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2009 Jeep JK Wrangler on 40 Inch Michkey Thompson Tires - Lowerforty

Posted in Project Vehicles on September 1, 2009
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When the Mopar Underground revealed their incredible "Lower Forty" JK Wrangler at this year's Moab Easter Jeep Safari, the fact that it rolled on 40-inch tires without any sort of suspension lift raised eyebrows. After all, this went against conventional school of thought. Heads were scratched and pondering ensued. The answer to the "How'd they do it?" question was simple: Body mods. Lots of 'em. Sure, any of us could make 40-inch tires fit under a JK by simply hacking off body parts until the tires have room to travel. That's easy. Problem is it would look like we simply hacked off body parts until the tires had room to travel. The result would probably be visually hideous. Which brings us to the crux of what makes Lower Forty unique; we all know that the rig had to be modified, but it's done so well that most have to study it for a while to figure out how and what they did.

The Jeep Underground wanted Lower Forty to be low and stable, but they also wanted it to look great. With that in mind, they made a sweeping number of mods to the exterior. For instance, they chopped the windshield height by 3 inches and then increased the rake to 46 degrees as opposed to the stock JK's 27 degrees. The meaty 40x13.50/20 Mickey Thompson Baja Claw Radial tires are mounted on a set of 9-inch-wide Mopar forged aluminum wheels. The rig was painted a custom color they call Red Eye No.3. Functionality was of course a major consideration, and that was the driving force behind many of the exterior mods. Here you can see the new tubular crossmember mounted just below the grille after the front frame horns and stabilizer bar were removed. These mods contribute to the rig's outstanding 90-degree approach angle. The custom carbon-fiber fender flares are one inch wider and four inches taller than stock flares, and they're coated with a rubberized bedliner inside to ward off damage from rocks and debris. Some of the less obvious mods include custom towhooks, and shortened rock rails to compensate for the larger wheel openings.

From this angle, you can see the cargo area and tailgate mods. Inside the cargo area is a securely mounted spare Mickey Thompson tire on a Mopar wheel. Directly above that is a Poly Performance cargo rack, which was affixed with cool body color-matched custom mounts. The custom drop-down-style tailgate gives a shout out to Jeep's heritage. The 'gate began life as a JK piece, but it was modified with the sheetmetal outer of a Jeep logo-embossed CJ-7 tailgate. The inside of the tailgate was resurfaced with a flat sheet of steel. The guys at the Mopar Underground reinforced the body structure under the tailgate so they could attach the hinges. Limit cables were sourced from a Dodge Dakota and the "shotgun-style" latches were custom made.

So, How'd They Do It?
There was no computer program or complex mathematical formula to guide the folks at the Mopar Underground when it came to the body mods. One afternoon, a few engineers, including Mark Allen, chief designer at the Jeep/Truck Design Studio, took this JK, which was actually an unsellable prototype that was used for all the early JK Wrangler brochure images, out to the Chrysler Proving Grounds, where they bolted up a set of 40-inch tires. They then broke out an arsenal of tools including a battery-operated Sawzall, a bunch of extra blades, and a few extra batteries. Allen says the rest of the afternoon consisted of "flex, saw, flex, saw, flex, saw, flex, saw." The flexing and sawing didn't stop until the necessary clearance had been achieved.

PhotosView Slideshow

From there, the JK was brought back to the Design Studio to have the new fenders and hood mocked up and then digitized. After that, the rig went to the Metal Fabrication Shop. It was there that a mind-boggling amount of work was completed. The windshield was chopped and raked; the custom roll bar was fabricated; the front and rear frame horns were removed; new front and rear crossmembers were fabricated; and custom towhooks were installed. New rear interior sheetmetal was fabricated; the seat riser height was reduced; a custom drop-down tailgate was added; a Dodge Challenger fuel door was integrated; the door corners were filled; the front fenders trimmed; and the rock rails were shortened. Nothing major.

Meanwhile, the Plastic Fabrication Shop was busy milling the molds that would create the new carbon-fiber hood/fender combo, front fender spats, and rear fender flares. When all of the body and paint work was completed, the JK was sent to Burnsville Off Road to have a pair of Dynatrac-prepped axles and a 5.7L Hemi engine with six-speed manual transmission installed using an American Expedition Vehicles installation kit.

PhotosView Slideshow

The Result
The upshot of all this work is that Lower Forty gains nine inches of ground clearance due to the increase in tire diameter, while at the same time retaining a low center of gravity due to the lack of suspension lift. But that's not all. The larger tires combine with the body and frame mods to create an incredible approach angle of 90 degrees, a departure angle of 60 degrees, and a breakover angle of 40 degrees.

We had a chance to spend a fair amount of time off-highway behind the wheel of Lower Forty. The power-to-weight ratio is incredible thanks to the Hemi engine, the crawl ratio is darn impressive thanks to the Rock-Trac 4:1 transfer case and the six-speed manual transmission, and the low center of gravity means the rig is stable whether it's off-camber or blasting over dunes.

All of this resides in a package that looks pretty darn good. Here's a detailed look at Lower Forty.

PhotosView Slideshow

Owner/Hometown: Chrysler Corp./Auburn Hills, MI
Vehicle/Model: 2009 Jeep Wrangler
Estimated value: N/A

Type: 5.7L Hemi V-8
Aspiration: Fuel injection, 2-inch-diameter exhaust, Magnaflow mufflers
Output, hp/torque (estimated): 375/375

Transmission: Getrag 238 6-speed manual
Transfer Case: Rock-Trac 4:1

Front: Stock coil springs, Bilstein shocks
Rear: Stock coil springs, Bilstein shocks

Front: Dynatrac-prepped Jeep J8 Dana 44, Bilstein steering stabilizer, Burnsville Off Road Rockproof Axle Upgrade Kit/ARB Air Locker
Rear: Dynatrac-prepped Jeep J8 Dana 60/ARB Air Locker
Ring and pinion: 5.38:1

Wheels: 20x9 Mopar forged alloy
Tires: 40x13.50-20 Mickey Thompson Baja Claw Radial

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