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Ultimate Jeep! How to Build It: J-Truck

Posted in How To on September 1, 2001
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Photographers: The ManufacturersChristian HazelJp Archives
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While the J-truck may not be the first Jeep to come to mind when thinking of an ultimate Jeep, it does have some qualities that the other models lack. Number one: It’s a fullsize. Number two: It’s got plenty of wheelbase, up to 132 inches (J-4000). Sure, driving a fullsize on the Hammer trails is like letting a bull loose in a glass shop, but just try and climb a greasy waterfall, pound over whoops and bumps, or tow a real trailer with your flexy flatfender. No siree, that’ll be a one-way ticket to crash town. Something else the J-truck has is a 45-degree factory approach angle. That’s better than an FC and the same as a flatfender, and of course, it increases with a lift and larger tires. Here’s how we’d build ours up.

Chassis

We would start with a ’71-’73 Thriftside J-2000 for its 120-inch wheelbase or a ’74-’84 J-10 with a 119-inch wheelbase. We’d keep the frame and body and that’s about it (this is ultimate, not budget). We dig the Thriftside bed because it can be easily trimmed to fit larger tires or we can remove the rear fenders altogether for tight-tree’d or rocky trails. We would build a full cage from bumper to bumper. The main bars would be made from 1 ¾-inch .120-wall DOM tubing (chrome moly is overkill) welded directly to the frame. The cage would increase the rigidity of the chassis and keep things from flexing and breaking. This is mostly for stability in the high-speed stuff but it would also provide safety in the event of a roll, uh, when we roll it. The suspension would consist of long GM truck rear springs on the front in a spring-under configuration with reservoir shocks like Bilsteins and a shackle reversal. We’d build a CORE-truck style four-link out back. We would keep the truck as low as possible and run a rear NASCAR-type sway bar and 2 ½-inch coilover shocks with the lightest coils possible on dual-rate hardware. Nitrogen-charged bumpstops would be found at each corner.

You might be wondering why we would bother with leaf springs up front and build a four-link in the rear. Simple, it’s difficult to make the suspension and steering work well on a long-travel straight front axle. We don’t want a full hydraulic steering system so a ram assist would be grafted to the front axle. We can easily get 12 inches of travel from the long GM rear springs and we can make the steering work well in this configuration. It wouldn’t really be cost-effective to gain a couple more inches of travel for the added expense. Sure it’s the ultimate J-truck but we’re not into way-overkill. However, coilovers on the front would allow more spring-rate adjustability. But we’d go for the leaves anyway. Out back a four-link will eliminate any possibility of axlewrap and provide lots of adjustability and suspension travel. Leaf springs just don’t compare for rear suspension.

Driveline

To keep the J-truck somewhat light we would run Ford 9-inch axles front and rear. To knock the weight down and get more ground clearance we would opt for chrome moly plate-style axles like the Avalanche Engineering Advantage series axles. We’d shave off an additional 40 pounds by using aluminum centersections and pinion supports and stuff them with 6.50 gears. The rear would have a 35- or 40-spline aluminum spool, again for weight savings. The frontend would get a 35-spline Detroit Locker and Dana 60 knuckles with 35-spline outers. Non-CV driveshafts with 1350 U-joints would connect up to an NP205 modified with twin-stick shifters. An SM420 would take care of tranny duties ‘cause we want a manual and no other tranny has a 7.05 First gear. In front of the tranny we would mount an Advance Adapters Ranger Torque Splitter. The use of this unit will fix two potential problems.

The suspensions on trucks with manual transmissions are often limited by the length and angle of the front driveshaft. The Ranger will increase the length of the front driveshaft by 7 inches plus provide a 27 percent overdrive when engaged, effectively turning our 6.50 gears into 4.75s, perfect for our 37-inch tires on the highway. We generally don’t like messing around with rebuilding an engine and we have often claimed a crate engine is the perfect rebuild kit for us. So, spinning all these hefty gear-drives would be a simple crate motor, the GM Ram Jet. With 350 roller-cam horsepower and 400 lb-ft of rock-chucking torque on tap the J-truck would be ready to light things up, namely the tires. The Ram Jet comes complete with the ignition and fuel-injection so all we have to do is run a wire or two, hook up a fuel line, and wham—instant throttle response at any angle.

Body & Interior

We’re into extremes. All the way left or all the way right. So for the body we would either spray on a real nice bright red or spray the truck down with pool acid, let it rust for a year, then clear-coat it. Although, to keep the weight down we would probably use fiberglass fenders and a hood from U.S Body Source. Carefully trimmed Ford bedsides could be used to replace the steel bed. If we went for the ’glass then we would paint it. We’d keep the inside pretty utilitarian with minimal gauges but plenty of dummy lights. We don’t need to know the temperature unless it’s overheating, and even then it doesn’t matter if it’s 260 or 270 degrees—it’s hot. A light can tell us that. Other interior items would include cloth high-back bucket seats like Beard’s or Mastercraft’s. Leather seems so snooty and vinyl would get too hot in the summer. Air conditioning would also be a sweet bonus. With A/C in the J-truck we could head out to the desert in the summer in comfort for off-road races and general jumping mayhem.

Tires & Wheels

We generally feel radials have weaker carcasses than bias-ply tires. However, there are few radials that are larger than 35 inches. So, for tires on our J-truck we’d go for some 37-inch mud tires that are relatively lightweight, have an aggressive tread pattern, are durable, and will work well on many different kinds of terrain. We’d mount them on M.R.T. three-piece carbon fiber wheels to keep the weight down. The M.R.T. wheels weigh about 13 pounds each. We would add aluminum bead locks since the plastic ones that come with the wheels tend to get chewed up in the rocks.

Good, Bad, & What It's For

The suspension would provide at least 12 inches of vertical travel in the front and around 18 inches in the rear, more than enough travel to hit bumps and jumps at speed. It should ramp just under 1,000 (probably in the 800-900 range) on a 23-degree RTI ramp, which is good for a fullsize. It certainly wouldn’t fit on tight trails but our ultimate J-truck could hit dunes, hill climbs, mud, and just about any other kind of ‘wheeling you could throw at it, even tough truck and mud bog competitions. With all the power and gearing options it should be able to tow a barn. However, tongue weight will be critical. With the soft coils in the rear a heavy trailer will make it squat like a lowrider. The SM420 has never been known for being a fast-shifting transmission and the 7.05 First is virtually useless on the street. So we essentially have a three-speed tranny. But the Ranger can be shifted while the J-truck is moving so we have more gearing options. It would be an eight-speed.

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