4 Link Rear
My leaf sprung rear
suspension had served me well for 5 years. It was simple and
rugged but as the trails got harder, I was a little rougher on the
leaf springs. The rear traction bar I installed helped keep
axle wrap down but in the end the downfall of my leafs came down to
their flexibility. To have a soft ride and good flex you need
flat, low spring rate leaf springs. But this also means they
can easily be damaged if flexed too far, or for instance, trying to
back into a vertical wall. If the tires won't climb, the next
path of least resistance is for the leaf spring pack to bend.
If this happens when the axle is drooped, the main leaf takes the
whole load and that's usually the end of that spring (it looks like
the spring above). After going thru 4 spring packs in the last
8 months (2 on the last trip thru Twisted) I was ready to go to the
I knew I wanted to run
either a coil spring or a coilover shock. I had decided
against a 1/4 elliptical spring, while that setup is great if you
want a ton of cheap droop, I didn't like how the spring didn't
provide any down force. So the decision was between coil and
coilover. The coilover won out for two reasons, one being
there is only one thing to mount as opposed to a coil spring where
you also have to shoehorn some shocks into the mix. Mounting
shocks doesn't sound hard but wait until I get to the links.
The second issue was adjustability. With the coilover you can
easily change spring rates, pre-load, the dampening rate and you can
have dual spring rates (think of it as a light spring for the small
bumps and a stiffer spring to help control the weight of the rig).
Once I had decided on
how the rig was going to be suspended the method of locating the
axle could be planned. This is probably the most challenging
part of this whole suspension and it can make or break the
performance of the rig. Now let me just say I knew nothing of
suspension design before starting this project. I'd also like
to say there are limitless ways to locate the rear axle in a link
suspension, in coming up with my link design I researched what
others had done, what worked, and made a plan. Next I got
under my rig and modified my plan to fit the constraints of my rig.
Here's what I wanted:
4 links, all the
same length (only need one spare)
All 4 links would be
triangulated to spread the loads amongst the rod ends and link
No link mounting
points would hang down below the transfer case on one end, and
below the axle on the other end.
The lower links had
to stay inside the frame rail as much as possible. The
theory here was since I typically dragged on my frame rails, if
the links were inside I would be less likely to hang up on them.
I wanted a to avoid
any squat or anti-squat, basically wanted the rear suspension to
remain neutral on acceleration. To accomplish this the
upper and lower links will be as close to parallel as possible
when viewed from the side.
There's also a few
characteristics that need to be considered and are directly related
to how you setup your links.
Roll Axis - The axis
the vehicle rolls along. For example, a low roll axis will
cause the chassis to lean more, a roll axis close to the center
of gravity (CG) will make the suspension less compliant.
Rear Steer - How
much the rear tires get tilted when the axle articulates.
Hard to describe, see the picture below and notice how the rear
tires are not parallel to the fronts. This is directly related
to the link length and also roll axis.
There's no way I
can adequately explain how all this stuff works, so here's some of
the helpful information I found on the web. This stuff all
came from the Pirate
4x4 BBS, if you use the search feature it is a wealth of
A good discussion
on 4 link theories and design.
on 4 link design with links to some excellant .pdf files that
really help explain some of the concepts involved.
on 3 link design
Here's the second
sketch I made of the links. The first version had the lowers
converging inboard of the transfer case flange but I didn't think I
would have enough clearance for the drive shaft due to my offset
transfer case. My second revision had the lowers tied into the
frame as close to the CV joint on the rear drive shaft as possible.
The uppers would be tied to the top of the rear differential and
angle towards the frame. The sketch shows my lower links
attaching to the axle tube well above the centerline. This
will place an enormous amount of stress on the upper mount, in the
end I moved the lower mounts to the axle centerline after running
some numbers using the formulas found here.
Page - Page 2