Sixty-some years ago, before our favorite four-wheel-drive vehicle earned the name Jeep, the Army’s 4WD ¼-ton scout vehicle was given the designation of “GP” for General Purpose vehicle. It was given this designation because the Army needed a small, agile vehicle that could do any job it was called upon to perform: carrying four soldiers, mounting a machine gun, towing a ¼-ton trailer, supplying ammo to the front lines, carrying wounded to the rear echelon, etc. This “GP” designation is one of the original reasons that GIs started calling it a “Jeep.”
“GP” was also in the back of our minds when we first conceived the idea for our latest Jeep project: a 2005 Jeep Rubicon Unlimited. We’ll be building it into the ultimate general purpose “GP” vehicle—an exploration rig that can be flat-towed behind a motorhome or tow its own camping trailer to and over the Rubicon. It may not be able to compete in the King of the Hammers, but it will be able to climb all of the Hammers—and drive there to boot!
We decided early on that we wanted an orange Rubicon LJ with an automatic transmission. We wanted a Rubicon because of its Dana 44 differentials, its heavier-duty transfer case with 4:1 low range, its selectable lockers, and its four disc brakes. The Rubicon Unlimited is one heck of a machine right off the dealership’s floor, but it still needs help to become an extremely capable trail runner.
In this first part of our two-part series, we’ll cover the gears, suspension, and tires. With the exception of a few parts, all the steps are interchangeable between the LJ and the TJ.
Axles and Gears
Starting with the strongest Wrangler available made our job much easier, but even more strength was our goal when we were deciding what our Jeep needed in its upgrade. We were keeping the original engine and transmission combination, but we opted to upgrade the axles.
We chose an RCV Performance front axle set. RCV axles use constant-velocity (CV) joints instead of the more common—and less expensive—U-joints. Not only are CVs smooth when at full lock turns, they are also far stronger. The RCV Dana 44 front axle kit comes complete and ready to install, including a tube of special-compound axle grease to fill the CV joints just before installation. The kit also includes a tool that aids in the installation of the orange urethane boots that protect the CV joints from moisture and debris while on the trail. (Hint: If you leave the axle ends exposed—i.e., don’t use the hub caps that usually come with new wheels—it’ll aid in the axles’ annual maintenance later on. You may need RCV’s Needle Grease Tip, Part #100-24, to shoot the grease into the axles’ ends.)
In past builds, other than adding trusses to the differential housings and installing deeper gears, we’ve pretty much left the rear differentials alone. However, this build was different, since we planned to hit the trails with a fully built Jeep rather than taking months to build it piecemeal. We chose a pair of Alloy USA chromoly axles, which are strong enough to twist our Yokohamas over Jackhammer boulders without snapping or even stressing an axle. The high-tensile 4340 chromoly rear axle shafts are heat-treated and induction-hardened to have twice the tensile strength of stock axles and are up to 40 percent stronger overall due to the utilization of the latest cold spline technology.
Why should you upgrade the rear axles?
The cost is the same or less than OEM replacement axles, but the big reason is that horsepower is useless if you can’t get it to the ground. And large, high-traction tires—such as our Yokohama Geolandar M/T 35-inch tires—can break or twist a decade-old OEM axle shaft.
But what were the axle splines going to slip into?
We could’ve left the stock gear set in place, but 35-inch tires can really overtax 4.11:1 gears on the highway. And we’ll be towing our custom camping trailer on most of our on-road excursions and many of our off-road expeditions, so we don’t want to lug the engine or cause the four-speed automatic transmission to be hunting for the right gear all the time.
We needed a gear set that kept the engine’s rpm at the same rpm or slightly higher than the factory-gear/tire-height setup, so we used Summit Racing’s gear comparison calculator. The calculator asks for the gear ratio, tire height, and miles-per-hour. Once you have the stock rpm, you input the variables that you are considering (tire size, gear ratios, etc.) and note the resulting rpm. With the Yokohama Geolander M/T’s 35-inch diameter, we determined that 4.56:1 gear ratios would give us a few hundred rpm above the stock gear/tire combo, which we thought was perfect for towing the trailer. Plus, we’d also get the advantage of increased rpm multiplication in the lower transmission gears for hill climbing and rock crawling.
We chose Yukon Gear and Axle for the two gear sets and differential rebuilding kits. The company’s master overhaul kits are what you need for a successful differential rebuilding. The Yukon master kits ensure a hassle-free install by utilizing the best components, including Timken bearings. Just remember, assembling gear sets properly is not for the inexperienced mechanic, nor for the experienced tech without the proper tools. A word of warning: Make sure you follow the manufacturer’s break-in procedures faithfully if you change your gearing! Today’s gears are cut closely to increase their lifetime and reliability, and to decrease wear and noise. If you don’t break them in correctly and carefully, they could very well seize and strand you in the middle of the desert.
Once you’ve changed your gears or tire size, you need a computer control module to recalibrate your speedometer and shift points. If you don’t wish to buy a module, most shops will have modules that will reprogram your onboard computer.
With a 4-inch lift, we also needed a longer rear driveshaft. The OEM front driveshaft is built with a double Cardin-style CV (constant velocity) joint while the rear is a slip-yoke/U-joint design. We decided to retain the front driveshaft, but we had it rebuilt with new joints by Precision Automotive in Kingman, AZ.
We contacted Powertrain Industries to replace the rear driveshaft. Powertrain builds double Cardin-style CV (constant velocity) driveshafts, which replace the OEM slip yoke. The 1310 series for Jeeps are somewhat light-duty compared the 1350 series or larger normally seen on ¾-ton or 1-ton trucks, but it is plenty strong for the Wrangler applications. And Powertrain’s design gets rid of the slip yoke and returns to the old-school design of the slip joint being near the differential end of the driveshaft.
TNT Customs of Cheyenne, WY offers a sophisticated, strong, stable Rock-Tek suspension for TJs and LJs. Quite simply, this is the most technical suspension system with which we’ve ever worked. According to TNT:
“The Rock-Tek suspension is specifically designed for tackling the most difficult terrain. The TRI-4 rear suspension is designed to cycle vertically throughout articulation to eliminate rear-steer commonly found in other suspensions. Aggressive geometry coupled with a flat roll center will make your Jeep stick to even the steepest climbs or radical off-camber situations. Lower arms are manufactured from 2- x0.375-inch 1026 DOM tubing, to handle anything they may come in contact with. Upper arms are 1.50- x0.250-inch 1026 DOM tubing. All control arms have 2.5 inches of adjustable length and feature rebuild-able Flex joints and bushings. Our high clearance pan gains 1 ¾-inch over stock on ‘97-‘02 models and nearly 3 inches on ‘03-‘06 model years. TNT Rock-Tek long arm suspensions come with a weld-on, full-length rear axle truss assembly that fits your rear axle. Our axle truss assembly includes our Tri-4 mount, coil spring mounts as well as new lower control arm mounts and shock mounts.… An added bonus of a weld-on truss assembly is an axle housing that is fortified against bending that can be caused by extreme terrain coupled with oversized tires.”
Keep in mind, though, installing this system requires a very well-equipped shop with a vehicle hoist, a plasma cutter, a MIG welder, several grinders, air tools, drills (both floor and hand-held), power saws, floor jacks, and a multitude of hand tools. Plus, you’ll need an experienced technician and patience (our Jeep was on the hoist for two months). In addition, many procedures require more than one set of hands.
Even with all those warnings, conditions, and requirements, the suspension is well worth it! It’s simply one of the best commercially built suspension systems we’ve ever had the pleasure of riding on. Not only is our Jeep more stable than stock at increased trail speeds, it is one heck of a rock crawler, too. While the LJ’s Rock-Tek Y-link high-clearance front arms are similar to the Jeep’s OEM suspension, changing the rear suspension from a track bar system to a Rock-Tek dual-triangulated four-link and relocating the shock mounts vastly improves the overall stability of the Jeep.
TNT recommends Fox 2.0 Performance Series IFP smooth body shocks for use with the Rock-Tek system. These shocks use race-proven dampening control to provide a comfortable on-road ride and predictable off-road handling in even the toughest conditions. According to Fox, the precision metal impact aluminum body increases cooling capacity and will never rust, and inside is Fox’s race-proven performance-valving technology. Fox’s Internal Floating Piston (IFP) mono-tube design separates the shock oil from the high-pressure, nitrogen-charged gas chamber.
As we alluded to earlier, we decided to go with Yokohama’s Geolandar M/T 35×12.50-17 directional tires. These tires can churn through the deepest of mud with ease because they feature beefy, deep tread lugs and a directional tread pattern for outstanding traction and easier clean-out. Plus, the tires have extra sidewall ribs and an advanced tread compound for added durability out on the trail.
We mounted our Geolander’s on a set of Raceline beadlock wheels from Summit Racing. The Raceline’s are designed, engineered, and machined to hold up to the extreme abuses of off-road desert racing and professional rock crawling. The fully functional beadlock wheels have a 32-bolt beadlock system built with durable outer rings and a strong inner lip to securing the tire’s bead to the wheel, even at extremely low air pressures.
In addition to strong wheels, strong steering is also needed for controlling the larger, heavier wheels and tires. We turned to Rugged Ridge for its heavy-duty steering components and stabilizer. Once the Jeep was finished, we took it to a Ted Wiens store in Las Vegas, NV for a full chassis alignment.
In Part 2 of our “GP” project, we’ll cover body armor and protection.