4 Link Rear Suspension

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 next level.

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 mounts.

• 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.

• Anti-Squat - This is a term used to describe how much the back end will raise or lower when accelerating.  The tendency for the suspension to either squat or raise is related to the suspension link angles. Here's a helpful diagram that TNToy posted on the POR board a while back:

• 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 knowledge. 

• A good discussion on 4 link theories and design.

• Another discussion on 4 link design with links to some excellant .pdf files that really help explain some of the concepts involved.

• Discussion 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.