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MadMaxxis Off-RoadRunner Part 2

Posted in How To: Tech Qa on October 28, 2015
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Building a 4-wheel-drive muscle car just because we are huge fans of the Mad Max movie franchise is one of the craziest projects we’ve ever undertaken at 4WOR, and that’s coming from the guys who built the double-ended Jeep Wrangler back in 2012! This 1973 Plymouth RoadRunner is going to be nuts. The car (in stock form) has no real frame but is akin to a Jeep Cherokee with a unibody design, and we are planning on stuffing a big heavy V-8 engine in this beast, so plenty of structural upgrades are needed. Plus, it’s pretty rusty. Not East Coast rusty where the roads are salted in winter, but still pretty bad in the eyes of most Californians. That’s probably why we could buy it so cheap after it had been on Craigslist for weeks.

So we started with a rusty old pile of Mopar and drug it to Chappelle’s Exhaust and Kustoms in El Cajon, California, where we began the destruction before the construction. The goal was simple: Build something cool and fill it with tires from Maxxis. Maxxis had a new mud tire coming out and was willing (or crazy enough) to sign up as a partner on this build. It couldn’t be a better fit. We need big tires, and calling the project the Mad Maxxis Off-RoadRunner just makes sense.

This month we’ll dive into more of the build, but you can see all the action on Dirt Every Day presented by 4Wheel Parts on the Motor Trend Channel on YouTube and on the new website.

The 1973 RoadRunner wasn’t perfect when we drug it to Chappelle’s Exhaust and Kustoms, but we had a solid plan to make it awesome. The rusty trunk lid was just the tip of the iceberg when it comes to the “patina” that had eaten away the body of this fine automobile.
We gutted the trunk, trunk lid, rear window, and dividing material and began tubing out the rear end. A 50-gallon diesel tank was sourced from a junkyard tow truck to feed the Cummins V-8 that will live under the hood.
We can’t say that our roll bar is conventional at all, but we feel it is strong by design, even though the design is wacky. The fuel tank sits right behind us and bolts to a recycled propane tank that was bolted to the floor and cut open to make a dog house in case our hounds want to ride along. We doubt they’ll want any part of this insane machine.
Behind the fuel tank we will stuff two spare Maxxis tires. Not because we think we’ll have failures, but rather because we are running different size tires: 35s up front and 40s in the rear. This will require different gear ratios in the axles: 4.56 up front and 5.38 in the back.
The RoadRunner is a unibody vehicle, and we knew it wouldn’t be enough for our drivetrain. We hit up the local Industrial Metal Supply and came home with a few sticks of 3x0.188-wall tube to build a solid subframe for mounting our suspension. The frame is welded straight to the bodywork, so it is still unibody, but now with thick steel instead of sheetmetal to build off of.
Our rear Deaver springs were left over from the 2011 Ford F-150 we built for an Ultimate Adventure a few years back. That truck was returned to Ford and sadly crushed, but not before we scavenged some goodies to recycle into the Off-RoadRunner.
A 3-inch boxed tube rear crossmember supports the rear of the leaf springs and ties through the floor to the rollcage in the bed. We had just five weeks to build the car, so the fabrication and suspension had to be dirt-simple, strong, and easy to assemble.
The rear axle is also simple and strong. It’s the Dynatrac ProRock 80 stuffed with an ARB Air Locker and 5.38 gears that was previously under the UA F-150. We also added a set of 14-inch Fox shocks with compression damping adjusters that mount to the bed cage through cutouts in the rear wheelwells.


Huntington Beach, CA 92647
Fox Racing Shox
Watsonville, CA 95076
Chappelle's Exhaust And Kustom
Industrial Metal Supply
Irvine, CA 92606
Suwanee, GA 30024
Columbus, IN 47202
Deaver Spring
Santa Ana, CA 92701

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