Suspensions have come a long way since the introduction of the first buckboard stiff, leaf- sprung flatfender Jeep over 70 years ago. Custom four-link Jeep suspensions have become increasingly popular among skilled fabricators, and for good reason. They provide an incredible amount of torque and axle control, articulation, and wheel travel. Unfortunately, a four-link requires a lot of forethought and knowledge of suspension geometry. Building a four-link that works properly isn’t exactly inexpensive or easy. We’ve actually seen some hack-job four-link suspensions perform worse than the leaf-sprung suspensions they replaced. After considering the advantages, disadvantages, and costs of several different suspension types, we settled on radius arm front and rear suspensions for our GPW project. Radius arms have been known to provide less articulation than a properly built four-link, but if you tackle the common radius arm issues, you’ll be surprised at what these easy-to-build suspensions can do.
There are three main things to address when building a functional radius arm suspension system. To achieve great articulation with traditional radius arms, you need large flexible bushings. Small and stiff bushings will limit movement and eventually become torn or damaged. Rigid rod ends at each radius arm mounting point will cause an incredible amount of binding that can tear your axlehousing and suspension bracketry apart.
The more vertical wheel travel you expect your Jeep to have, the longer your radius arms should be. As a radius arm suspension cycles, the steering caster and pinion angle change. This change can be a detriment to handling and driveshaft survivability. Longer radius arms cause less caster and pinion angle change than shorter radius arms over a given wheel-travel measurement.
Ground clearance is the third consideration. Many radius arm designs can impact ground clearance so much that they are not worth considering on a Jeep that sees off-road use.
Our radius arm suspension expedition started when we inadvertently bumped into a half-stripped ’90s Land Rover in the wrecking yard. We noticed the huge, high-quality, contoured rubber bushings that were clearly designed to specifically increase articulation. We also admired the beefy forged-steel construction of the arms. After a quick search, we found a local Land Rover wrecking yard. We asked the guy running the yard if he had ever seen the bushings wear out. He laughed, shook his head, and pointed to stacks of used radius arms, all of them with bushings in great shape. With a price tag of $40 for each complete used arm, we were sold. A single quality rod end, commonly used for a four-link, costs more than that!
We created a cardboard template for the radius arm mounts on the axle ends. This was carefully traced onto 3⁄16-inch-thick plate steel and cut out with a Miller (millerwelds.com) plasma cutter. We smoothed the edges with a flap-wheel grinder.
We looked at several different factory front and rear Land Rover radius arms that were offered over the years, but we eventually settled on the front arms from a ’99-’04 Discovery II for all four corners of our flattie. These arms seemed to offer a little more ground clearance than some of the others, and the bushings seemed like the easiest to adapt to our Jeep. Each bushing featured a molded-in steel sleeve with a 5⁄8-inch hole for mounting.
The idea is to keep our Jeep low for stability reasons and still have the suspension set with around 50 percent uptravel and 50 percent downtravel. This will provide a good ride on- and off-road at speed and while crawling slow. Since we’re only working with about an 81-inch wheelbase, there is no need for a ridiculous amount of wheel travel. It would simply make the Jeep overly tall or unstable on precarious sidehills and climbs. Most shorter Jeeps don’t need any more than about 10 inches of vertical wheel travel from full compression to full droop, so that’s where we set our goal.
We originally considered using compact and simple air shocks on our project. But after talking to the suspension specialists at Fox, calculating the weight, and considering the way we planned to use this Jeep, we were convinced it was best to go with a coilover shock—Although, it had to be a compact coilover to fit in the limited confines of our flattie. We are trying to keep the original body lines and don’t want shocks protruding through the hood or into the interior. Here is how it all came together. Stay tuned for the future segments where we’ll tear into our axles, set up the steering, and slap in a power plant among other things.
Step By Step
It’s important to consider the location of other major components when mocking up the axles and suspension links under the frame. We fit our Jeep with an FSJ steering box, draglink, and tie rod. We clamped the box to the frame to figure out exactly where the front axle should be placed.
The cast-in leaf-spring mount on our ’74-’79 Wagoneer front axle was exactly where our Land Rover radius arm wanted to be mounted. For strength reasons, we prefer to not weld to the cast iron Dana center sections. We chose to bolt a custom mount to this side of the axlehousing.
Half of the passenger-side radius-arm mount ties into the cast center section in several places. It’s made from gusseted 3⁄16-inch-thick steel plate and is bolted to the reinforcement rib of the axle. The forward mount ties into the differential cover bolts.
Our Land Rover radius arms are about 40 inches long, allowing the front and rear arms to nearly meet in the middle of our flatfender frame. This is a huge advantage because it places most of the suspension stresses on the strongest part of the frame. Heavy-wall tubing pierces both sides of the frame, creating a mounting boss for each radius arm. The arms are cinched in place with 5⁄8-inch bolts. A double-shear plate will be added to the outside of these arms to help prevent the bolts from shearing off or bending.
We mocked up the front and rear track bars using heavy-wall tubing and stiff wire to hold them in place while cycling the suspension for clearance. Be sure to leave plenty of room for the engine and other components.
Up front, you want your track bar to be about the same length as, and parallel to, your steering drag link. Because we are trying to keep this Jeep low for stability, we had to have some complex bends in the track bar to clear the differential and engine. Fortunately, we had a used aftermarket JK front track bar made from 11⁄4-inch, 0.250-wall DOM lying around. We cut it up and rewelded it to fit our brackets for the mockup you see here.
This is our rear suspension at full compression. Our rear track bar is made from 11⁄4-inch, 0.250-wall DOM and 3⁄4-inch Ruff Stuff Specialties rod ends. Always build the front and rear track bars to head down from the frame to the axles in opposite directions. Ideally, the front and rear track bars will be parallel with the ground at ride height. This provides the best handling. In reality, this is not always easy to do or even possible. You have to make some compromises when fabricating your suspension. The compromises you decide to make will be dictated by how you use the Jeep. Once you have everything mocked up, cycle the suspension in as many ways as you can to check for clearance issues.
We fabricated 3⁄16-inch-thick steel bumpstop mounts so that the suspension would compress to metal-on-metal before the axle makes contact with any engine vitals. These cool, compact, bolt-on foam bumpstops (GM part number 15712438) can be found on the rear of a ’07 Chevy 1500 4x4 pickup as well as other years and models. We ordered ours new from GM Parts Outlet (gmpartsoutlet.net).
At ride height, the equal-length steering draglink and front track bar on our Jeep are nearly flat. This will eliminate bumpsteer. We later added a bit more gusseting to the radius arm mount that ties into the diff cover.
We didn’t want to hack up the sheetmetal or build custom fenders to fit bigger tires, mostly because we like the factory flatfender body lines. So, we opted for OE-style body panels from Omix-Ada. The new GPW grille, Willys script hood, and CJ fenders mated to our battered GPW tub should keep people guessing as to what kind of Jeep it actually is.
To properly check for clearance around our fresh fenders and Fox shocks, we mounted up a set of 15x9 ATX Slab beadlocks to 33x15.50-15 Interco Super Swamper SX tires. It’s a beefy combo that will help put a lot of tread on the ground, especially when aired down into the single digits. They keep the Jeep low and wide for stability. We’ll be counting on a light overall weight, maneuverability, and a flexible suspension, rather than extreme ground clearance, to get us further up the trail.
Since we didn’t have a lot of space to work with, Fox recommended these 2-inch-diameter, 10-inch-travel emulsion coilovers. We really wanted to run the new Fox 2.5-inch DSC coilover shocks. They provide nearly unlimited compression and rebound damping control via two separate dials, but we just didn’t have the real estate to fit them. Flatfender Jeeps simply don’t have a lot of room to work with in the wheelwells, and we didn’t want the shock reservoirs intruding on the already-cramped engine bay and interior.
We estimate that the sprung weight of our Jeep will be about 500 pounds per corner completed. We wanted the 10-inch travel Fox shocks to sit in the middle of their travel at ride height. When you calculate the numbers, you end up with a 200-inch-pound spring rate. We ordered two 10-inch-long 200-inch-pound QA1 coils for each shock from Summit Racing. Summit offers many different coilover coils and rates so you can easily tune in your Jeep’s suspension.
It’s incredibly important to cycle the suspension and check for clearance and binding issues. We like to mock up the shocks in place with one coil installed. This gives us a better idea of where extra clearance might be needed, while still allowing the suspension to be compressed and articulated. Our upper rear shock mounts will sandwich the body and attach to the frame-mounted rollcage. Removing the nitrogen from one of your shocks simplifies the mockup process. In most cases, it’s best to start with the suspension fully compressed when selecting shock lengths, finding ride height, and figuring out the amount of wheel travel you want.
Up front, we bent up a pair of simple shock hoops from 11⁄2-inch, 0.120-wall DOM using a JD2 Model 3 bender. Even though our Fox shocks appear to be kicked back at a steep angle, this mounting doesn’t affect the motion ratio all that much and certainly nowhere near as much as mounting a shock in the middle of an A-arm or on a trailing arm. After checking for clearance and cycling the suspension, we switched to longer Ruff Stuff Specialties tabs and turned the lower shock mounts 90 degrees to eliminate binding during articulation. To protect coilover shocks from damage, install limiting straps that limit the axle droop about 1-inch before the shocks top out.