
has to move to allow for suspension travel. Now we get to figure out
just how long a shackle needs to be to work properly with our suspension
setup.
Its
best to start this while actually installing a lift, or whatever other
work you may do that involves taking apart your suspension. Get a
length of string, any ole string will do, its just going to be our
measuring stick for this operation. Tie it to the fixed spring mounting
bolt, good and tight so it wont come off. Now, lay the string along
the main leaf of the spring so it follows the arch. Mark the string
where the shackle bolt would pass through the spring eye. You now have
the total length of the main leaf. Pull the string tight, and rotate
your shackle forward and see if it reaches the mark you made for spring
length at a 45-50 degree angle. If it doesn’t, then measure from the
mark on the string to the shackle mount, and this is how long you need
the shackle to be for your suspension. Be aware of the other things a
longer shackle will do to your suspension, and be able to account for
them when you make the change.
What the heck is shackle inversion, and why is it a bad thing?
Shackle inversion usually happens on the front of jeep YJ’s and CJ’s when the suspension is drooping to its maximum point. As we discussed earlier, this is when the shackle is in line with the arch of the main leaf of the spring pack. While
at this point, if something hits the shackle while moving forward, its
possible, and probable, that the shackle will fold backwards instead of
returning to its normal position when weight is put back on the wheel. The shackle folding back against the frame, shortens the eye to eye length of the spring, possibly bending it. Shackle
inversion is usually pretty simple to remedy, a lever of some kind is
used to pry the shackle away from the frame and past the point of max
droop, and then the spring will do its job and spring back to where it
should be. Be careful tho, as the spring attempts
to spring back, it has some stored energy that will be released pretty
quick like, possibly taking your lever and throwing it into the woods. Shackle
inversion can be avoided by installing stops on the frame to limit the
shackles travel, or designing a stop into the shackle itself. Some
folks use boomerang shackles in the front with the short leg at the
frame end so that the center brace acts as a stop, but its not what
boomerang shackles were designed for, we’ll get to that in a minute.
Why do some shackles have braces welded in while others don’t?
Bracing between the two sides of a shackle can help or hurt, depending on what your goals are. To explain we’ll have to go back to how a leaf spring works a bit more. As
the axle attached to the springs articulates (one side in compression,
the other side in droop), the spring still has to do its two jobs,
locating the axle, and supporting the weight of the vehicle. As the axle twists, the springs must change length, as well as twist with the axle. That’s
the reason for rubber or polyurethane bushings in the spring eyes, they
allow the spring to twist in relation to the bolts holding it in place. While
this twisting is taking place, the sides of the shackle will also move a
bit in relation to the spring, one side being slightly ahead of the
other as its pushed by the bushing. Adding a
brace (or a bolt with sleeve) between the two sides of the shackle will
stop the shackle from twisting forcing the bushing to do more work, and
transferring some of the twisting force to the shackle mounting point on
the frame. Remember I said a shackle brace can help or hurt depending on your goals? If
your goal is road handling like a sports car (don’t know why you would
want this from a jeep, but whatever), then the brace is a good thing as
it will help to firm up the suspension as the body rolls around corners. If your looking for the last bit of available wheel travel, then the solid braced shackle can hurt a bit. When is a braced shackle really needed? When the shackle length increases over about one inch over stock. As
the sides of the shackle get longer, they will flex between the bolts
as force is applied to them, adding a brace will reduce this flex, which
can be a huge negative with long shackles, as they induce a vague
feeling to the handling and allow the springs side to side movement to
increase.
Now on to shackle shapes, and how they affect things.
Up until a few years ago, shackles were simple, strait pieces of metal, then came the tow shackle, and the boomerang shackle. The
tow shackle is pretty simple, still 2 pieces of metal, but provisions
have been made to allow the hookup of a tow bar for flat towing. Sometimes
they are run backwards so that the tow bar attachment point is towards
the axle of the vehicle, and with the pin installed will act as a
shackle stop to reduce the chance of shackle inversion. The boomerang shackle was origionally designed for the rear of YJ’s. In
the rear of YJ’s with lift springs, as the suspension compresses, the
longer length of the main leaf and the added travel the shackle has to
account for it would cause the shackle to contact the rear cross member
limiting travel. Back in the day, before boomerang shackles, we’d just notch the crossmember where the shackle hit and things would be good. The boomerang shackle was just a bit more BLING way of getting around the problem. Boomerang
shackles are also used by some in a front application with the short
leg towards the frame to counter shackle inversion. Some
mistakenly think that the different shape of the boomerang shackle will
alter the angle of the shackle or how it travels through its range of
movement. This is not true, the shackles angle is
measured in a strait line between its mounting points, it can be a
boomerang, zig zag, strait, or round, the shape does not change the
angle of the shackle. On an unrelated note, this
is why bent drag links and trac bars don’t change bump steer, the
mounting points dictate the angle, not the shape of the bar.
Shackle Mechanics
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 shackle?s effect on spring rate. This second effect occurs whenever
the shackle swings in its arc and moves the rear spring eye vertically.
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.
