The 1997-2006 Jeep Wrangler (TJ series)
Prototypes for the Jeep Wrangler “TJ”
There were three prototype vehicles competing to become what became the 1997 Jeep TJ (the 1996 model year was skipped). The preceding Jeep CJ and YJ (in that chronological order) were live axle designs with Hotchkiss suspensions, commonly known as a “parallel leaf spring” suspension.
The production group at Jeep/Truck Engineering (JTE) proposed a long-spring version of the YJ suspension to increase mobility, the lowest cost upgrade possible for the YJ. The Vehicle Development group proposed adapting the XJ/ZJ link coil suspension (Cherokee/Grand Cherokee) as the medium cost solution. This was eventually approved for, and used in, the TJ.
Evan Boberg and I, from PreProgram Group (this was the R&D group for Jeep and Dodge trucks), were tasked with hacking together an independent design to approach the Jeep performance standards for production. We were able to greatly exceed those standards, exceed on-road standards, anduse a narrower track width. We got 17 patents on the design, which had a floating axle to eliminate the main issue of an independent drive axle—that of half shaft joint angularity. This set up was used on a mule called “Li’l Blue.”
The downside to our design was cost. Not just a little bit more expensive, but a lot. I am sure we could have reduced the cost, given more time, but we only had just under ten weeks to design all the parts, build the vehicle, and get it to Moab, Utah for testing.
TJ Jeep Wrangler final-production suspensions
The TJ Jeep Wrangler used a Quadra-Link front and rear suspension, similar in basic design to that of the Cherokee; modifications were made to the Dana 30 front axle (also used in the prior YJ), and a Dana 44-3 axle was added in the rear to accommodate the Quadra-Link (link-coil) suspension.
The Dana 30 front axle has an under-slung carrier and is identical to the ZJ’s. The steering knuckle and ball joints have the same service features as the previous YJ model. The brake caliper adapter is part of the steering knuckle and non-adjustable steering stop. The lower ball joints are lubricated for the life of the vehicle and have no grease fitting due to space constraints.
In the Quadra-Link suspension, four control arms are connected to both the frame and a driving axle – the same ones used on the Grand Cherokee (the only difference being the bushing rates). Anti-dive effects were incorporated into the control arm angles, improving braking performance. The front and rear control arms use rubber bushings that help isolate road noise; the control arms are mounted to the axle and frame through the bushings. The front and rear lower control arms are identical, but the upper control arms have isolator bushings in both ends.
Instead of leaf springs, the suspension uses single-rate coil springs, which provide minimal friction and thus more free articulation and smoother operation. Four spring rates were available – computer-selected depending on the option packages ordered on the Jeep. Variable-rate rear springs improve the Jeep’s ride height and ride control at full load. The rear variable-rate springs were available with three spring rates (also computer selected).
The standard twin-tube, 1.18-inch (30-mm) diameter shock absorbers have deflected disc valves that control ride motions and oversized reservoir tubes. The oil passage area varies through the deflection characteristics proportionally to the shock’s velocity; faster motion results in a larger passage area and limited dampening force. The shocks are low-pressure gas charged to help prevent air pockets (cavitation) during rough road operation. The rear shocks are attached to the fuel tank cross member using weld nut
High-pressure monotube shocks were also available in the optional heavy-duty suspension package. Compared to the standard shock absorbers, the option has larger-diameter pistons, more fluid capacity, and higher gas pressure. Ride height is the same as with the standard shocks, and the mounting is inverted (compared to the 1995 Wrangler), putting the shock’s body lower in the air stream to improve cooling.
The TJ Jeep’s frame was fully redesigned since the YJ, increasing thickness by 25% and torsional stiffness by 15%; the full-perimeter frame, chosen due to high off-road use, was retained. A two-piece box-section fuel tank cross member with corner gussets contributes to the increased torsional stiffness, along with a thicker, flanged transfer case skid plate that supports the transmission mount.
A front track bar controls front-axle lateral movements, and body roll is minimized due to the high-roll centers based on the track bar’s axes. The angle of the front track bar and the steering linkage angles also reduce steering effects from vertical movement. A rear track bar is attached to the left axle bracket and the frame rail on the right side, using rubber bushings at both ends.
Concealed by a valence panel, the spring-steel stabilizer bar is mounted across the chassis’ underside to the rear axle. Links connect the stabilizer bar to the axle brackets, and sleeve bushings are on the upper and lower ends of the links. The mounts are isolated by rubber bushings.
Urethane, micro-cellular jounce bumpers on the front and rear engage with the axle early in jounce travel. The progressive, dual-rate bumpers have a quick rising rate that help minimize the possibility of bottoming out. The jounce bumpers are in steel cups that act as full-travel stops, and a solid urethane core gives a second stage rate.
The TJ Wrangler’s diagonal articulation was greater than that of the YJ at about 7 inches (178 mm), helping off-highway capabilities to return to roughly the space of the last Jeep CJ.
Birth of the Jeep Wrangler Rubicon
The Rubicon had the largest tires ever installed on a non-CJ Jeep. During testing, the combination of these tires and the 4:1 reduction in the transfer case caused a great deal more gear tooth-bending stress, which damaged the ring gear and shattered the C-clip holding the axles to the spider gears inside the axle.
The first thought was to simply replace the axle assembly with a wider, old style full floating Dana 44 from a CJ. Unfortunately, the cost for this axle was not in the budget for Rubicon, so Chrysler and Dana went to work on a cost reduction for the rear axle. The end result was to use just enough of a 44 to solve the stress issue, while still allowing line interchangability with the lighter duty 35C, which was still adequate for an unmodified TJ with standard tires. Axle tubes, brakes, semi floating hubs, axle shafts, and bolt circles were all interchangable with the 35C. The only major difference to “non standard parts” were the special tools used by Spicer for sizing up the tube ends to fit the Dana 44 carrier housing, and the special spider gears in the differential to adapt the 35C axle shafts.
On the front axle Rubicon needed a matching 44 to replace the 30, for the same reasons. The same approach was used with one hitch: the link-coil overconstrained suspension. The carrier housing sat right where the left hand upper control arm axle bracket wanted to be. Luckily, the bushing (suspension isolator in the upper control arm) fit by clearing the housing, so some fancy footwork in casting the carrier housing and a process change before production allowed this issue to be sidestepped neatly.
Now you had a slightly stronger setup that fit and solved the issue of shattering gears. It didn’t come cheap, though — in production terms it cost about $110 per axle ($220 per vehicle) more. Multiply that by the total number of Rubicons built on TJ for some staggeringly high dollar amounts.
- Jeep Wrangler Sport (2000) and Rubicon (2003)
- Jeep Wrangler Rubicon (2007)
- Jeep Wrangler Unlimited Rubicon (2007)
1994 Jeep Wrangler updates
In mid-1993, an optional Add-A-Trunk was made available; this was a steel storage container that mounted behind the rear seat, bolted to the rear wheel houses. It had a hinged lid which was flush with the tops of the rear wheelhouses and had a carpeted cover; with the gate open, the trunk lid could be held open with a prop rod.
An easy-operating soft top was made optional on Wrangler during the 1994 model year; it came with the hardtop's full doors rather than the standard soft top's half doors. The top included over-center tension bars to make folding and raising one person operation. A boot system covered the folded top sides when not in use; rear side curtains and the rear window were attached in the same manner as the standard soft top. Regardless of top, Wranglers in 1994 got a tail-mounted extra brake light, posted to the right height by a gooseneck bracket.
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