Extreme CJ 1/4 Elliptical Rear Suspension


My extreme suspension modifications did not end with the SOA front end, in fact it was only getting started. My SUA 4" lift rear springs had sagged a little and it was time to do something different. I knew I wanted a 1/4 elliptical setup because I had seen how they work on the trail.


Gathering the parts

I ordered the Rod ends from National rod ends: Part number RMX10T and RMXL10T. These are 3x4" x 5/8" high strength rod ends in left and right hand thread. A hoop kit for the rear axle and 1/4 elliptical springs were ordered from Avalanche Engineering. I picked up some 11/16ths id. x 1 1/8th OD. D.O.M. tubing from a local stock car chassis fabricator. Various sizes of mild steel flat stock, channel and angle were also used.

Stripping to the Frame

I started by stripping everything from the transfer case back completely off the frame. This included the gas tank, rear bumper, rear axle, rear drive shaft, rear springs, rear shackle hangers, and the rear brakes. The only brackets left were the original main spring eye hangers which were left because the Avalanche Engineering 1/4 elliptical spring packs utilize them.


Rear Axle Brackets

Because of the number and complexity of the brackets that would be welded to the rear axle it was recommended that the rear axle be completely stripped. To further reduce the weight of the axle assembly and make moving it easier, I also disassembled it, except for the ring and pinion gears while I was working on it. After this the rear hoop assembly for the axle was mocked up. Avalanche Enginering supplied the gussets and the hoop. Because the gusset plates that come with the hoop get welded directly to the rear differential we tested the position several times before finally welding in place. The hoop fit great but we did have to do some additional minor work to the gusset plates to achieve just the fit that we wanted.


Calculating the Axle Position

The next step was to put the axle in place and figure out where the control links would tie into the axle and the frame. I used the output of the transfer case as the center line for the pivot point. With that in mind the frame mounts are dictated by your engine and transfer case combination. After getting an idea of how the frame mounts would be laid out we moved on to the axle. I started by fabricating the mounting points for the lower links. To ensure the mounts would be sturdy we used 3/16" steel plate to fabricate all of the brackets. I located them as close to the outside of the axle as I could and still maintain adequate tire clearance. Then I moved to the frame end of the lower links and fabricated some mounts out of 3/16" thick 2" tubing. I used an 5/8ths sleeve through the tubing as a bolting point for the link.


Cross Member for the upper links

For the upper links I made a new cross member that mounted on top of the frame above the rear output of the transfer case. 3" U-channel was used for the cross member. It needed a U-shaped bend in the middle to clear the rear output on the transmission. Myron, the person who was kind enough to allow me to use his shop, is a blacksmith. Myron fired up his forge and they spent the next few hours putting the bend in the U-channel. Angle brackets were used to attach the U-channel to the frame. The channel was centered over the output and attached to the frame. After the cross member was in place the upper links were test positioned to check the link geometry.


Upper Link Brackets at the Axle

There were about three angles that had to be considered when fabricating the brackets to connect the upper links to the axle with the curvature of the hoop. Cardboard templates were made first and then they were used to fabricate the metal pieces. After several slight changes in shape they were welded to the rear axle hoop assembly.


Mocking Up the Axle and Checking Alignments

When the brackets for the link arms were complete the rear axle was moved into place and the correct position for these components was determined. Several measurements were taken to get the wheelbase, pinion angle, and approximate ride height just right. After we had the axle well placed we made measurements to determine the length of the locating bars. The next step was to cut the D.O.M. tubing, true it on the lathe, drill to 11/16ths, and then tap a right and a left hand thread for the rod ends on each end. Then the rod ends were screwed in each end and the rods assembled into the Jeep. After assembly it was clear that we had missed on the angle on the upper cross member brackets by a few degrees. So, out the cross member came and then reinstalled with the angle reset correctly.


Mounting the Springs

Finally it was time to turn my attention towards the the rear springs. I ordered the springs with a box that allowed me to use the existing spring hanger as the attachment point for the rear springs. The springs were put into place and pre-loaded. Then the springs were clamped in place and the shackle position was calculated. A rear shackle pivot point was attached on the rear axle using some plate, D.O.M. Tubing, and some urethane CJ shackle bushings. The shackle was bolted to the spring and the axle set to ride height.


Bump stops for the Front End of the Springs

The Jeep was sitting to low at this point and the springs had literally no up-travel. A spacer was needed to bring the Jeep up to the desired ride height, so it was back to the drawing board. We tried a few bump stops from various vehicles but none were to my liking so it was decided to fabricate something out of steel. This turned out to be difficult because the angle of the frame and the angle of the spring were different. After much trial and error the brackets had the correct angles and were bolted through a sleeved hole in the frame.


Testing

With all the links mounted and the springs in place the Jeep was driveable. The first order of business was to take the Jeep out and ramp test it on my trailer. The deck height on my trailer works out to equal about the 1000 mark on a 20 degree RTI ramp. Clearances were measured for bump-stops and shock mounts, then it was back to the shop to fabricate them. Another cross member was added above the rear axle housing out, made out of 2" 3/16" wall tubing. Shock studs were placed in the center, about three inches apart. The cross member was worked into place and when the position appeared ideal it was welded to the frame. Then it was back out to the trailer to ramp it again and decide on the ideal place for the lower shock mount on the axle tube. After one aborted try at mounting on top of the axle tube the mounts were moved the front side of the axle tube, the angle set and welded in place.

Driving and Ramping Impressions

I think the pictures of it on the ramp tell the story of how it flexes. I won't elaborate more except to say it is just an unreal sensation with it flexing this way yet feeling completely stable. You wouldn't believe the contact pressure the rear tire that is dropping still has. My major concern was how would it handle on the street. After a maiden voyage after getting the exhaust done I would have to say my fears were misplaced. The suspensions feels ultra stable, tracks straight, and gets no more body roll than any other spring over Jeep. But - get on the throttle a bit from a standstill and man does the rear end hook up, you can just feel the tires plant and the Jeep just moves smoothly forward with amazing control. In the end was it worth it. I spent in excess of 120 hours working on the Jeep in an incredibly well outfitted shop. This is not my daily driver. If you are contemplating this type of suspension for your daily driver and you don't have a shop and tons of fabricating ability this is not the suspension modification for you unless you are going to have a professional shop fabricate it and install it for you. Even after all the time and effort that I ended up putting in, I would say that yes this suspension is worth it. The flex is unreal and it still drives well on the road. More pictures of testing the flex are available on my web page.

Acknowledgements

That said I would like to thank several people for their help in the project. I would like to thank Myron Hanson for letting me take over his shop for two months. My wife for not complaining too much that she hasn't seen me in those two months. Steve at Avalanche Engineering for fielding my many calls. John Nutter for bouncing ideas off of. And lastly the guy the rolled his CJ at Moab for putting the fear of good in me and making me make sure I over engineered every part of this system. The power of picture is just amazing.

Year Two thousand Updates

Well after running the 1/4 elleptical a year there were some shortcomings with the originol design. Some of these were addressed on the maiden voyage to Moab and were promptly fixed. It was discovered that the D.O.M. I had used for the lower bars was not thick enough. These were promptly sleeved with 1/4" wall piece of D.O.M. I also became apparent that the lower bolts had a tendancy to bend. This was remedied by adding a strap to capture the lower bolt on the front mount. Also the 3" u channel for the upper mount was not strong enough and had bent. This was replace by steel tubing 3x1.5x.250 wall. This was configured the same as the previous crossmember and bolted in place.

This fixed the problems with the initial design and things would have been okay at this point. After all the Jeep had awesome flex and looked great when taking pictures. But on the trail it didn't feel very stable and it just wasn't performing as well as I thought it should. So when it was decided to upgrade the rear end to a HP 60 axle some changes were made.

At first thought putting the lower link out by the tire seemed to make good sense. But I soon found out that this just made the lower bars giant rock catchers. Also the lower mounts were below the frame and this further aggreviate the problem. So it was decided that the bars needed to be moved inside the frame and moved up higher. A box was built on the frame to capture the front end of the bar and to act as a skidplate mount for the t-case. This brought the front mount up below the bottom of the frame. The lower links were welded to the axle just below the centerline of the axle and much farther in than previously.


The second area that was addressed was what I felt was the uncontrolled droop in the rear axle. I came to the conclusion that flex like all things should be enjoyed in moderation. I didn't like the way the swinging spring mount provided no resitance to the axle drooping. I decided the front of the spring should be solid mounted. I tested this by welding the previous design togther and testing it. I was very happy with the results. It made the Jeep feel much more stable and the drooping was more controlled. But this was to much stress for the stock spring hangers and they soon ripped from the frame.

It was decided that the box spring mount would stay but it would need to be solid mounted. The mounts would have to be strong and durable. I fixed the damage to the frame and then added a 1/2" plate that was drilled to accept the box. These were then welded on both sides of the frame. Two 5/8" bolts were then used to attach the box. The front hole utilized the same hole that was already in the spring box so it was located were the stock spring holder located it. A small piece of D.O.M tubing was added to the top of the box and the secong hole was located. By moving this hole will allow for some adjustment in ride height. Also the D.O.M. could also be moved closer or further to the box to gain or lose ride height.


This year I have been much happier with the way the thing is handling on the trail. On the street I can't really comment as it doesn't see enough road time anymore to matter. But I feel those slight changes made all the differance in the world. No revisions are planned for this year.

Important Disclaimer:

This information is a personal account of modifications done to one particular Jeep and is not necesssarily endorsed or encouraged for any other application. As with any suspension or steering modification the handling and safety of the Jeep is affected. In some driving circumstances this or other modifications may be dangerous. The author only provides commentary and opinion based on his experience, your experiences could be significantly different. Mike Knorr and BC4x4.com cannot predict how any given modification will be used or what it will be subjected to. We do not recommend or endorse this or any product for any specific or general application. Anybody attempting this or any other modification does so at their own risk, we do not assume any liability. Accept no compromises in any part of a custom suspension setup. It must be perfect, your life and the lives of those around you depend on it.