of the 4-link suspension is due primarily to its ability to let
the race car turn freely in the middle of the corner without compromising
forward bite. To understand how a 4-link can be made to provide
such handling, you must first understand a few basics about rear
that you can increase forward bite on any type of rear suspension
by angling the trailing arms upward toward the front of the race
car. Trailing arms mounted in this manner cause the rear tires to
try to drive underneath the chassis as the rear axle pushes the
race car forward (See illustration 1). As a result, the loading
of the rear tires (during acceleration) is quickened and forward
bite is enhanced.
There can be a handling trade-off, however, to the forward traction
gained by running the trailing arms upward to the front of the race
car. During chassis roll, trailing arm/s mounted upwards will cause
the right rear tire to move rearward (until the arm/s reach a level
position) and the left rear tire to move forward. The condition
is referred to as "loose roll steer". (See illustration
Loose roll steer causes the rear axle to steer towards the outside
of the race track. If excessive, loose roll steer can cause a loose
handling condition that negates the benefits of the forward bite
gained by running the trailing arms upward towards the front. However,
the right amount of loose roll steer can help a race car to turn
the corner correctly. At best, any trailing arm arrangement is a
compromise between forward bite and roll steer.
The 4-Link Difference
A well designed
4-link provides good forward bite and the proper amount of roll
steer. The two most critical factors to the performance of a 4 link
suspension are the link lengths designed into the suspension and
the angles to which the links are adjusted. The key to correctly
designing and tuning a 4 link is to understand the significance
of these two factors.
We stated earlier
that trailing arms mounted upwards to the front of the race car
enhance forward bite by using axle thrust to quicken the loading
of the rear tires. We use the upper links on a 4-link suspension
to enhance the forward bite. Upper link angles from 15º to
18º on the right and 10º to 15º on the left provide
good forward bite. A good starting point for both links is 15º
upwards (to the front).
in mind that chassis roll causes the link angles to change. If the
link angles become more upward on the left than on the right, the
left rear tire can become loaded more quickly than the right during
acceleration (due to the axle thrust effect). This condition may
cause a gas pedal push. One fix is to position the links so that
the right side link is from 3º to 5º higher than the left
when the chassis is at ride height.
Be aware that
trailing arms angled uphill too steeply can hold the chassis up
during acceleration which can reduce the effectiveness of the shocks
and springs. This condition will cause loose handling-especially
on rough race tracks. Keep in mind that trailing arm angles can
become excessive if the rear of the chassis lifts a lot during acceleration.
The length of
the upper links should be at least 17" . We can reduce loose
roll steer by making the lower links shorter than the upper links
(more on this later). If the upper links are shorter than 17",
the lower links have to be extremely short to minimize loose roll
steer. But extremely short links change their angles radically whenever
the suspension moves. When the rear links are too short forward
bite and roll steer are overly affected and handling becomes inconsistent.
We can use the
lower links of a 4-link suspension to help offset the loose roll
steer tendency caused by the steep angles of the upper links. The
following examples and illustrations should help you to understand
this important function of the lower links. You should pay close
attention to how the lower link adjustments change the paths traveled
by the bottom of the birdcages during chassis roll. Keep in mind
that any change to the path traveled by any trailing arm will affect
in illustration 2A, both the top and the bottom links move the birdcages
(and the rear tires) rearward on the right side and forward on the
left side during chassis roll. This action will cause loose roll
We can reduce
loose roll steer by lowering the bottom links at the chassis. You
can see how this adjustment works in illustration 2B. We've lowered
the bottom links to a level position and now the bottom of the right
side birdcage moves forward during chassis roll instead of rearward
as in illustration 2A. On the left, we have reduced the forward
movement of the bottom of the birdcage. As a result, loose roll
steer is reduced.
Basically, we've position the bottom links to counteract the forward(L.S.)
and rearward (R.S.) movements of the birdcages caused by the upper
links. As a result, we reduced loose roll steer. We can reduce loose
roll steer further by lowering the bottom links further as shown
in illustration 2C. Notice how this adjustment, positioning the
lower links 5ºdownhill, causes the bottom of the right side
birdcage to move forward more during chassis roll than in illustration
2B where the links are level. On the left side, the bottom of the
birdcage now moves rearward (until the link reaches a level position)
instead of forward as in illustrations 2A and 2B. Consequently,
a further reduction in loose roll steer results.
Generally, bottom link angles from 0º to 5º downhill (to
the front) are used to help control loose steer. Some forward bite
may be lost when the bottom links are lowered but the effect on
forward bite is usually minor relative to the overall handling improvement
that is realized by reducing loose roll steer.
used to reduce the loose roll steer of a 4-link suspension is to
shorten the bottom links. Notice, in illustration 2D, how the shortened
bottom link pulls the bottom of the right side birdcage forward
during chassis roll more than the longer links in the other illustrations.
The bottom of the left side birdcage does lose some of its rearward
movement because of the shortened bottom link. But since left side
birdcages typically move down much less than right side birdcages
move up during chassis roll, the overall effect, when shortening
the lower links, is a reduction in loose roll steer. However, if
the left rear of your chassis hikes up during cornering, loose roll
steer may increase whenever both bottom links are shortened!
We could reduce loose roll steer even further by combining the long
bottom link arrangement of illustration 2C on the left side and
the short bottom link arrangement of illustration 2D on the right
side. The preceding paragraphs should help you understand why.
The length of
the bottom links are dependent on the roll steer and traction characteristics
desired by the chassis tuner. For most track conditions, bottom
links 2æ shorter than the upper links work well. Short links(
from 3æ to 4æ shorter than the upper links) generally
work best for tight, flat race tracks or on any track where the
chassis tends to be loose. Long bottom links (equal in length or
no more than 1æ shorter than the upper links) work best for
fast tracks or on any track where the chassis tends to push. You
should use the information in this article to determine the correct
link lengths for your application.
However, a proven
4-link arrangement includes 15 1/2æ bottom links, mounted
5º downwards to the front, coupled with 17 1/2æ top links,
mounted 15º upwards to the front.
A 4-link birdcage
rotates or "indexes" on the axle tube whenever the suspension
moves (unless both upper and lower links are equal in length and
parallel to each other). Indexing is greatest when there is a lot
of length and/or angle difference in the upper and lower links.
causes the coil-over mounts, if located on the front of the birdcages,
to rotate against the shocks and springs during suspension bump
(compression) movement. As a result, the springs and shocks are
compressed from both ends at once and the suspension becomes very
stiff. (Try to bounce the rear of a car with a 4-link rear suspension).
roll, indexing loads the right rear tire and unloads the left rear
tire and wedge is reduced (40 lbs to 80 lbs is typical!).
improve driveability by keeping the race car flat in the corners.
However, indexing can cause the rear suspension to be too harsh
on rough race tracks. When selecting springs for your 4-link, you
should keep in mind the effect that indexing has on suspension stiffness.
are used to mount the coil-over units directly to the axle housing.
When clamp brackets are used in front of the axle, axle wrap-up
during acceleration causes the rear axle & chassis to separate.
The rear axle (and tire) are forced towards the race track.
are sometimes used on short, slick tracks to improve initial forward
bite. Mounting the left coil-over unit ahead of the axle (on a clamp
bracket) generally tightens corner handling. Mounting both coil-over
units on clamp brackets and ahead of the axle can improve forward
bite on stop and go or slick race tracks. On extremely slick race
tracks, you can tighten overall corner handling by using clamp brackets
to mount the left coil-over unit ahead of the axle and the right
coil-over unit behind the axle.
usually increases when the coil-over units are taken off birdcages
and mounted to clamp brackets (since there's no longer any indexing
of the springs). Consequently, it may be necessary to increase rear
spring rate when making this adjustment.
You should keep
in mind that any loading of the rear tires caused by clamp brackets
during acceleration will be accompanied by an unloading of the rear
tires during deceleration This unloading can upset the race car
upon corner entry -especially when both coil-over units are positioned
ahead of the axle and attached to clamp brackets. You may be required
to make chassis adjustments to correct any corner entry handling
problems caused by clamp brackets.
The 4-link is
a relatively complex rear suspension that is very sensitive to adjustments.
A link length change of 1" or a link angle change of 5º
can make a noticeable change to handling. When designing or tuning
a 4-link, it is important to understand the relationship between
link lengths and angles and how the relationship affects roll steer
and tire loadings.
We highly recommend
that you build a full-scale working model of your 4-link, or use
the design parameters mentioned in this article, to help you to
better understand the 4-link suspension. You can use cardboard,
wood, aluminum strips, etc. The idea is to trace the paths actually
traveled by the centers of the birdcages during chassis roll. You
should draw the paths to include at least 3" of rebound movement
for the left birdcage path and at least 3" of compression movement
for the right birdcage path.
You can evaluate
the roll steer characteristics of different set-ups by comparing
the different paths drawn on your model. You can also check the
indexing and the link angle changes during roll or bump. In short
you will speed up your learning process by working with a model.
As we stated
earlier, the 4-link is a fairly complicated rear suspension. We
hope the information in this article, combined with your efforts,
will provide you with an AFCO advantage!
the upward angle (to the front) of any link will enhance forward
bite and increase loose roll steer.
• Decreasing the upward angle (to the front) of any link
will decrease forward bite and reduce loose roll steer.
• Suggested angle adjustment parameters:
Links: 12º to 20º (upward) RS 10º to 18º (upward)
Links: -5º to +5º
• You may need to reduce link angles when using clamp bracket/s
and/or when running on rough race tracks.
• You may need stiffer rear shocks when using clamp brackets
(to control wheel hop).
• Angling the links inboard (at the front) tends to increase
loose roll steer.
• You can correct roll steer handling problems by leading
or trailing the right rear tire (or left rear).
• Shortened bottom links (especially R.S.) tend to reduce
loose roll steer.