It's a well-known fact that cars with unibody construction—aka unit body, subframe, or stub frame design—are designed to satisfy an automaker's need for both strength and low manufacturing cost. Yet in spite of some very clever engineering, both of these goals are ultimately in contradiction with each other. As you lower the cost of production, the stiffness—and thus the handling and ride quality—goes down. In the 1960s, Chrysler was arguably the most successful of the Big Three at walking the cost/performance tightrope, but as the decades rolled on, the compromise of a unibody chassis comes increasingly to light.
The compromise we're talking about manifests itself as shaking, twisting, and flexing. If unchecked, this shows up as cracks in paint, wrinkles in sheetmetal, misaligned body panels, and cracks in glass. The cause is that the front suspension cradle is not linked solidly to the rear suspension structure—except through the thin sheetmetal of the floor and greenhouse. When the front suspension moves one way and the rear moves another, the old body pays the price. But that's not all. The suspension is unable to do what it's designed for and the car can handle in unpredictable, nonlinear, and unsafe ways. And we haven't even begun to talk about adding more power, bigger brakes, better suspension, or stickier tires.
In the 1960s and 1970s, the Mopar A-, B-, and E-Body chassis offered some of the best performance available at that price, but over time, the weld points and stressed areas of the body shell begin to yield. Even the best-kept cars fall victim. Since rebuilding the Mopar's unit body from scratch is impractical, the best bet it to install a set of weld-in subframe connectors from US Car Tool, sold through Reilly Motorsports (RMS) for $199 a pair. These are beefy form-fitting C-channels that connect the front and rear frames of the unibody Mopar, and tie to the body at the same time. Moreover, these are available for Mopar B- and E-Bodies, as well as A-Bodies.
Since our 1968 Plymouth Valiant (a 1964-76 Chrysler A-Body) is already equipped with the RMS Alter-K-tion front coilover suspension and the RMS Street-Lynx rear four-link coilover conversion, we would be committing structural suicide by not installing a set of subframe connectors. It was a no-brainer to order the subframe connectors, and it only took a half day to install them using our Millermatic 211 MIG machine.
Ron Aschtgen of Outlaw Motorsports (Riverside, California) is helping us assemble Project Valiant, and he handled the subframe connector installation for us. As Aschtgen discovered, the US Cartool units fit our A-Body very closely, their contoured sides needing very little trimming and grinding to create a gap small enough to make effective MIG welds with the Millermatic 211. Some minor fine-tuning with the fit is to be expected on something like this, and along those lines, we also had to relocate the service brake, parking brake, and fuel conduits. Nevertheless, the end product of vastly improved chassis stiffness was worth that small amount of trouble.
As a result of this half-day's work, our Valiant is for all intents a full-frame chassis—in fact, it's even better than that. We have given up absolutely none of the existing strength in the Mopar's gusseted and boxed unibody—a strong point that traditional full body-on-frame cars—such as GM's A-, B-, and G-Bodies—never had. Now we've got the best of both worlds, with plenty of stiffness to handle the torque from our 657hp Indy low-deck wedge, as well as the higher cornering and braking loads from our RMS suspension and Wilwood disc brakes. This ol' Mopar chassis now has another half century left in her!
1. In this "before" photo, it's easy to see the front suspension structure—or subframe—and the rear suspension rails. Note how they are only superficially connected to each other by the body and floor. As it sits, all of the engine torque and suspension loads must pass through the body, which easily twists in its advanced age.
2. While simple looking, these formed and contoured steel rails are going to transform pretty much all driving-related aspects of our Mopar project, from cockpit noise and road manners, to cornering, braking, and launching. These A-Body pieces are made by US Car Tool and sold by Reilly Motorsports for $199 a pair. The left and right rails are unique and will only fit one side of the car.
3. Installation begins by trial fitting the subframe connectors to the car. Note how nicely they conform to the shape of the floorpan from the start. You'll still need to grind and tweak them to get the gap down small enough to safely weld them.
4. In the previous photo, you can see where the service brake line snakes through the connector toward the front. Here's a close-up of the channel Ron Aschtgen made for the line to pass through. The parking brake assembly was also removed—we'll be relocating that at a later date, but most often it will pass over or under the connector.
5. Trial fitting the subframe connectors is an iterative process, and here Aschtgen is grinding away a high section that he marked with a Sharpie marker. The connectors also have a series of breakout tabs that conform to various years and models of Mopar A-Body, depending on the shape of the floor. These are either broken off or welded solid as needed.